264 
FXUS65 KBOU 031845
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
1145 AM MST Wed Feb 3 2021

.UPDATE...
Issued at 1145 AM MST Wed Feb 3 2021

A couple changes to make at this hour though the forecast looks
good in general. We're outperforming on temperatures today, so
we'll raise highs by a couple more degrees. This will bring Denver
close to the record for the date of 70.

As far as the snow, we're seeing a leading band (really a band of
bands) develop from north of Grand Junction to Rocky Mountain
National Park, with the main band trailing it from south of Salt
Lake City to near Rawlins. There's plenty of instability and
shear, with quite a bit of lightning where this band has
intersected the Wasatch mountains this morning. Most of the models
show a complex banded structure like this with quite intense snow
in the bands, though I wouldn't bet too much on the minor 
details. There's a general emphasis from southern Grand and Summit
counties mid to late afternoon to areas south of Denver by late
evening. Main impacts would be I-70/Berthoud Pass this afternoon,
foothills W/SW of Denver for rush hour, and possibly I-25 south of
Denver mid to late evening. For now, we'll raise PoPs and snowfall
in these areas a bit more. Probably not a lot outside of these
areas though everyone could wind up with a quick rain/snow shower
this evening.

&&

.SHORT TERM...(Today and tonight)
Issued at 340 AM MST Wed Feb 3 2021

An upper level trough will move towards the area from the northwest 
today with WSW flow aloft.  Moisture will increase ahead of this 
system with rather steep lapse rates. With position of upper level 
jet this should lead to areas of heavier banded snowfall across the 
mtns by midday thru the aftn hours. Will likely see snow squall 
activity in some mtn areas this aftn into the early evening hours 
with rapidly changing road conditions.  

Across nern CO, sfc low pres will be in place with downslope WSW low 
level flow.  This will allow for another mild day with highs in the 
upper 50s to mid 60s across the plains.  In addition a few light 
showers may move off the higher terrain late this aftn which may 
briefly enhance wind gusts. Wouldn't be surprised to see a few brief 
gusts up to 50 mph across portions of the plains.

For tonight, the main upper level trough will move across the area 
with the flow aloft becoming more WNW.  Cross-sections continue to 
show decent lapse rates over the area with favorable orographics in 
the mtns.  Thus should continue to see additional snow in the mtns 
with amounts by 12z Thu in the 6 to 12 inch range.

Across nern CO, a cold front will move across the area by mid to 
late evening.  The low level flow will be mainly northerly behind 
the front, however, some of the high res data is turning the winds 
more NNE. Meanwhile, as the upper level trough moves across, 
there could be an enhanced area of precip behind the front. Right
now it looks like areas along and south of a line from Denver to 
Akron could see a brief period of heavier snow late this evening 
thru 2 am or so. In addition gusty north winds may cause areas of 
blowing snow and poor visibility over the plains as well.

.LONG TERM...(Thursday through Tuesday)
Issued at 340 AM MST Wed Feb 3 2021

By Thursday the center of the trough is across North Dakota and the 
ridge axis extends southward across the Central Plains. The core 
of the cold air aloft thus gets shifted east throughout the day 
and drier WNW flow will likely put an end to the orographic snow 
in the mountains for all but the highest ridges of the Park Range 
and Continental Divide north of I-70, where just enough moisture 
hangs around for light snow. No significant travel impacts are 
expected and additional accumulation should be 0-2" through the 
evening hours. Across the plains, the surface pressure gradient 
picks up and with some clearing resulting in vertical mixing, 
northwest winds could gust over 35 mph across the far northeast 
plains and along the Wyoming border. Despite some modest warming 
aloft temperatures will remain below normal with highest in the 
upper 30s to around 40 across the plains, upper 20s in the 
foothills, and low 20s in the mountain valleys. There may be a few
flurries before midday across the eastern foothills, Palmer 
Divide and to a lesser extent south of I-76, but no additional 
accumulation is expected.

Strong Northwest flow continues Friday and moisture advection 
begins in earnest early Friday ahead of the next embedded trough 
aided by a 120 kt jet. More favorable WNW flow will result in snow
picking up across the west slopes and the 700 mb flow becomes 
saturated west of the Divide by Friday morning. Lapse rates just 
above the mountains are poor until later in the day on Friday. 
Nevertheless, light snow is expected along favored WNW facing 
slopes throughout most of Friday and all of Saturday with 
persistent, fairly moist northwest flow aloft. On Saturday 
afternoon the left exit region of a 120 kt jet streak passes over 
northern Colorado, but the atmosphere is drying out by then so the
vertical motion boost may not make much of a difference in snow 
amounts. Upslope flow becomes westerly late Saturday into Sunday, 
but dries out significantly which will bring an end to the snow in
the mountains by early Sunday. Synoptic support for lift is 
lacking because even the weak troughs are missing us to the north 
Friday and Saturday. Thus, snowfall amounts may stay below 
advisory levels and travel impacts minor, though the Park Range 
(zone 31) may see a decent snow over a 36 hour period. Much of the
high country along and west of the Divide should see accumulating
snow. Across the plains, downslope flow combined with no synoptic
or mesoscale support for lift means just a slight chance of snow 
on Friday, with the best chance along the Wyoming border and far 
northeast corner. The rest of the area including the I-25 urban 
corridor may just see a few flurries, if that. Highs Friday will 
be near 40 across the plains, with 20s in the mountain valleys. 
We've been advertising an arctic front on Saturday for a few days 
now and not surprising, the models are having a hard time figuring
out how to handle it. One thing is certain: the coldest air of 
the year, by FAR, will be across the central and northern plains 
this weekend, driven south by a long wave trough over Hudson Bay 
Canada. The trough axis is over 1000 miles east of us, making it a
challenge to get the real deal arctic air into Colorado. The EC 
and Canadian models keep the arctic air mostly east of us, while 
the NAM and GFS slosh the cold air most of the way to Denver but 
just the northeast corner gets the really cold air under that 
solution. Even the ensemble solutions are downplaying how far west
the coldest air gets on Saturday, with just a handful of members 
of both the GEFS and Euro ensemble bring the coldest air west of 
I-25. Given all this, the cold front will move across eastern 
Colorado Saturday but may not bring the true arctic air much 
further than the far northeast corner. Will keep highs well below
normal but aside from the northeast corner, highs for now are 
forecast to be in the upper 20s to low 30s, other than the low 20s
for the northeast corner. With a weaker front PoPs are not 
significant for most of the plains, though the far northeast 
corner is close enough to frontogenetic forcing early Saturday 
that they could see up to inch of snow or so.

Sunday and Monday northwest flow continues and any embedded moisture 
should stay northeast of Colorado.  Can't rule out very light snow 
across the mountains given orographics but lapse rates are poor. 
Sunday morning a jet max moves across the state and there is a 
pretty good inversion between 500-600 mb.  With 75 kts of flow in 
that layer, a mountain wave could develop which forces winds down
the east slopes. Too far out to say how far east the strong winds
get, but the mountains and eastern foothills could see gusts over
70 mph if this trend holds. Temperatures both days will remain 
below normal with the coldest temps across the northeast corner of
Colorado, closest to the arctic airmass.

Tuesday and beyond the pattern looks cold with embedded moisture in 
WNW flow resulting in chances for mountain snow, but so far not 
seeing anything significant snow-wise for the plains and I-25 
corridor.

&&

.AVIATION...(For the 18Z TAFS through 18Z Thursday afternoon)
Issued at 1145 AM MST Wed Feb 3 2021

West winds will develop across the Denver area this afternoon,
between 20z and 00z, with gusts to around 25 knots causing
potential crosswind issues for KDEN/KAPA for a few hours. Winds
will become northerly this evening, with gusts to around 30 knots.
Scattered rain and snow showers will develop early this evening,
with a few hours of snow showers likely 04z-08z and areas of IFR
visibilities in the showers. Most places will have an inch or
less, though locally heavier showers are expected south and west
of Denver. Quick clearing is expected between 08z and 12z with
lighter winds.

&&

.BOU WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory until 9 AM MST Thursday for COZ031-034.

&&

$$

UPDATE...Gimmestad
SHORT TERM...RPK
LONG TERM...Schlatter
AVIATION...Gimmestad