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PSA#9 - mainly above 8,000
feet! Fri 3/23/2007 5:40 PM
Good evening everyone,
I can't believe it has been almost two months since the last PSA
(31jan), but while February remained cold, the snows just 'nickeled and
dimed' us towards a near-normal month, and March has been an
unmitigated disaster so far (from a snow lover's perspective). This is
about to change for folks higher up, flat-landers will have to deal with
a good amount of rain instead! A storm that had dropped into the Baha
region over the last few days is now 'yoyo-ing' back towards us, and
should pass right over us within the next 24h. The first (big) December
snow was a bit like that, with the difference being the warmer
temperatures that one gets three months later, but the subtropical
moisture source, and the forward speed of this storm is quite
comparable. Low-level easterlies have already started to blow towards
the Front Range, and a lot of moisture is coming towards us. The main
low is still sitting in southern Arizona, but a piece of this storm is
quickly moving our way along with a jet streak that will put us right
under the left front quadrant - very favorable for enhanced upward
motion! The rumble outside attests to the decent vertical instability
associated with this storm system, so a good amount of rain (0.5-1")
could fall with a few evening thunderstorms that are drifting in from
the south. Almost all the cooling with this storm will come from the
precipitation itself (and associated evaporation from falling
precipitation), since no cold air to speak off is within a long distance
from here - no real cold front in sight! Deep upslope should remain in
place overnight until the upper low tracks either right over northern
Colorado (towards the northeast). It will weaken as it crosses over us,
so the main precipitation will take place by tomorrow morning.Night-time
cooling should slowly lower the snow line from about 10K right now to
possibly 7K by tomorrow morning. While the weather service (and various
forecast models) has been waffling this week on how to call this storm,
they have now issued a winter storm warning for 6-12" in the foothills,
with all rain expected in the plains. I have no overall disagreement
with this forecast, except that the amount of liquid that could fall
over the foothills (Larimer County in particular) could translate into
up to two feet of snow in the highest (east-facing) elevations. I fully
expect to see up to 2" of moisture with this storm, and the earlier it
transitions into snow, the more you going to get.Ironically, the amount
of moisture in the air (dewpoints in the 40s just east of here) is
reducing the "wetbulb-cooling" effect that will bring temperatures down
- if it had been drier to begin with, the cooling might have been
dramatic! Enjoy the first thunderstorm of the season/finally a spring
storm, and, maybe, a foot or more of heavy wet snow above 8K - it will
warm right back up early next week, ahead of the next storm that looks
like bora right now (should be good for powder west of the divide,
though, most likely on Wednesday) -Klaus
P.S.: El Niño is dead, long live theanti-correlation tendency of wet
Aprils to follow dry Marches, such as in 1995, 1997, 1999, 2004, and
2005!
PSA#8 (=1-2 punch + extreme cold on Friday) Wed 1/31/2007 12:22 AM
Good morning,
I will keep this one short. Today (Wednesday) looks like yet another
"little snow storm" day that will nevertheless pack a punch since it
will be cold enough to stick all day even in the plains. Temperatures
will be dropping all day, with the potential for sub-zero readings by
midnight/early Thursday in the higher foothills, and just above that in
Boulder et al. Most of the moisture that we will wring out from the
clouds should fall in the form of fluffy dendrites. Therefore, I am
going for 3-6" even in the lower elevations, with the potential for a
few inches more in the foothills with good north(eastern) exposure. The
more interesting situation will set up for Thursday night with a true
arctic cold front that will usher in sub-zero temperatures for most of
Friday both in the plains and foothills (yes, I am going colder than
most other forecasts out there). Depending on a couple of factors that
are not settled yet (how much spill-over of the arctic air to the west
slope, how much moisture is left in same region to be blown over the
dome of cold air that will set up for a day or so, how fast the front
rushes in & through), we might wake up on Friday to not only sub-10F
readings in the higher foothills and sub-zero in the plains, but also
another 2-6" of snow. Right now it looks like the cold air will only
hang in for a day in the foothills and maybe two days along the Front
Range (Saturday should be warmer in any case, although the wind chill
may make this warm-up miserable higher up), with warmer (and dry)
conditions into next week, but the models have been quite fickle about
this, so the amount of warming will depend very much on how much
westerly flow we will get to flush the cold air eastward (say, to
Greeley). Would not be surprised if we stay at or just below normal
early next week rather than a true Chinook situation. What a winter!
And, yes, this arctic snap is NOT typical for El Niño winters (despite a
couple of precedents in '72-3). Meanwhile, might as well cheer on our
attempt to set all kinds of seasonal snowfall records (more on that in
my next PSA/postmortem) -
Klaus
PSA#7 (don't blame the
messenger!) Thu 1/4/2007 5:03 PM
I will keep this one short, too busy to do this one justice, and it
won't be as big as the last two. Another eight days have passed, and it
is time for yet another storm. This one is a fast mover, it has a very
impressive jet streak (>150mph) associated with it, and we should have a
few hours under the left front flank of it later tonight (good setup for
heavy snow showers). A cold front is trying to slip right in from the
north, there is plenty of moisture around (recycled from melting snow in
recent days?!), and we should have at 6-12h of decent upslope starting
right around midnight.
Bottomline: except
for jet-induced showers that could streak out onto the plains, we should
keep most of the snow right against the foothills, more or less like the
last storm. I am expecting 6-12" from Boulder westward, with some lucky
(?) folks measuring up to 18" by tomorrow around noon. If you get 2"+/h
sometime after midnight, you might sit underneath one of those heavier
snow bands, and end up with those higher amounts.
Since this will be mostly overnight, and fairly cold air should get
entrained by tomorrow, there won't be any rain this time.
Happy Snowy New Year!
Klaus
P.S.: No time for long postmortem, but here is a nice web site to see
snowfall totals for the last two storms (yours truly is NOT the snowiest
location on these maps, believe it or not).
http://www.crh.noaa.gov/bou/?n=winter2_storms
P.P.S.: We are on record-pace for seasonal snowfall totals, in many
locales exceeding the '97-98 pace in particular, and already ahead of
last year's seasonal totals, and we are not even at the usual half-way
point!
PSA #6 - on record snow&PSA-pace... Wed
12/27/2006 3:48 PM
Wow, here we go again, just eight days after the last HISTORIC December
snow, here is another storm knocking at our door, and it has the
potential to give us almost as much snow as the last one! If you can't
get 'ready' today, you may have most of the morning tomorrow to run
last-minute errands. Given the state of our roads right now, it will
take less snow than last time to create poor to impassable road
conditions.
Hopefully, DIA has gotten some of the cobwebs shaken off, and is better
prepared for the next storm...
This storm is coming on a more traditional, if slightly southern, track
at us, including a (south-of) Four Corners low at higher levels, and
with slightly less moisture to begin with (although a moisture plume
from the eastern Pacific appears to be feeding into it already). It will
start fairly mild tomorrow morning (if you see any precipitation in the
plains in the morning, it might be mixed rain/snow!), but will wrap in
colder air than in the last storm by Friday. What makes this storm quite
unusual (aside from being a spring-like storm in December!) is that a
second short wave will drop in from the north on Friday, which should
slow down the progression of the main storm to the point of feeding yet
another moisture plum from the Gulf of Mexico back into our region by
Friday evening, giving us a good 'back-wash' scenario.
Of course, this
scenario, while quite possible, is not 'nailed down' just yet. In fact,
these 'Fujiwara'
situations (of two lows circling each other) are notoriously difficult
to predict, but the potential for a drawn-out snow storm into Saturday
night is quite real, and the fact that the southern storm is much bigger
than the northern wave dropping in argues that there may be less
uncertainty than usual.
Bottomline: foothills might see some early snows starting tomorrow
morning, but the heavier snow should hold back until around (or even
after) noon.
There should
be 12h period or so of moderate-to-heavy snow into tomorrow night, with
a fairly pronounced focus on the (northern) foothills and adjacent
plains rather than far out east - we will have less moisture to work
with, and there appears to be more of a direct (easterly) upslope
component. This first batch of moisture should be good for at least a
foot of snow from Boulder westward, with some favored spots approaching
two feet of fairly fluffy snow by Friday morning. Friday should see
lingering snow showers while we wait for the 'backwash' to kick in. Even
if this second part of the storm fizzles/misses us, the snow from
Thursday into Friday morning should be sufficient to make this the
snowiest/wettest December since 1913. If the prolonged 'backwash'
into Saturday evening comes through on top of this, we could see another
foot or so to round out a December that might rival or even displace
December 1913 as the snowiest and wettest winter month (Dec-Feb) in
recorded Front Range history.
In terms of impacts of tomorrow's snow, I am concerned that less snow
than last time might make for comparable impacts - the roads are not
completely cleared, and while the winds won't be as strong as with the
last storm, drifting snow on the eastern plains might create big snow
drifts anchored by remaining snow berms along the roadways.
I may not 'get around' (literally) to update this tomorrow, so enjoy
this one (if you can), and think of the good skiing conditions we should
see well into spring (in the higher foothills) after this storm!
Happy New Year('s snow) -
Klaus
P.S.: Almost everybody had two to three feet of snow in the last storm,
rivaling or even exceeding (Boulder northward) the Xmas'82 benchmark.
Only 1913 saw more snow in December! Larger variations in melted totals
than snow fall totals, and I can think of two reasons for that: 1. There
was a lot of wind on the plains (and even some foothills) with the last
storm, so smaller gages in particular were 'undercatching' the snow; 2.
If you tried to follow the 'Nolan Doesken approach' of measuring as few
times as possible per day, you had to deal with crowning of the gages,
or even overspill. And then there was DIA, reporting UNDER 1" of liquid
from a 20" blizzard - no way! But this keeps their ranking as one of the
driest years on record going...
--
Update #2 - PSA5 - biggest December snow in 19 years?! Tue 12/19/2006
11:24 AM
Good morning, y'all,
while I am not 'seeing the white in the eye of the storm' just yet,
indications are very good for a rare December 'dump' in the Front Range.
This is actually perfectly consistent with El Niño, despite some of the
media reports you may have been
hearing: we often get 'schizophrenic' winter months with El Niño, where
nothing happens for weeks on end, only to be followed by big snow storms
even in mid-winter (spring is another story, for another PSA). It has
been a while since we have had such a big 'El Niño storm' in December,
but we saw two of them in the '80s, the infamous Xmas blizzard of '82
being the first one (two feet even in Denver, but almost nothing in Fort
Collins), and just after Xmas in '87, when more than a foot fell in and
around Boulder, in a storm eerily similar to the one that I am expecting
for tomorrow (yes, weather history can repeat itself!).
The storm that battered the Pacific Northwest last week swept through
Colorado on Sunday, dropping up to 9" around Estes Park, 2-4" from my
neighborhood to Allenspark, and next to nothing in Boulder and
Nederland, is currently reorganizing over Arizona.
It is expected to 'yo-yo' back into our state by tomorrow morning, with
the potential for a deep upslope storm from tomorrow morning into
Thursday morning. Ahead of it, we may see a few snow showers drifting in
from the south today and tonight, but it should not amount to more than
a reminder to get your errands done today. There is a slight chance that
heavier snow band will pass through later this evening, but it should
not be mistaken for the main storm.
Moderate to heavy snowfall rates (1"+/h) may hold back until late
morning tomorrow, but could start as early as before sunrise, sorry
about that uncertainty. It should last for at least 12h, but has the
potential to continue well into tomorrow night, depending on how slow
the cutoff low will leave the 'sweet spot' in SE Colorado, which in turn
will depend on the next storm's speed and track upstream. Backwash snow
bands should keep us at least cloudy with occasional flurries on
Thursday, before the next round of snow comes in some time on Friday.
How much are we going to get? Consensus of most models is now from over
0.5" of moisture (respectable for any December snow) to almost 2" in the
'best-case' scenario, with strong upslope flow through at least 18h.
Thus, everyone in NE Colorado should see at least half a foot all the
way out to Kansas, but Boulder/Denver/DIA (!) have a good chance of
seeing a foot or more. Flights in and out of DIA may see minor impacts
through tonight/early tomorrow, but might get bogged down by noon or so.
'Sweet'
spots in our foothills (northeastern exposures should do best) have a
shot at measuring two feet by Thursday morning. If all the ingredients
come together, with lingering upslope and just the right temperatures
for fluffy dendrites near the end of the storm, I would not be suprised
to see even higher amounts.
What could go wrong? Yesterday's U.S. models advertized the 'evil twin'
version of this storm, with placement too far to the east, and deepening
low pressure giving us the dreaded northwesterly downslope flow pattern.
But even that scenario gave us 3-6"
of snow (which is still the weather service line this morning, watch
them upgrade this storm to much higher amounts later today...).
No really cold air to entrain, so plain 'vanilla'
cold through the rest of the week, the
plains might even see above freezing when the sun peaks through off and
on, but no real melting either. Given the school break, foothill roads
may remain barely plowed through Xmas (and we won't seen any melting at
all up there, just plain settling).
If I don't send out another PSA before Xmas, expect only snowfall
amounts through early next week, guaranteeing snow on the ground, and
even a small chance for some snow falling again Sunday/Monday as well.
Season's greetings,
Klaus
Update #1 to PSA#5 Sat 12/16/2006 4:15 PM
Good afternoon,
looks like "Stage 1" of this storm is going to be a bit late, and weaker
than hoped for yesterday. I still think that everyone on this PSA-list
will see some snow between about midnight tonight (northern foothills)
and early Monday morning.
My best guess
right now would be 1-3" in lower elevations, and 2-5" in foothills,
enough to make it look wintery again. Due to the banded snowfall
patterns with this storm, some lucky folks could still see more than
that, again, more in the foothills of Larimer County than further south.
Then a lull in the action, but remaining chilly (with a little day-time
melting at lower
elevations) on Monday and Tuesday, while we are waiting for the main
storm.
While Stage 1 looks somewhat diminished, Stage 2, the cutoff low to lift
out through Colorado around Wednesday is still on track! That one could
be good for some fairly intense snowfall, watch for Update #2 ,
hopefully with PSA-worthy forecast amounts!
Another storm appears to be taking aim on us just under a week from now,
but it is too early to get excited about that one. However, if all of
the above pans out, a White Xmas with more than half a foot of snow on
the ground is in the cards for the plains, even though it does not look
like any fresh snow will fall after next Saturday.
Think snow!
- Klaus
Tentative PSA #5 Fri 12/15/2006 5:51 PM
Good evening,
interesting weather situation unfolding for the next five days or so.
Very wide range of possible outcomes, anywhere from just a few flakes to
more than a foot all the way down to plains by next Thursday. The
wettest scenario bears more than a passing resemblance to the Xmas'87
storm, for those that remember that one.
The same storm that has pounded the Pacific Northwest in the last two
days is working its way inland, and is starting a complicated sequence
of 'ripples in the flow' that will impact us both this weekend and early
into next week. In fact, we have a decent chance to get hit by the same
storm twice, since a lot of energy gets dropped into the bottom of the
trough after the initial short wave passes over us (or, unfortunately,
to the north, as the latest models want us to believe), leading to a
cutoff low near Baha (!) that will later on lift out, and could hit us
again around Tuesday/Wednesday if the ECMWF track holds up.
So, the broad-brush scenario is that a cold front will sag south over
the next 48h, giving the lower elevations upslope flow by tomorrow
evening, but it will have a hard time getting deeper, fighting the
150mph winds at the jet stream level will blow from the southwest from
tomorrow evening through most of Sunday. This pattern often results in
more or less stationary precipitation (snow) bands that give one area
much more snow than another one, generally favoring Larimer County over
Boulder County, for instance. Too early to tell whether we will just a
few flurries by Sunday evening, or somewhere in the vicinity of 4-8" as
last night's models were predicting. Cold air will cover us early next
week (below freezing day-time highs in the foothills, but hardly anyone
will see below 0F). Then the suspense about the cutoff low track - will
history repeat itself as on December 26th, 1987, with a good snow storm
on Tuesday/Wednesday, or will it miss us? Stay tuned!
Hope you enjoyed today's warm weather, not to be repeated at least past
Xmas - I will update this forecast on Saturday if I see clearer which
way we are heading -
Think snow/season's greetings!
- Klaus
P.S.: PSA#4 verified nicely with about 6-18" up and down the Front
Range. A more reliable start to the winter snowpack than the October
snows. Low temperatures dipped down to near 0F in the plains, and near
-10F higher up. If this El Niño winter continues true to form, those
cold days in late November/early December might have been the coldest
for the season!
--
PSA # 4 Tue. Nov 28
2006 1:31 PM
Good afternoon,
hard to believe it has been more than a month since my last PSA, but
even my 'trigger-happy' pre-Thanksgiving forecast self did not find it
worthwhile to mention a "very small chance for flurries" last week. The
incoming storm will be more noteworthy for its wintery character than
snow totals, although there is another uncertainty/smaller-scale jet
dynamics to give us an outside chance to reach the magic foot by
tomorrow. Cold air has been "festering" over central Alaska for
most of November, in an odd repeat of last year's pattern (despite El
Niño), and has finally shifted into the Yukon and points south over the
last few days. This is an exceptionally cold air mass for so early in
the season, with numerous -40F to -50F reports (well to the north of the
border). Sub-zero air has now spilled all the way through Montana,
single numbers cover northern Wyoming, while we are still hanging on to
above-freezing temperatures for a few more hours. While the arctic high
air pressure is starting to nose into our state, a surface low is now
setting up over southeastern Colorado (close to Pueblo), and our wind
has shifted to the northeast. Unfortunately, there is a batch of really
dry near-surface air being brought in that will reduce our snowfall
chances (in lower elevations) for at least another 3-6h, but the
deepening upslope flow (not as intense as I would like to see) should
overcome this handicap, while higher-level moisture should keep us
cloudy for the rest of the day. While the coldest air of the season is
waiting to spill into our backyard, it will have to come across a pretty
long fetch of bare ground (most of eastern WY), and the actual duration
of upslope will be 1-2 days at best, opening the door for modification
starting already tomorrow night.
Meanwhile, much of the cold air has
spilled across the divide and dropped snow down to the sea level in
places like Seattle and Portland, feeding into a decent snow storm in
our mountains, with highest ski resort reports from the San Juans
(Silverton: 18" of powder!) as of earlier this morning. As the storm
tracks eastward, mountain snows should start to favor northwestern
exposures more than the current southwestern exposures, so that everyone
west of the divide ends up with more than a foot of powder, followed by
bitter cold, well below zero.
Bottomline for the Front Range below 9K: spill-over snow
coverage will increase in the foothills this afternoon, and should
become steady moderate snow by sunset. Should last through much of the
night, but could wind down before sunrise, depending on how fast the
cold air rushes in. Most of the foothill dwellers should measure 6-12"
by this time tomorrow, with a few lucky folks seeing even higher fluffy
totals.
Temperatures will drop the rest of the day, reaching sub-zero lows in
all but the lowest foothills to the south of here. Would not be
surprised to see close to -10F in higher northern foothills, possibly
the coldest of the season in this young El Niño winters. Boulder, Fort
Collins, etc., may not see sticking snow until sunset, but should have
lingering snow into the morning, to the tune of 4-8" is my best guess
right now. Due to cloud cover, low temperatures may not get much below
10F by tomorrow morning. This cold snap will modify tomorrow night, as
we might see a bit of a bora situation, with Boulder hitting its low
close to 5F early in the night, while Longmont and other protected lower
elevations could slip down below 0F, given that there is more than a
trace of snow on the ground.
Another short wave will drop in around Saturday, looks weaker for
now, but should reinforce the wintery character of our weather (higher
foothills might stay at or below freezing for more than a week from
today), followed by another, potentially PSA-worthy storm about a week
from now.
Enjoy!
- Klaus
P.S: Temperatures may drop low enough by tomorrow morning to render the
liquid de-icer useless, so watch your driving tomorrow morning in
particular (hope you have not forgotten to drive in snow after our long
dry spell).
P.P.S.: The PSA3 storm in late October delivered in spades, lots of
1'-2' reports, with the highest numbers from Nederland southward. It
capped an exceptionally snowy October, surpassed only by 1997 in higher
foothill elevations. Too bad we had such warm and dry spells in
November, otherwise this could have been a record-early onset of winter
snow pack below 9K!
PSA#3 Wed 10/25/2006 2:04 PM
Good afternoon, everyone,
by now most of you have heard about the impending, quick-hitting snow storm
to crash our Indian Summer weather tonight. Before you get too excited, note
that the storm will (1) last less than 24h, (2) has to bring a lot of moisture
on short notice (current dewpoints are in the 20s around here), and (3) could
easily be too warm well into the night to allow for large accumulations in the
lower elevations. Having said that I have to admit that this one was fun to
watch as it was toying with us, and certainly confused the weather service until
Monday...
Clearly, we will see at least 0.5" of moisture tonight, but it is not so
clear who will get the most - the upslope component has been advertized as more
northerly than easterly, and if the surface low deepends radially over SE
Colorado, we could see enough downsloping wind from the NNW to keep
Boulder/Foothills on the low end, while DIA/Longmont could see more moisture (9
March 1992 and another one in early January 1992 were both a bit like that,
although the difference in Boulder was between a trace in January and over a
foot in March). In any case, the latest tendencies in the models tell me that
this storm is not bringing in any extra cold air from the north, so that much of
the cooling will have to come through evaporation (from falling snow) and
vertical overturning as the cold air makes its way across the mountains from the
west. So, I would be surprised to see snow in the plains before midnight, and it
might have a very hard time sticking to the roads. Trees are another matter,
however, so there could be some pretty hefty tree damage (power outages?) where the snow
is heavy enough to bring down branches. The good news is that it also seems that
the winds might not be quite as strong as earlier expected - the system is
coming in just a tad slower, and will be just a bit slower kicking out as well,
making for slightly lower wind speeds from the north at the height of the storm.
Bottomline: expect precipitation to commence in the foothills first this
evening, could be light drizzle off an on in the lower elevations, but could
also hold off until midnight. Between about midnight and noon, the bulk of the
precipitation should fall at a good clip (some places will get more than 0.1"/h,
translating into snowfall rates up to 2"/h in the higher elevations). It will be
all snow above 7K, but could remain mixed until the early morning hours in the
lowest elevations. Total snowfall hard to call - if the upslope remains
favorable for more than a few hours, I would expect to see more than a foot in
the higher foothills, but some lower elevations may have trouble measuring more
than an inch or two (typical location for that would be Lyons, for instance).
We should start warming up again by Friday, so this is about as quick as it
gets around here. Roads around DIA could be messy tomorrow morning, since this storm could be
more intense to our south and east! What an exciting month - keep it coming!
- Klaus
P.S.: 9" from the non-PSA last weekend, a great start to the season
continues...
PSA # 2 2006/2007
Tue 10/17/2006 11:46 AM
Good morning, everyone,
if you have looked outside in the last hour or
so, you have likely seen some snow flakes,
even down in Boulder, and about 6-8h ahead of
time! Have been holding back on this
one until I could see 'the white in the eye of
the storm', but did not mean to miss the
beginning of the snow. Just goes to show you how
volatile the atmosphere has been
lately - models have had a hard time figuring out
how much moisture to give us, and
how much of it as snow down in the plains.
We will have about 12h of decent upslope flow (up
to just over 10K or so), the air is
obviously already cold enough for snow in the
plains, but it will have a hard time
sticking until sunset. Snowfall rates should
increase between now and then, reaching
1-2"/h in the foothills, and then the whole
short-lived event should more or less
wind down by midnight (earlier to the north/later to the south). We won't have a
real cutoff low to keep things lingering, but a decent overrunning situation
with moisture gliding up from the south, while cold air is (apparently eagerly)
working its way in from the north. Snowfall totals from 6-12" in the foothills
are in line with weather service expectations,
except that I think it will happen
earlier and faster than their original
pronouncements. If the whole system stalls
a bit later tonight, we could have lingering
0.5"/h snowfall rates until the early
morning hours which could boost some lucky folks up to 15" or more, but that is
an outside chance scenario.
In the plains, grassy surfaces (and car tops,
etc.) should see a few inches accumulate
(I guess for some this is the first snow of the
season), and temperatures should
plunge well below freezing by tomorrow morning. If it clears out as early as I
expect, higher foothills could see singe degrees F by sunrise tomorrow, while
some places in the lower elevations could dip in the high teens (most will just
get solidly into the 20s). Untreated roads in the lower elevations will become
icy by tomorrow morning, but main roads should be o.k., and will dry out by this
time tomorrow. Higher up, north-facing driveways may set up their winter snow
surface (hope you got your snow tires!) for the season, but only south=facing
exposures may have some icing problems by tomorrow, most others will just become
snow-packed. Again, many higher-elevation roads won't see much melting today, so
the snow could stack up to more than 8" on unplowed roads (just a heads-up for
low-clearance vehicles).
Finally, the typical El Niño-October snow connection seems to work (remember
1997 and late October 2002?!), and the pattern is not done either, we could see
a virtual repeat (except for much lower starting temperatures) by Saturday -
THINK SNOW!
Klaus
P.S.: For those new to this, my rule for issuing
these PSA's is that I have to see
a reasonable chance for a foot of snow in the area of interest before issuing
one, that's why I did not write anything about our 'dud' a week+ ago!
--
Klaus Wolter
Updated PSA#1 - looks like the "dry" scenario wins!
Thu 9/21/2006 6:34 PM
As I was saying - it is almost never a good sign when the weather service
jumps the gun... Wave #1 has passed and dropped up to an inch of rain just south
of Denver, leaving Boulder and Larimer County with 0.2", more or less, and no
snow below 9K.
Wave #2 is taking aim at us, and it looks like the upslope scenario is becoming
less and less likely. A few spill-over snow showers higher up and certainly
another cool down is on tap for Friday and Saturday, but no big dump below 9K.
Don't get me wrong, I could see some areas (especially with northern exposure)
getting a few inches of snow between now and Saturday morning (there is enough
jet dynamics around to allow for a few northwest-southeast oriented stripes of
heavier precip), and the snowline should lower to 8K by tomorrow morning, and
about
6K(+/-) by Saturday morning, but the amounts just aren't there to get too
excited about this. Different story west of Peak-to-Peak highway where the
mountains should get a decent spill-over event with a foot of snow or more by
Saturday morning.
Oh well, it is very hard to get a decent snow storm during El Niño Septembers
(last 2"+ September snow of this type in Boulder was in 1965!), quite contrary
to what often happens in October (remember 2002 and 1997?!). Should have trusted
my statistics rather than the (more optimistic) forecast models...
But we are going to have yet another cold (early) weekend, and there is a
distinct chance for a freeze both Saturday and Sunday morning down to the
plains.
Until the next PSA, hopefully before November -
First PSA of the 2006-07 season
/ Wed 9/20/2006 8:35 PM
Good evening,
after a few flurries above 8K last Saturday, it looks like the higher foothills
may get some measurable snow both late tonight as well as from late Thursday
into early Saturday, with the latter storm lowering the snow levels to the lower
foothills.
The weather service has just come out with a winter storm watch for the second
period in the higher foothills, never a good sign so early in the game...
Complicated weather pattern evolving right now, with a leading wave coming from
the west, very similar to last weekend - good for snow WEST of the divide and
some spill-over to the east, aided by good vertical (in)stability and a brief
potential period of upslope (northeasterly) flow tonight.
However, we are starting out with a very dry airmass, and a surface low that
might deepen too rapidly over southeastern Colorado, thus tweaking our upslope
flow to a more northerly direction (ending up with less moisture). This might be
one reason that there is a large spread in the model forecast precipitation for
tonight, depending on when the model run was started (four per day) and which
model is used.
IFFF we are lucky, this could be good for a few hours of moderate to heavy snow
showers (above
8K) and get us off to a good start.
The second, more important storm is dropping in from a more northerly direction,
dragging in colder air, and leading to a more prolonged period of precipitation
for us from tomorrow night (or so) into Friday, maybe Saturday. Still lots of
question marks on that one as well, not least because it could interact
(negatively) with the first wave - if the first one deepens too much and slows
down, we could get a longer period of downslope winds tomorrow, reducing our
moisture potential yet again.
However, it has the potential of producing deep upslope flow from the northeast
(not quite a full cutoff low circulation for us, since the main low parks itself
over the norther plains), which could lead to 24h or so of moderate to heavy
snow above 7K or so.
Since I probably won't have time to update this until tomorrow evening, here are
the two bottomline scenarios for now:
1. DRY: occasional sprinkles tonight, with a few flurries late above 8K, gusty
winds from a northerly (to northwesterly) direction later tonight, as the cold
front passes. Blustery Thursday, but mostly dry all day. Thickening clouds
tomorrow evening, but only weak upslope (if any). Mixed rain/snow showers on
Friday above 7K, totaling less than half an inch of moisture from now until
Saturday morning. No snow in the plains or lower foothills.
2. WET: good showers with front tonight, rapidly changing to snow above 7.5K,
with some damage to weaker fully-leafed aspen (have no idea how much the recent
aspen disease outbreak has weakened the affected trees, but almost all leaves
are still up below 9K). Some lucky and intrepid observers could measure half a
foot by tomorrow morning (not on open ground, just grassy areas). Still mostly
dry around midday tomorrow, but cool enough for some of the snow to survive the
day (in the shade/where the ground is not too warm). Lower elevations might get
some good rain showers, even with the potential for a late evening thunderstorm
with the front.
Tomorrow night things could get interesting: if the precipitation gets heavy
reasonably early AND we have good upslope flow, snow might mix in down to the
lowest elevations by Friday morning (although the odds are much better for
Friday night). Heavy snow is possible for the higher foothills, a real foot of
wet snow being not out of the question, especially by Saturday morning.
More by tomorrow evening, hopefully not a cancellation...
As always in this first PSA (Public Service
Announcement) of the season, feel free
to let me know if you want to be taken of this mailing list, and report any
significant snow measurements back to me - or tell me which CoCoRAHS observer
you are!
Think snow, this season can't be as bad the last one, especially with El Niño
back in the game -
PSA#8 - better late than never! Sun 3/19/2006
2:28 AM
Good morning, y'all,
today's moisture (apparently close to half a foot of heavy wet snow in some
foothill locales) was just the harbinger of things to come: a genuine spring
snow storm is coming to town, just in time for the March 2003 (and 1998)
anniversary! While many models anticipated the general idea of a storm hitting
Colorado this weekend, the details remained murky until Saturday, and last
evening's precipitation was actually quite underforecast, so there is still
quite a bit of uncertainty.
Basically, we have a low dropping in on us from the northwest, while a
blocking high is residing near Hudson's Bay, providing us with the necessary
cold air to turn this into snow by tomorrow evening. Moisture is ample, both
from the Pacific aloft, and from the Gulf near the surface. There is a good
chance that this storm will indeed cut off near the Four Corners, giving us
about 12-18h of good upslope flow tonight into Monday, although some of the
highest precipitation totals may fall on the eastern plains (the models are
still waffling on that one). One inch H20 (or more, if you trust the Canadian
meso-scale model) are in store for us, on top of what already fell last evening.
In the foothills (above 6K or so), snow should accumulate off and on through
Sunday AM, with the heaviest snow commencing in the afternoon. In the plains, it
might not be before later in the afternoon that the snow will start to stick (it
should fall as snow almost everywhere). Snow totals by Monday rush hour: easily
8-12" in the foothills, 4-8" in most of the plains (Boulder probably right in
between). "Best case scenario" could drop 12-18" in favored locations (northerly
exposure should help), including possibly the Palmer Divide, but that is outside
my region of interest.
I will be out of town from Monday afternoon until Friday, but there is an
interesting little storm that should pass through about Wednesday. While
undercutting the upper level ridge that is trying to build over us, there is the
possibility that it could briefly cut off and pass to our south which might give
us another chance at more than a few flurries... Compared to last week's storms
(which dropped a total of more than 18" and 1" of moisture on our house), this
one is not as cold (no sub-zero lows!), and the snow will be a lot 'heavier'
(snow/H20 ratio) this weekend than last.
Think snow,
Klaus
PSA # 7 Sat 3/11/2006 5:58 PM
Good evening,two bouts of snow later, we are still looking at another snowy period from
tonight through tomorrow evening. The snow that is trying to materialize right
now stems from one last ripple in the flow ahead of the main trough lifting out
right over us tomorrow. Does not look like strong upslope flow during this
period, but decent cold air aloft (allowing for good showers during the day-time
tomorrow), good moisture at all levels, and some dynamical forcing due to the
jet stream to our south should allow for off and on moderate snow to continue
for more than 24h.
Since a fair bit of this snow will be forming at favorable temperatures for
dendritic growth (-10/-15C), we could also see some pretty fluffy snow
(yesterday morning's snow was 40:1 in many foothill locations) thrown in. The
joker in this set-up is that as the low lifts out tomorrow, we might get some
good backwash in the PM, which combined with very cold air aloft could allow for
some heavy snow-showers as well. While it probably won't really clear out Sunday
night, we could see a repeat of near-0F in the higher foothills and low 10s in
the plains.
The weather service is going for 3-6" in Boulder and places north, and 5-15"
in the foothills, with northern locales being favored. I lean towards slightly
higher totals in the plains (it should remain cold enough that untreated roads
will be a mess tomorrow morning/evening), but would agree that the foothills
would be lucky if a foot accumulates. This goes along with about 0.25-0.5" of
moisture, so all in all not quite the moisture relief that I had hoped for
earlier this week.
However, the rebound into milder temperatures should get interrupted mid-week
with yet another system passing thru and affecting mostly the mountains.
Speaking of the mountains, this storm set-up has been kind to the southern
mountains for a change, with Wolf Creek boasting 42" in the last 48h, and they
should get at least another foot by tomorrow evening. The mid-week 'storm'
should revert us to the familiar "northern-central mountains being favored"
pattern. If you can avoid avalanche-prone regions, back-country skiing tomorrow
should be about as wintery as it can get around here (the late calendar date
notwithstanding).
Enjoy!
Klaus
P.S.: PSA#6 rendered a total of 0.60/11.1" for me from Wednesday thru Friday,
while Niwot Ridge (at C-1) had 0.85/15.6" (thanks, Mark). This is the most snow
I have had since the October snow!
PSA#6 - Update Thu 3/9/2006 6:02 PM
Good evening, y'all,this is a quick update on tomorrow's storm: we are still on for another bout
of day-time snow around here. Main differences to yesterday's snow (which
underperformed somewhat in the foothills but dropped close to 0.5" liquid on
Boulder and other communities close to the foothills): the ground has been
cooled down considerably, so the roads should become snow-covered much faster
than yesterday, the air mass will be colder than yesterday as well, so that this
time there is NO chance of rain, it will be all snow (we had some rain in the AM
below 6K yesterday), and right now it looks like a smaller event, say, from
0.25-0.5" H2O rather than twice the amounts I expected for yesterday. This being
spring-time, who knows we might be lucky and get more than expected for a
change. In any case, tomorrow's totals should reach 2-4" in the plains and 4-8"
in the foothills, with a slight preference for northern locales. The next event
after tomorrow will come in a bit faster than expected, say, around Sunday
morning, and might be good for another PSA, stay tuned! Think snow!- Klaus
P.S.: Since people are reporting less and less to me ( and I encourage you to
report any reasonable snow fall amount you measure, regardless whether I
predicted it or not), you can check county by county snowfall (and moisture)
amounts at the following web site:
http://www.cocorahs.org/
Just click on "Maps", and pick what you want to see. For instance, this
morning's reports for Boulder County showed quite a few 0.5" reports from
Boulder, but only two reports at 0.40" for the foothills (the northern one is
mine).
PSA # 6 Tue 3/7/2006 5:12 PM
Good afternoon y'all, snow is on its way, and it will feel like winter (again) for at least the
next seven days! There is a storm brewing west of us that should track more or less right over
us tomorrow, and will bring a good chance for moderate to heavy amounts over
much of Colorado (the southeast corner will get the least). Interesting details
to worry about: heaviest precipitation will fall during the day-time for us,
which may leave paved roads in the plains just wet (it should all fall as snow,
however); it looks like a fast mover right now, but it involves a short-lived
cutoff circulation which, if it stalls, could easily double the expected snow
amounts; models are still all over the place with the exact placement of the
heaviest precipitation (anywhere from southeastern WY to the Palmer Divide), but
someone could easily wind up with an inch of moisture out of all this; today's
down slope winds brought in some awfully dry air (yet again) that needs to be
replaced/moistened up which will partially accomplished by seeding from above ('virga'
= wasted precipitation that evaporates into the air below) and from low-level
flow from the north and east that will bring in moister (and colder) air by
tomorrow morning.
Bottom-line: expect snow to begin tomorrow morning (the earlier, the better
in terms of how long it will take to moisten up the atmosphere) and should
fairly showery most of the day; heaviest snow bursts could occur just before
evening rush hour, making for highly variable road conditions. Total amounts
should be highest in the east-facing foothills (the higher, the less wastage to
melting from below) and reach at least 3-6" in the plains and easily twice that
higher up.
Would not be surprised to see 2"/h for some of the better pockets of snow,
giving totals of up to 18" in best locales. In terms of moisture, most people
should see half an inch or more, giving us a nice break from the fire danger.
The pattern will remain cold and unsettled through the weekend, giving us at
least two more chances for several inches around Friday and early next week (if
I see a chance for another foot, watch our for PSA#7), while temperatures should
remain below freezing through the period above 8K, and with fresh snow lows
could reach near 0F once again. If it all pans out, we might end up with a
normal March's worth of moisture by this time next week!
Think snow, those wild fires have been flaring up way too early!
- Klaus
P.S.: I hope you still have your gloves handy, snow tires on your cars, and
remember to disconnect the garden hoses from their outside faucets.
PSA 5 Update Fri 2/17/2006 5:29 PM
Good evening, minor updates for tonight and this weekend:
1. This air is quite a bit colder than predicted, especially on the plains.
Expect to see lows at or below -5F tonight in the plains and below -10F higher
up.
For Boulder and Denver, the daily records are just a few degrees below 0F, so
we might see a few record lows around here.
2. Today's cold push also came in earlier than advertised (keeping
temperatures in the plains steady or dropping all day with a stiff (feeling)
northerly breeze).
It has now filled up to at least 9000' feet, but Niwot Ridge is still
'basking' in the 10s right now, and may not drop below zero at all tonight.
Since the cold air came in so early, we have less of an upslope push to work
with this evening, but there still is moisture coming in aloft from the
southwest that should be good for at least a dusting tonight - just don't expect
several inches. Mind you this subtropical moisture fetch is giving Wolf Creek
Pass heavy snow right now - one of the best storms of the season so far!
3. The last bout of snow and cold on Sunday now looks weaker than expected
two days ago, but should keep everyone below freezing and with a final upslope
surge give most of us some light snow.
4. Downsloping flow should return by Monday, but may not be strong enough to
scour out the arctic on the plains (especially in places like Greeley and
Longmont). Of course, the mid-February sun is going to do its part to help us
warm up a few degrees every day. Since there still is the chance for another
cold surge from the north around Wednesday, a rebound into the 50s may have wait
for quite a while.
Let's hope that my snow forecasts are too conservative this time for our
region.
The change to west-northwesterly flow by Monday should resume the favorable
regime for the northern and central mountains next week, although the moisture
fetch does not look too impressive right now.
Have a nice and wintery President's weekend -
Klaus
P.S.: Two day totals so far are in the 4-8" range from Boulder northward,
with some lower foothill locations slightly above that.
PSA # 5 Wed 2/15/2006 6:18 PM
Good evening, y'all -better late than never, here is tentative PSA for the next five or six days
that promises no big dumps around here, but plenty of winter weather, including
snow off and on through the period, and continuous below freezing temperatures,
reaching close to 0F even in the plains.
Tonight, a storm that has been brewing to our west, and brought overrunning
precipitation to southeastern Wyoming for the last 24h (up to 18" north of
Cheyenne!!!), will make its move across the mountains and give us a few hours of
moderate snow overnight. Since there is a strong jet streak involved, some
east-west oriented heavier bands of snow could bring twice the amount of snow to
some lucky folks, but pretty much everyone should get at least a couple of
inches. The whole storm should also sag from north to south as it translates
across the divide, but northern foothills should on average get more snow than
southern ones (say, south of Boulder). So, by tomorrow morning, some reports
could be up to 10", yet again (although the highest amount I saw last time was
"only" 7.5", with widespread 5"
amounts in western Boulder County - where were the reports from Boulder Heights,
Coal Creek Canyon, and around Estes Park???), while others may only measure 2".
However, it has been cold all day, and should remain cold enough for any snow to
stick around into at least early next week.
A second surge of cold air and upslope flow should materialize on Friday (PM)
and be good for another couple of inches (possibly more in the foothills). And,
finally, the last piece of energy trapped as a cutoff low over the Great Basin
should eject across Colorado in the Sunday/Monday time-frame, and be good for
another round of snow, possibly more than just a few inches. Since I will be out
of town over the weekend, that one will probably be the biggest one...
Considering that arctic air from the Hudson's Bay area has been moving
steadily southwestward and is projected to reach Montana over the next few days,
where it will get well below zero (possibly into the -20s and -30s after highs
in the 50s just three days ago), our expected lows near 0F (o.k., -10F higher
up, if it clears out long enough) are pretty puny in comparison.
Enjoy our return to winter over the next week, it should remain relatively
calm as well, which is a nice change of pace from more than two months of almost
relentless downslope winds!
Think snow -
Klaus
P.S.: If I see a big change in this scenario by Friday, expect an updated
forecast by then!
PSA # 4 Thu 2/9/2006 4:46 PM
Good evening, y'all -winter is knocking at our door again after a few weeks of warm/windy weather
(although my last PSA verified o.k., especially from Nederland south). We had a
pretty impressive frontal passage around noon, with a 15F drop in a few minutes
and wind gusts up to 40mph
(NCAR) from the northwest. It has taken another four hours or so to cloud
over, and the moisture situation is a bit underwhelming, but should be
sufficient for snow to start in the next few hours.
This is going to be a fairly quick affair, with most of the snow falling
between about 6pm and midnight, and anywhere from 0.1" to 0.5" of moisture,
depending on your locale (eastern plains will get the least, northeastern
exposures the most, probably in lower foothills rather than higher foothills)
and which model you believe. This translates into an inch or two for the least
favored locations and up to 10" of fluff for the lucky winner. There will be
little spill-over into Western Colorado with this storm, for a change. All
models do agree on a very fast temperature drop to near 10F (0F) in the plains
(foothills) by tomorrow morning, and little warming during the day tomorrow. In
fact, the air may be unstable enough to allow for some scattered snow showers to
fall during the afternoon tomorrow as the coldest air passes overhead.
With fresh snow on the ground, the plains should see its lowest temperatures
tomorrow night (close to 0F), while the higher elevations may get too breezy to
drop much below 0F.
Since the air aloft is going to be the coldest since mid-December, some
protected spots higher up could drop below -10F nevertheless.
After a slow warm-up over the weekend (especially if we get more than a
dusting), there is potential for another PSA-worthy event in about a week that
could make tonight's event look wimpy by comparison.
Stay tuned, it looks like winter is not going to remain AWOL for the next
week or two!
Think snow -
Klaus
P.S.: With all the recent warm weather, I would anticipate some nasty icing
to form by tomorrow morning's rush hour, even on treated surfaces (since the air
temperature will be so low).
PSA # 3
03/28/2007
Good morning, y'all,it has been a loooong time since my last PSA, despite some great snows above 9K
in the last two months. Too many downslope wind storms have left even the higher
foothills below par (with the exception of good spill-over pockets like
Nederland and Peaceful Valley), and at the base of the foothills, Boulder and
Fort Collins have not even cracked the one foot seasonal total marker (as of
this morning).
Meanwhile, our weather patterns have been amazingly repetitive and yet
unpredictable for almost a month now, with repeated wind storms on the east side
of the mountains, and occasional "teaser snows" that either managed to spill
over the divide or developed in the wake of a passing storm, with just a few
hours of upslope flow to squeeze out any available moisture. Remember the large
wild fire southwest of Pueblo some 10 days ago? It got put out by up to 8" of
snow that fell overnight in one of those passing storms that otherwise only
favored the mountains. Unfortunately, the models have been producing more false
alarms with this flow patterns than not, so that I have been very hesitant to
put out a PSA until I am virtually certain that something reasonably big is
going to happen.
Today is such a day, with a couple of inches already on the ground from a minor
disturbance ahead of a storm, and the stage being set for at least twelve more
hours of northerly to northeasterly flow, it is virtually certain that most
people reached by this PSA will see at least half a foot of snow (except on warm
surfaces like paved roads), and some lucky folks (with more of a northern
exposure and/or being located south of here (say, in Jefferson County) that
should receive a longer duration event than further north. This storm will
remain cold enough that even the lowest elevations will see all snow, and the
air aloft will be just in the right temperature range to promote nice snow
crystals/fluffy snow this evening.
Flies in the ointment? This storm has at least three: unfortunately, it is one
of those fast movers (as so many storms before it that just resulted in wind for
us), so the upslope flow situation will be short-lived, fairly weak, and much of
its moisture is being deposited west of here. Nevertheless, enough moisture has
snuck in from the high plains to allow for snow to continue through this morning
when there has not been much upslope flow to speak off.
Bottomline: widespread snow should pick up this afternoon (possibly worst around
rush-hour), and linger into the night, especially south and west of Boulder.
Should be a pretty, nearly calm, and cold sunrise tomorrow, everything being
coated with 4-8" of fresh snow, and some lucky folks ending up with up to a foot
of fluffy powder.
Hard to tell whether elevation will make any difference between the lowest
foothills and higher up, except for maybe slightly fluffier snow at high
elevations.
Our dreaded downslope winds will return to the foothills by Friday night, but
another weaker storm should drop in on us by Saturday evening, allowing for a
lull in the windiness, and a few more flurries into Sunday morning. Right now,
it does not look like this one should be as strong as today's storm, but that
could change. Beyond that, it looks fairly murky right now, but there is
potential for yet another storm around mid-week next week, and you can kiss the
60sF good-bye for at least the next week - ruining a perfectly good chance at a
record-warm January...
Think snow,
Klaus
PSA #2 11-23-2005/2006 snow season (POST-Thanksgiving!)
Sorry about the late/short notice, but there is a good
chance for snow this
weekend (Saturday night in particular) that could become PSA-worthy. No risk of
rain instead of snow, as we saw in October's storm, and continued cool and
unsettled conditions for most of next week as well. The northern mountains (that
got off to a great start earlier this month) should do well next week as well.
I will probably not get around to updating this, so my current estimate is 3-6"
in the plains, and twice that in the higher foothills for this weekend. I seem
to recall several post-Thanksgiving storms that gave us good snow totals, in
particular last year, and in
1995 (both over one foot
for me).
Again, sorry about the short/late notice, and have a great T-day tomorrow (two
more dry and warmish days to "contend" with...) -
Best,
Klaus
P.S.: The October storm verified big time in some places (south/east Denver, and
around my neck of the woods had over a foot), but Boulder only got rain!
Snowfall totals around 30"
near Breckenridge were probably (close to) monthly records!
PSA #1
10-07-2005 2005/2006 snow season (don't ignore this
one!)
Good afternoon,I hope you all enjoyed a very pleasant and warm fall season so far. After a
near-miss snow storm in early June (that verified my last PSA nicely above 8K in
north-central Larimer County and into North Park), we had a pretty hot/dry
summer, and an extremely dry September (my 3rd driest in 16 years). In fact, the
temperatures from October 1st to 3rd were record-breaking in much of our area (I
measured 73F at 8500' on two separate occasions, setting and tieing a new
October record). Then, there was a little interlude earlier this week when many
of us in the higher foothills managed to get our first measurable snow (quite
late, I might add), and it has been nice to see how slowly the first few inches
of snow have been melting above tree line.
For about a week now, first the ECMWF (European) and the GFS (Global Forecast
System) have been advertising a significant storm to hit us early next week, and
as of last night all models have converged on the timing (if not the strength)
and approximate temperature of this storm. I mention temperature since this
storm will be warm enough for rain in the lower elevations for a good portion of
it. But I am getting ahead of myself.
Essentially, this storm is dropping in from the northwest (it is currently still
offshore), and will form a cut-off low as it approaches CO on Sunday morning.
That cut-off low (at the upper
levels) should travel slowly past the Four Corners from Sunday night into Monday
night, leaving us in DEEP easterly upslope for around 36h. Apparently, the
remains of "Stan" will be feeding in some upper-level moisture from Baha, so
that might explain some of the precip totals I have seen in some models (giving
the San Juans a very good dose of rain initially as well).
If you want to think of an analog case, remember what happened around October
24-25, 1997 - the infamous October Blizzard that shut down DIA (i.e., Peña
Boulevard). Main difference to that storm: even though it tracked similarly, it
had a tighter circulation, AND colder air on its backside, leading to an
all-snow event even in the lowest elevations.
So, since this is 2-3 days out, take this with a grain of salt, a lot can happen
between now and then, but I gather that you would rather have an uncertain
forecast today rather than an accurate forecast on Monday morning, when it might
be too late for folks in the higher
elevations:
SUNDAY: Clouds will be building over the divide all day, with isolated
thunderstorms possible by noon or so. Rain showers should start drifting
northeastward over the foothills in the afternoon, with some locales receiving
up to half an inch of rain even above 8K. Easterly flow should commence around
that time and intensify towards the evening, creating a more general upslope
event by nightfall. Around sunset, this should result in a steady moderate
rainfall over the plains (if you watch the Broncos play, you will need good (and
warm) rain gear), and moderate to heavy (1-3"/h) snow above about 7K. Since a
jet streak will be rounding the base of the large-scale trough all night into
Monday, there could be dynamic enhancement of snowfall rates due to being in the
left front quadrant of its exit region (lots of jargon that translates into
slowly moving heavy snowbands).
The snowline will lower very slowly
all night into -
MONDAY: Heaviest widespread precipitation in the morning, with snow replacing
rain all the way down to the plains at times. The key to this will be the
precipitation intensity: the more moderate to heavy precipitation falls, the
more likely it will cool the lower levels sufficiently to change into snow. If
the Canadian model is right and we "only" get 0.5-1"
of moisture, the plains might only see a trace of snow, but that is the most
extreme dry/warm scenario. If the wetter scenarios (2-4" of moisture are
possible in areas with initial thunderstorms/intensified snowfall bands later
on) verifies, the heaviest showers from
midnight till noon on Monday should be all snow down to 5K. Given the current
foliage, this could do all kinds of damage to deciduous trees (and power lines).
Precipitation should remain widespread from the divide to the eastern plains
until at least late morning on Monday, but could rekindle/linger well into the
night, if the cut-off low stalls out as "planned". Even if it just rains in the
plains, it should be a very chilly day, hardly out of the 30sF.
Bottomline: a trace up to potentially a foot of heavy wet snow in the western
suburbs (if it rains on Monday morning, no need to worry); 6-18"
in the lower foothills; and one to three feet in the higher foothills/near the
divide, depending on (1) eastern/northeastern exposure to upslope winds, (2)
elevation (close to and above 10K should be almost all snow), and
(3) vicinity to one of those jet streak-enhanced snowbands.
Aftermath: lingering cool air should slow down the warm-up until late next week,
and there may very well be a second, smaller storm that could drop into our area
around Wednesday and add a few inches of snow (PSA #2 will be issued if it could
approach a foot).
Enjoy - this could easily be the second biggest October snow in more than 20
years (since the "Bronco Blizzard" of 1984, and behind October 1997) -
Klaus
P.S.: A reminder: if you want to be taken of this list, just send me an e-mail.
I would recommend doing the same, if you change your e-mail address (I lose a
few "customers" each year due to that mechanism). Also: I only issue a PSA if I
see the possibility of a foot of snow (or more) in the Front Range foothills,
not every time I see snow coming around the corner. This is completely
voluntary/non-operational, and when I travel, I will typically not send one out,
unless I see something coming way in advance. So, "buyer beware!", I assume no
liability for this.
PSA # 10 04-15-2005
Good evening,
This is a tad early to send this out, but I see the potential for a prolonged
wet period for our region next week, and I will be out of town for the rest of
the month. The airmass will probably not be quite as cold as during the last
storm, so that a lot of the precipitation will not fall as snow, never mind
stick to the ground in the lower elevations. Above 8K, we could see enough to
turn around the sub-par snowpack in our region. As a matter of fact, the last
storm actually put enough snow on Niwot Ridge to briefly surge back to 100% of
normal. Since the first storm probably won't get going until Tuesday
evening/Wednesday, the details are still quite iffy, but I would expect a cold
front to drop in from the north on Tuesday, while a cutoff low spins to our
west, probably lifting out to our north or right over us. In mid-winter, such a
track could completely miss us, but this is spring, and things don't have to
line up perfectly to get decent precipitation amounts. Since the pattern will be
quite blocked, again, the storm could easily last two days, with lulls in
between, and remaining unsettled through the remainder of the week. Temperatures
should be below normal, with mostly 40s in the plains and 30s in the higher
foothills. A second surge is possible by the end of the week, which might
reinforce the late-season cold air enough to bring snow to the lowest
elevations, if this has not happened during the mid-week storm.
Total precipitation should rival, if not surpass, the last storm, especially in
the mountains, and may give us a "wet" month for a change, and keep backcountry
skiing in pretty decent shape at least to the end of the month. This storm
should also continue to general tendency (in spring) to shift the storm track
northward, so that, for once, the northern mountains might actually do better
than the southern ones.
In the spirit of PSA's, I would not be surprised to see more than a foot of new
snow measured in the higher foothills during the mid-week storm, with more after
that. As I mentioned in PSA #9, the period around the 21st of April is almost as
likely to see good snowfall amounts higher up as the 'singularity' around April
10th (posting our 9th storm around a foot or more in 17 years!).
Too bad Eldora is closing down this weekend, the snow by the end of next week
could be the best of the season -
Think snow (yes, the drought is still not over!) -
Postmortem to PSA # 9: April 16 2005
All in all about what was expected, except, yet again, the
plains south and east of Denver got more than their fair share - closing down
DIA and numerous highways due to near-blizzard conditions. Roughly from southern
Boulder County into Jefferson County, the storm delivered between 1.5 and 2.5
feet of heavy snow, mostly due to its long duration (higher foothills saw
continuous snow from Saturday evening through early Monday), not any
particularly heavy snow.
I "only" measured 15.1" (1.22 H2O), but have to admit that it was nice to see
the fourth one-foot+ storm of the season, especially after a frustratingly slow
start last Saturday night into Sunday morning.
Update to PSA # 9
-
April 9th-April 11th(?) 2005
Good afternoon,
we are on track for a decent spring snowstorm around here. In my original PSA
statement, I articulated a few caveats, some of which are still unresolved,
while others can be removed.
First the removals: this event will be mostly a snow event all the way down to
the plains, especially on Sunday. While we are not tapping into any really cold
air to our north, the initial short wave that is passing through right now (see
the towering clouds to our west?) will get a flow from the north going that
should bring in air cold enough to allow for a fairly quick change-over to snow
(especially, since the lower levels will be fairly dry, which will allow for a
large drop in temperature due to evaporative cooling).
In fact, the air may become
cold enough for some nice dendrites to mix in. So, while the plains will
probably not see any powder snow out of this, the foothills could see some
pretty fluffy stuff towards the end of the storm.
Timing and intensity: precipitation should start in earnest some time after
sunset tomorrow and around midnight (earlier showers can be counted as "icing on
the snowstorm"). There has been a bit off a wobble in the intensity (and
location) of the heaviest precipitation, including the exact mid-level wind
direction and speed (very important for the orographic effects), but it seems
right now that there should be a twelve-hour period from about midnight until
noon on Sunday that should feature some heavy snowbands (in excess of
1"/h) anchored against
the foothills, especially where northeastern exposure is most favorable.
Duration and location of heaviest snows is still a bit iffy - in fact, today's
ECMWF run is still showing good upslope over us by Monday morning, while the ETA
(now "NAM", an unfortunate
acronym) shuts us down by Sunday afternoon. Canadian ensembles show a large
spread as well, with some storm totals of barely 0.5" (and over by Sunday
afternoon), while others keep going into Monday, with 2" liquid totals. This
hinges essentially on two "flys in the ointment": #1:
the lead wave crossing over us right now may "sap" the energy available for the
second storm (the one discussed above), and may linger close enough to us (in
the
Dakotas) to continue to
interfere with our upslope situation; #2: once the cutoff low forms near Grand
Junction tomorrow morning, it should keep moving to our south and not linger
over, say, SE Colorado/NE New Mexico too long, mainly because there is another
wave trying to move onshore early next week to keep the pattern progressive. As
the ECMWF suggests, the next wave may hold back just long enough to keep the
upslope going for, say, a total of 36 hours instead of 12.
IOW, if it is still snowing on Sunday evening, don't be surprised if it keeps
doing it until Monday morning, while a clear sunset on Sunday should indicate
the end of this particular storm.
How much will we get? Based on the above, the most likely amounts are just under
a foot on grassy areas (and TREES - watch for tree damage to those that are
leafing out already!) in the plains to one to two feet on north- to east-facing
foothill locales. If the slower scenario pans out, this could be doubled, but
seems unlikely. A hard freeze should occur on Sunday night in particular.
PSA#10 may follow in about six days...
Think snow -
Klaus
PSA#9 for this weekend
April 7 2005 1:44 AM
Good evening, y'all,
don't get fooled by two storms that narrowly missed us (by less than 100 miles
each), and the warm weather of the next few days. The next storm appears to line
up more in the classic spring storm sense. This means that it should cut off
WEST of our longitude (for a change), that there should be a prolonged period of
deep upslope (at least 24h, possibly twice as long), and that the foothills
could indeed end up with well over a foot.
The usual caveats this far out (2.5-3 days) apply, for instance, it is still
unclear whether most of this will fall as snow all the way to the plains, or
whether you have to go above 7K to enjoy this as a heavy snowfall. More
importantly, there is a fairly wide spread in the model runs with regard to the
duration/speed/intensity of this event
- it could be over by Sunday
evening, or it could last well into Monday. Right now, the central precipitation
value for us seems to hover around 1", but quite a few runs give us twice that.
Interesting background: "climate singularities" are a fact of life around here,
such as the "mini-dry season" at the end of June/early July before the monsoon
kicks in, or the (in)famous "Halloween anomaly" (first time of season to have
decent odds for snowfall).
Well, the last best chance
to get a good snowstorm in the plains appears to center around the 10th of
April. To wit, in my sixteen Aprils in Boulder (since 1989), we have had eight
storms in the foothills that dropped around a foot or more of snow on us between
April 9th and 12th. Last year and 1991 are the "high-snow-marks" for this
phenomenon when around two feet fell in my neck of the woods. Incidentally,
comparable good snowfall odds for me follow almost two weeks later, but by then
the snowline often settles around 7K or 8K, so that the plains often only see
lots of rain (the dump of 1997 notwithstanding - I saw close to four feet in
that one, while Boulder picked up around two feet if memory serves correctly).
Let's hope that three is indeed the charm, and this one finally delivers what
the last two withheld - watch for an update by Friday afternoon, by which I hope
to narrow down the amounts...
Klaus
Update PSA #8
Mon
3/14/2005 10:15 AM
"not much for the next few days!"
Good morning,
Quick update: storm #1 appears to have delivered (through this morning, I have
measured just under 17" so far, at 20:1 not a spectacular moisture producer).
Storm #2 looks like it will be going too far south. However, this opens the door
for storm #3 on Thursday into Friday that was supposed to go to our north in
earlier runs. Would not be surprised if that one delivered more than the few
flurries than earlier advertised. I will be out of town through Thursday
evening, so no update will come from my end. Don't believe the forecasts that
warm us up significantly by the weekend, the weather will remain more
winter-like than we have seen in quite a while at least through the weekend.
Think snow (just not at DIA, please...),
Klaus
PSA #8 Fri 3/11/2005 12:22 AM
Good evening, everyone,
I am writing this a bit earlier than usual, since I may not be able to update
this tomorrow.
Yes, I know I have been quoted in the media as saying that I expected a wet
March (or, in the proper lingo: that the odds were favorable for a wet March).
Of course, that interview was originally recorded almost a month ago, and it
took so long to get out that one should have affixed a disclaimer that 'reality
bites' - we had an unusually dry spell in the last few weeks.
However, the dry (and mostly mild) weather will come to an end this weekend,
with the potential for quite a bit of snow all they way down to the plains from
a prolonged snowy spell that could last from Saturday night to Wednesday
morning, along with continuous sub-freezing temperatures above about 8K. While
the U.S. (GFS) model has been quite variable in its total precipitation
predictions, the Canadian ensemble model forecasts have been pointing at a
substantial event since last Saturday (!), anticipating at least an inch of
moisture for at least six days now. The latest run from this evening continues a
modest upward trend that would give us 2"+ for the five day period from Saturday
evening through Thursday afternoon.
Interestingly, the upcoming storm may be part of the reorganization of the
planetary waves that were locked into a very blocked pattern for some three
weeks, anchored by an extremely long duration (about three weeks) high pressure
cell over the North Atlantic that kept most of Europe in a very impressive
winter pattern for so late in the season, with record snows from the Netherlands
to the Moscow, and all-time record low temperatures for so late in the season.
This pattern appears to be coming to an end right now, with an overall
"retrogression" (westward movement) of the Atlantic block, leading to the
establishment of a western U.S. trough (instead of a ridge) over the next three
days or so. By the way, it is too late in the season to get really cold air over
us with this pattern, especially since the air upstream (in Canada) is not
particularly cold either.
Also, there is not a particularly good fetch of moisture with this system, even
though the dynamic forcing might compensate for this, along with periods of
upslope wind.
Bottomline: expect two more warm days with a fair amount of sun and wind, and
then a good cold front from the north Saturday night. We should wake up to a
steady snow that might last all day, and into Monday (in the Canadian scenario),
followed by a possible lull during the on Monday. A second surge follows the
first one by about 48h, leading to more accumulating snow. There is the distinct
possibility that the second wave may lead to a cut=off low in the vicinity of
the Four Corners region. If it had not been so dry lately, I would predict about
a foot for each of the two surges (more in the foothills/less in the plains,
with good melting from below initially, and warmer temperatures), with the
potential for more snow with the second storm in particular.
Given the recent dry
weather, I will be happy to see a total of more than a foot out of this. This
should also be a good storm for the mountains on both sides of the divide (after
the initial snows that will focus on the east side of the Front Range), so that
the statewide snow pack numbers should go up again. If the Canadian ensembles are
right, this could hoist the snow pack in the South Platte basin very close to
close to normal by the end of the week.
Think snow, and watch for a possible update on Monday morning, before I have to
go on a trip (unfortunately),
Klaus
Update PSA #7
Tuesday Feb 15 2005
Good afternoon, y'all, it looks like the main event is already over, so most of
us will only see light snow for the remainder of the evening. I have no idea how
much snow fell in my neck of the woods, but the weather service wrote that up to
a foot fell in Larimer County foothills! The second storm within two and a half
weeks that behaves more like a spring storm than a mid-winter storm. Hopefully
this means even more interesting storms by next month!
I may still have to write PSA #8 later this week, if the next storm sets up
properly. Meanwhile, there is still some room for at least light snow over the
next day or two, with a break around Thursday into Friday, and then more of the
same (overrunning moisture from the southwest and low-level
upslope) over the weekend into next week. - THINK SNOW (and report it,
please!)
Top Of Page
PSA #7 Tuesday Feb 15 2005
Sorry about the late notice, but this one is evolving stronger than expected -
yet again! And again, the only model that has so far flagged this one as a
potential dump is the Canadian Ensemble model! In short (I have an appointment
right after this), the persistent split flow pattern has shifted enough to be in
our favor, with colder air from Canada infiltrating our area in the lower
elevations, and moisture from the Pacific overriding this at higher levels. The
overall configuration looks pretty stable into at least tomorrow, so the snow
should continue at least until then.
Given this morning's snowfall rates, this should translate into a foot of snow
(or more) in the foothills of Boulder and Larimer County, with the best snow so
far being reported from locales west of Fort Collins, where it may approach two
feet by tomorrow. Boulder et al. will be lucky if more than 8" accumulate.
Temperatures should remain roughly the same as they are now for the rest of the
day, ie., just below freezing in the plains and upper 10s/low 20s higher up
(arctic air will remain well to our north).
So, the treated roads should remain just wet until sunset, and even after then,
it may not be too bad, depending on the amount of mag-chloride used. The higher
foothills may become quite messy late tonight, since they probably won't get
plowed after 4pm, and there won't be any melting up there.
The pattern remains potentially quite interesting into next week, with a lull
around Thursday. Will issue another PSA later this week, if we are still on
track for the weekend snows. No more 60s and 50s in the plains for quite a
while!
Again, sorry for the delay, but most models were just giving us a few inches out
of this...
- Klaus
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PSA #6 /
Postmortem #5
Good evening,
The storms are starting to roll in regular progression (or so it seems), and we
definitely seem to have the best snowpack in the Front Range in about 7 years
(without the benefit of an October four-footer!). Looks like another storm is
just about to hit us. This one is yet another "funny" one, with a huge moisture
plume moving in (aloft) from the southwest, and surface arctic air from the
north and northeast, which will get moderated somewhat by the lack of snow from
South Dakota to us. Nevertheless, the stage will be set for a decent overrunning
event (warm air above cold air) for tomorrow into Wednesday. If you believe the
ETA model (with its superior terrain resolution), we should barely get 0.2" of
moisture, while the coarser GFS model spits out more than three times as much
for us. This would be quite high for a January event, but I am leaning towards
the GFS because it appears to have a better handle on the moisture
supply/advection in the next 24h than the ETA. To wit, we should be saturated
and precipitating right after midnight (at least in the foothills), while the
ETA holds back at least six hours and never gets as moist as the GFS. The
Canadian Ensembles from last night support a bigger event as well. Fly in the
ointment: the circulation will never settle into a straight, deep upslope event,
so that we are dependent on the unusual amount of moisture as well as some
dynamic forcing (jet streaking rounding the cutoff low over Utah) to get the
higher amounts that I anticipate/"wishcast".
Bottom-line: this event should last for about 24-36h (petering out during the
middle of Wednesday), temperatures should stay below freezing once it gets going
(mostly 20s in the lower elevations, and 10s higher up), and the second half of
the event should benefit from layer temperatures in the low 10s which should be
good for fluffy snow. Some of the really early snow may fall as mixed
precipitation in the lowest elevations(!). Since I expect close to 0.5" liquid
(it is very hard to get more than half an inch of moisture in January!), I can
see widespread amounts of 5-10" in the lower elevations, and up to a foot or
more higher up (fluffier snow!). Due to details of the dynamic forcing there may
be some SW-NE oriented banding as well that could double localized amounts.
It will be cold enough for the snow to stick during the evening rush hour
tomorrow, and might refreeze/become icy on Wednesday as well.
The weather service is not that different in terms of the forecast snowfall
totals (4-8" in the plains close to the foothills (more to the EAST!), and 6-10"
for the foothills), but they have gone hogwild about the arctic air that is
supposed to flood the area. Might be a case of forecasting (again) the last
event that just happened before Christmas. I just don't think that sub-zero lows
are in the cards, especially if we keep as cloudy as it looks to be right now.
Happy (Snowy) New Year,
Klaus
P.S.: Postmortem - a nice pre-Christmas snow dropped just under a foot on some
lower foothill locales (Boulder Heights, near Eldorado Springs), while higher
elevations got less fluff (7" total for me). Temperatures dropped below zero
virtually everywhere by the evening of the 23rd, only to rebound nicely on the
next day, without too much wind. What a short-lived (and reasonable
predictable) arctic episode! I recorded -13F which might well be the low for the
winter.
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PSA #5
(Belated Update)
Dec 22 2004
Good evening, y'all,
Another round of snow is coming our way, similar to last night's fluff (7.4" is
the highest report so far, from Boulder Heights, but only 0.17").
However, the final
arctic surge will be about 10F colder than last night's which means that higher
elevations may get cheated out of some nice dendrites and get the dreaded DENSE
arctic snow ("ice needles") instead. One observation of interest is that this
arctic airmass is not of the mature, stable variety, where one can observe
almost isothermal temperatures from the surface to the mid-troposphere. Since
the incoming air mass has not had much time to "mature", and since there are even
some regions to our north that are still only covered by a thin/non-existent
snowcover (eastern Montana!), this means that the air is somewhat unstable,
leading to snow showers/-bursts with occasionally sharp boundaries, as witnessed
last night in our drive home into the mountains.
Bottom-line: expect more snow from late this evening until late morning, fluffier
in lower elevations, and about the same amount of liquid (0.1-0.3") as last
night.
This means around 2" higher up (say, above 8K), and up to 6" in the lower
foothills.
Temperatures should dip well below zero higher up (-10sF is quite possible, if
there is any hole in the cloud cover), and close to 0F in the plains. I am not
sure about tomorrow night - there will be plenty of warm air trying to scour out
the arctic air by early Saturday. Could result in good winds higher up/ a wave
cloud to keep the plains from cooling off too much tomorrow night. So, the
"worst" of the cold
may be over after tonight.
Season's greetings,
Klaus
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PSA 5 Xmas week 2004
Outlook / Postmortem #4
Good evening, y'all,
After almost three weeks of the 'winter doldrums' (on the eastern plains), next
week promises at least winter-like temperatures next week, possibly with some
moderate snows as well. As is typical for these medium-range forecasts, the cold
temperatures are much more predictable than the amount of precipitation. The
first cold front is scheduled to arrive by Monday evening, and should get us
well below freezing, while the secondary/more important push should arrive about
two days/nights later, with the potential for true arctic air (temperatures
hovering around 0F even in the plains) over eastern Colorado.
The weather service is rather skeptical about the chances for snow (they put in
a measly 10-20% for Tuesday through Thursday), but I am more optimistic. This is
for two main reasons: one is based on my own experience with cold waves around
here, the other is based on the Canadian and European model forecasts. Starting
with my own experience, this is one of the few places I know where arctic air
has a way of generating its own, focused snowfall patterns, while many other
regions around the country (with the notable exception of the Great Lake 'snow
belts')
that tend to dry out as arctic air moves in. The main reason for this is
topography: having the mountains to our WEST means that arctic high pressure
cells that move south from Canada towards Colorado push the air against the
mountains, and "make their own snow" as these upslope conditions continue for
longer stretches. The amount of snow we get from this depends on (a) the amount
of upslope (intensity, depth, and duration), and (b) the amount of upper air
forcing (overrunning aloft and/or disturbances in the upper level flow). The
latter can be quite fickle, since relatively small-scale features can make all
the difference, while the former (upslope) is a safer bet. Since we should be in
an overall upslope regime from Monday night at least into Thursday, I am less
concerned than the weather service that this will result in precipitation of a
long duration. Typically, these situations have separate diurnal pulses that
appear to synchronize around sunset (which is earlier to our north), and come in
from the north, making for messy evening rush hours (probably not before Tuesday
next week).
Reason #2: the Canadian, European, as well as the U.S. Navy forecast models give
us more intense dynamic (upper level) forcing around Wednesday that yields
around half an inch of moisture (or close to 10" of fluffy snow) - compared to
the anemic looking weather service model (GFS).
Bottom-line: the stage will be set for wintry, occasionally snowy weather, but
it is early to call for snowfall amounts. Look for an update on Monday!
Have a good weekend,
Klaus
P.S.: quick postmortem on the post-thanksgiving storm: forecast verified nicely
with widespread 6-12" amounts in the plains (maximized around Loveland), and
12"+ amounts in the foothills (maybe close to two feet straight west of Fort
Collins). Subsequent cold weather kept the snow on the ground for more than a
week even on the plains, and almost everybody experienced sub-zero lows.
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PSA 4
Nov 24 2004
Good afternoon, y'all,
We appear to be stuck in a cold pattern into the foreseeable future.
Any snow that will fall
over the next week or so will not melt with the usual swiftness (in the
lowlands). After a quick moving system tomorrow night into Friday, the more
interesting situation is shaping up for the weekend, around Sunday. As has been
common this fall, the models are all over the place with this one, with some
solutions short-changing us yet again by tracking most of the action to our
south and west, while others put us right under the cross-hairs of a major
winter storm. After a brief warm-up tomorrow, apparently with less wind than
originally expected (?), it is quite possible that we will see some snow falling
out of the sky from tomorrow evening right into next week. Around 9000'
and higher, the present light spill-over snows should continue off and on
through the next 30h, leading to an even longer stretch of accumulating snow.
After last weekend's "dud", I am hesitant to put numbers on this one, but my
best guess right now is for around a foot of snow (powder) in the higher
foothills from tonight through Monday, and about half of that for the lower
elevations (plus/minus 50%).
Enough, in any case, for some travel problems. For those of you that have to
travel this weekend, I would advise to check the latest forecasts in case the
"wet" scenario comes to pass.
If we do get more than a few passing showers (like yesterday), and it clears
out, this will be our first chance of getting close to 0F in the higher
elevations, and down to about 10F in the plains (most likely around Monday).
West of peak-to-peak highway, the current freeze should continue well into next
week, allowing for a nice/overdue build-up of a more seasonal snowpack.
Happy/snowy/safe T-day/weekend,
Top Of Page
POST MORTEM
PSA 2 & 3 Nov 22 2004
Looks like we had one good Halloween forecast (PSA #2), and one mixed forecast
for last weekend (PSA #3). Regarding the first one, I had promised localized
amounts of up to one foot on the evening of Halloween, and it looks like there
were 13.2" near the top of Coal Creek Canyon, as well as 10" near Red Feather
Lakes.
Otherwise, there
were widespread reports of 6-8" in the foothills, with lesser amounts (initial
RAIN) further down.
The weekend storm ended up tracking further west than expected/hoped for,
dropping a cool 29" on Wolf Creek Pass in 48h, vs. just a couple of inches in
and around Boulder to Eldora and Sugarloaf, while northern Boulder and the
foothills of Larimer County received at least twice as much (yours
truly: 5.8" out of 0.43", Niwot Ridge just under 7"
out of the same amount of moisture), and there was even an 8" from west of Estes
Park.
Unfortunately, the same system that only gave us a glancing blow over the
weekend will track eastward too far south of us to affect us any further. A
decent low-level surge from Montana and Wyoming will cool us down for tomorrow,
but looks to shallow and dry to give us more than clouds (riming in
foothills?!). Thanksgiving looks dry and windy for now, with some spill-over
snows possible in the favored locales (NED, etc.). Should also warm us up again
after tomorrow's cool-down. From Friday into Sunday looks like another cold push
with potential for upslope snowfall that could become PSA-worthy - stay tuned, I
will update this by Wednesday.
Looks like winter-weather is here to stay into at least next week, hopefully the
snow will not be far behind -
Klaus
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PSA 3
Nov 19 2004
"Weekend storm"
Good evening, y'all,
looks like the flow patterns will finally favor us again, sort of like the
Halloween storm, but with more (and longer) oomph behind it. By the way,
tomorrow is the 25th anniversary of the 2nd biggest November snow storm on
record (officially about two feet, but according to some eye witnesses closer to
30"). In case you are wondering, close to 40" fell in early November 1946 for
the bigger one.
Getting back to the storm du jour, a decent-looking wave is dropping into the
intermountain west right now, and will carve out a fairly long-lasting cutoff
low to our southwest that will finally "kick out" around Tuesday. In between
tonight and Wednesday, the best-case scenario keeps the cutoff low close enough
to us to keep us in an overrunning pattern (cold air near the surface vs. warm
air aloft) for most of this period. It does not look likely that the flow will
get much of a moisture fetch from the pacific, so that much of this period may
not get heavy precipitation rates. For that matter, it was disheartening to see
how long it took to cloud over with near-surface upslope winds today. Oh, and
the worst case scenario drops the wave too far to our southwest (and too fast),
and then kicks it out too far to our south early next week to do us any good
then.
Other pertinent information: 1. It will be cold enough for snow at all
elevations and from the beginning of this storm; 2. The weather service has just
issued a winter storm watch, with 4-8" in the plains and 6-12" in the foothills
and higher up by tomorrow night; 3. All models agree on at least light snow to
fall from early tomorrow into tomorrow night.
Bottom-line (focusing just on the next two days, more on Monday, if "part II" of
this storm materializes): watch for snow to commence before midnight in the
higher foothills, followed by snow at lower elevations well before sunrise.
It should be coming
down at a pretty good clip by sunrise, accumulating to 4-8" by sunset in the
plains, and more than that higher up. This is one of those storms where the
upslope is not terribly deep, so I could see the middle foothills (~7000')
receiving the highest amounts. Alright, places higher up with good southeastern
exposure should also do well.
Tomorrow night , the snow should continue, albeit at a slower pace, adding
another few inches to the total. Grand totals should be between 6-12" in lower
elevations, and up to 20" in the most favored locations. Even though most models
do not give us more than 1" moisture, I am still optimistic about snow totals,
since the temperatures should favor good crystal growth (-10 to -15C).
More on Monday (including postmortems for this and the last storm), especially
if part II looks good by then -
THINK SNOW,
Klaus
P.S.: I sent this out one hour ago, but it bounced back because of one faulty
address, sorry!
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PSA 2
Oct 29 2004
Halloween Edition
.GIF)
Hello everyone,
quick thumbs up on Sunday evening snow for EVERYONE, including the lowest plains
elevations! Range of modeled outcomes is still quite large, but all of them spit
out at least a few 1/10" moisture, along with temperatures that should quickly
drop below the freezing mark. Best case scenario: 12h of decent upslope, with
moderate snowfall rates could lead to localized (close to) one foot amounts in
the foothills. Timing is settling in on Sunday evening, so dress your
trick-or-treaters warmly, and prepare for snow on roads. Should be melting on
lower elevation pavements, at least initially.
A little bit of statistics thrown in for "Halloween" anomaly connoisseurs
(Boulder is tricky, since the cutoff time for daily observations is at 5pm (!)
rather than midnight, or early morning). If you take the daily data since 1948,
and allow for snowfall to occur either on the 31st of October and/or the 1st of
November, there have been 14 (out of 56) such occasions, or exactly one every
four years. In the last 15 years, the odds for snow in Boulder did improve to
40% (1989, 1991, 1995, 1996, 2002, and last year), but that includes 0.2" in
2003, while all others were at least 1". Two of the more memorable occasions
were in 1972: 10.5" on those two days, plus 9.6" on the 30th (15" on ground by
the 1st), and just two years ago (2002), with 6.7" on those two days, but a
total of 20.8" from the 29th to the 3rd (only made it to 10" on the ground,
however).
In Denver (measuring midnight to midnight), the odds for snow on the 31st are
only once every 8 years, but go up to over 50%, if you allow for snow on the
1st, and/or snow on the ground (the Denver statistics can be found at:
http://www.crh.noaa.gov/den/awebshtml/halosnw4.shtml).
In the higher foothills (yours truly), the odds are over 50% for snowfall (8 out
of 15 since 1989) on the 31st, improving to over 70% (11 out of 15) if one
allows for snow on the 1st as well. IOW, almost double the odds seen in Boulder.
BTW, my record snowfall for the 31st and 1st combined is "only" 10.5" (in 1995),
so the upcoming event could challenge that.
So, despite what one hears about Halloween always being snowy in Boulder, this
is not quite correct, to say the least, but it certainly qualifies as the first
period in the fall season to have winter-like (or better) odds for snowfall.
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There is potential for a more prolonged wintry period from the middle to the
end of next week, but more on that as we approach it.
Happy Halloween, and let's hope that the election results are more certain than
my forecasts (I believe I commented that hell froze over after the 2000
election, in case you are wondering about my political tilt, or was that a
comment about the weather?) -
Best,
Klaus
P.S.: My first PSA, back in September verified pretty well - measurable snow was
confined to elevations around 7000' and up, and I had one report of 8.6" of wet
snow (Coal Creek Canyon, just under 9000'). And we had indeed close to 1" of
moisture for most of us.
There was another event earlier this month (around the 12th/13th) that dropped
almost as much moisture on us, possibly a little more snow (I had 4.6" in the
September event, and 6.8" in the last one). I did not send out a PSA for that
one, expect amounts far below 12". Turns out that event was a bit more than
expected. Always good to see that happening early in the season (as opposed to
false-alarm-storms that don't deliver).
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PSA 1 Sept 20 2004
"First snow of the season, with a small chance for significant snows in
higher foothills"
Good evening,
after a wet and cool summer, fall may skip a season over the next few
days, by dropping our temperatures some 30F or more, as well as some of
the white stuff down to the lower foothills. Based on today's model
runs, the spectrum of possible outcomes is still quite large, ranging
from a few showers (and flurries higher up) for tomorrow to a prolonged
precipitation event from later tonight through most of Wednesday (which
would allow for accumulating snow to fall all the way down to western
Boulder). Too bad that the moisture from "Javier"
managed to skip over us ahead of this storm, otherwise we could be
talking fairly large precipitation amounts.
Compared to other PSA's (for newbie's, that refers to "Public Service
Announcements"), I won't go into detailed scenarios, partially because
I need to get out of here, but also because I do not have a favored
model this time around. However, the wettest outcome could result
in an inch or more of moisture (if it falls in
upslope regions overnight, this would be found in the higher
northeast-facing foothills; if more of it falls during the daytime,
with isolated thunderstorms, anyone might luck out), and most of it
would fall as snow in the highest foothills, while lower elevations may
have to wait until Tuesday night for any frozen precipitation. If the
clouds and precipitation linger around tomorrow night, the plains may
escape a freeze for that night, but I believe there is a high risk for
at least a light freeze by Wednesday (and Thursday) night for all but
the most protected locales (like downtown Denver...).
Cross your fingers (if you like snow), it seems that one in three
Septembers is good for a PSA-worthy storm, and the last one was in 2001
(in the higher foothills)!
Think snow -
Klaus
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