National Weather Service Forecast Discussion

000
FXUS65 KBOU 090323
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
823 PM MST Sun Dec 8 2019

.UPDATE...
Issued at 822 PM MST Sun Dec 8 2019

Water vapor satellite imagery is showing the best lift associated
with the upper level system has shifted east of the mountains
across northern Colorado. This has caused the snow to decrease for
most areas. Orographic lift resulting from a west to northwest
flow across the mountains combined with moisture will keep light
to moderate snow going through the night in the mountains. An
additional 2-6 inches will be possible into Monday morning. Will
keep the Winter Weather Advisory in effect for the mountains.
Across the lower elevations, a cold front will bring northerly
winds and low clouds after midnight. Winds decrease after the
frontal passage, but low clouds will linger into Monday morning.

&&

.SHORT TERM...(This evening through Monday)
Issued at 245 PM MST Sun Dec 8 2019

As the main trough axis with the current storm system pushes into
northwest Colorado this afternoon, starting to see an increase in
precipitation coverage across eastern UT and northwestern CO in
the wake of the dry slot which was over that region earlier this
morning. Observation points in this region indicate showers of
light to brief moderate rain across the lower elevations with
light to moderate snow coverage over the higher terrain. Forecast
soundings still indicate an increase in instability through
tonight as mid-level temperatures cool 7-10degC while in a
saturated to near saturated environment. Strong orographic
westerly flow at mountain top along with a few hours of better
instability will be the main drivers through the end of the event
midday monday. With these features tonight, can`t rule out a few
brief bursts of moderate to heavy snow overnight across the higher
terrain.

With northern Colorado under the gradient of 100-120kt jet riding
across the CO/NM border this afternoon, weak mid-level QG fields
indicate a fairly rapid transition over to strong subsidence
behind the area of best lift later tonight. Snow will begin to
taper off in the high country after 12z Monday with scattered
snow showers persisting over the highest terrain through late
morning and afternoon as flow aloft weakens and dries out.

Across the lower elevations tonight, dry conditions and mild
temperatures are expected this evening ahead of a cold front
expected in the 08z-09z timeframe. With strong net westerly flow
aloft there`s a good chance for a few showers to pull of the
higher terrain and onto the Urban Corridor this evening and
overnight. At this time, forecast sounding indicate precip should
be in the form of rain or rain mixed with snow tonight as the
column gradually moistens. Showers that develop after the frontal
passage overnight will be in the form of snow. Soundings across
the lower elevations mostly fail to fully saturate below 600mb
overnight. With the meager saturation, frozen precip likely won`t
attain best dendrite growth, while in a fairly mild airmass.
Soundings hint at a possible brief period of freezing drizzle
overnight initially with the frontal passage. Cross sections also
indicate that precipitation tonight will be banded, so there will
be the likelihood for some locations to receive up to a half in
inch of snow, while other locations remain dry. The atmosphere
will begin dry in the 15-18z timeframe and this trend continues
through the rest of Monday with temps remaining cool.

.LONG TERM...(Monday night through Sunday)
Issued at 340 PM MST Sun Dec 8 2019

Moderate zonal flow Monday night gives way to strong north-
northwesterly flow aloft on Tuesday. Northwesterly flow aloft
decreases Tuesday night with an upper ridge moving across the CWA
Wednesday. The next, weak, upper trough moves across Colorado
Wednesday night. Most of its energy is south of the CWA. Downward
vertical motion dominates the CWA according to the QG Omega
fields Monday night through Tuesday night. Upward energy is
progged Wednesday and Wednesday evening, with subsidence after
midnight. The low level winds look to adhere to pretty standard
normal diurnal patterns all five periods. There isn`t much
moisture progged in the lower and mid levels through the periods;
just some in the upper levels. There is no measurable precipitation
on the QPF fields. There are fairly decent winds along and east
of the divide, mostly up high, but not of highlight criteria. No
pops. For temperatures, Tuesday`s highs are 0.5-2.5 C warmer than
Monday`s highs. Wednesday`s highs are 1-3 C warmer than
Wednesday`s. For the later days, Thursday through Sunday, models
have north-northwesterly flow aloft on the increase Thursday. By
Friday, a strong northwesterly jet maximum is progged over
Colorado. This continues through Saturday and becomes more due
westerly Saturday night and Sunday. Speeds at 250 MB are in the
120-150 knot range Friday afternoon into Sunday morning. Models
are showing a broad mean upper trough developing across a good
portion of North America later next weekend. There is decent
moisture on the cross sections through the weekend.

&&

.AVIATION...(For the 06Z TAFS through 06Z Monday night)
Issued at 822 PM MST Sun Dec 8 2019

Surface winds will remain from a west to southwesterly component
ahead of a cold front expected 08Z-09Z. This cold front will bring
a sharp switch to northerly winds. Areas of low clouds will
develop behind the front after 09Z. The best chance for seeing low
clouds will be along the foothills. Where low clouds form,
ceilings will fall to around 1000 feet. Low clouds will linger
into mid to late morning with the clouds clearing or scattering
out by 18Z.

&&

.BOU WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory until 11 AM MST Monday for COZ031-033-
034.

&&

$$

UPDATE...Meier
SHORT TERM...Fredin
LONG TERM....RJK
AVIATION...Meier

NWS BOU Office Area Forecast Discussion