Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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FXUS65 KBOU 081728
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
1028 AM MST Sun Dec 8 2019

.UPDATE...
Issued at 1023 AM MST Sun Dec 8 2019

A fairly sharp dry slot along with breaks in the overcast is
currently traversing across western and northwestern Colorado this
morning bringing sharp subsidence and a temporary pause in the
precip from earlier this morning. Another round of instability
will still be expected for late this afternoon and evening as the
main trough axis moves across the region. Current water vapor
channel indicates some lift and a light convective nature to the
precip associated with this axis out over northern and central UT.
At this time, the current winter highlights look on track as an
expected 7-11degC of cooling in the 600-500mb layer is still
expect through today and into late tonight while in an environment
of strong westerly flow for orographics. Best cooling and snow
production indicated in the short range models exists from 6pm
this evening through 3am Monday. Otherwise, only minor adjustment
to todays temps and winds. Scattered showers will still be
expected to detach from the continental divide region, in strong
westerly flow aloft, and move onto the plains this evening and
into the overnight period when the front arrives.

&&

.SHORT TERM...(Today and tonight)
Issued at 400 AM MST Sun Dec 8 2019

An upper trough dropping down over Northern California has
brought west-southwest flow aloft over Colorado. Moisture has
deepened over the last few hours as snow has entered into the
northern and central mountains. Good snow is falling over Summit
County and the I-70 corridor, and is expected to spread north
through the day, before winds turn more westerly this afternoon.
It becomes a bit more unstable this afternoon as colder air moves
in aloft, and QG upward motion increases. Could see a few
localized areas of heavy snow, increased QPF and snow amounts a
little over zone 31. Overall, advisory amounts still look good for
the rest of the mountains, even though zone 31 may see localized
areas of warning criteria. Winds will then turn northwesterly
tonight to bring good orographic snow and colder air to the
mountains. Temperatures will drop into the teens.

Over the plains, a mountain wave and downsloping winds still
exist this morning, keeping the foothills and into the urban
corridor breezy and warm. High clouds streaming over the area will
continue through the day, but the thickest look to stay to the
south of us. Even though forecast soundings show a very shallow
inversion which doesn`t break, am thinking the warm overnight
lows now, and assuming some sun will make it through should break
the inversion. Therefore, warmed the forecasted temperatures up
today across the plains by several degrees. As the lift from the
weather system moves across the area this afternoon, a few rain
showers may form quickly moving northeast. The associated cold
front isn`t expected until after midnight, with northerly winds
pushing down through the early morning hours. These winds will
then veer to the northeast and east, creating some weak upslope
flow towards sunrise, to bring a slight chance of snow across the
plains and into the foothills.

.LONG TERM...(Monday through Saturday)
Issued at 400 AM MST Sun Dec 8 2019

Colder air will continue to filter over the forecast area Monday,
in the wake of tonight`s cold front. Snow will gradually taper off
by late morning in the mountains as moisture depth decreases, but
a few spots could see a couple more inches of snow. The Winter
Weather Advisory for the northern mountains until 11 AM Monday
looks to be on track, especially considering there will be snow
covered roads and reduced visibility over the higher passes due to
blowing snow. On the plains, shallow anticyclonic upslope is
expected to remain in place. This would keep a slight chance of
light snow showers or a few flurries in the forecast. Moisture is
quite shallow and airmass is strongly subsident, so it surely
doesn`t look like much if anything. Temperatures will struggle
into the 30s on the plains, while mountain locales will be in the
upper teens and 20s with much colder wind chills.

For Tuesday through Thursday, we`ll see a gradual moderation in
temperatures, although a weak wave or two moving through the flat
ridge could bring weak backdoor cold fronts onto the plains. At
this point, hard to time any of these so we`ll go with the model
averages here, pushing to above normal levels Wednesday and more
so Thursday.

The main feature in the extended period is the prospect of more
significant snowfall in the Colorado high country starting by
Friday and then continuing through much of next weekend.
Obviously, that far out there is more uncertainty here, but
medium range ensembles are becoming in better agreement that a
powerful Pacific jet stream will plow across the Central Rockies.
This would open us up to a plume of Pacific moisture, and
combined with jet dynamics and strong orographics result in
potentially heavy snowfall for the Colorado High Country. Taking a
look at Ensemble tables, there`s an extended period of 2-4
standardized anomalies of Integrated Vapor Transport (IVT) into
Colorado late Friday night through Saturday. These coincide with
a plume of 700-500 mb specific humidities of 2-3 g/kg, also well
above normal. Some of this snow may also spill onto the plains as
we should get a cold front and push of at least shallow upslope.
If this pattern manifests itself, then Colorado skiers should be
getting excited about the prospects next weekend, while travelers,
in contrast, may be in for some rough driving conditions ahead.

&&

.AVIATION...(For the 18Z TAFS through 18Z Monday afternoon)
Issued at 1023 AM MST Sun Dec 8 2019

VFR conditions are expected through today across the metro Denver
terminals. Surface winds today will be predominantly from a west
to northwesterly component ahead of a cold front expected late
tonight. This cold front will bring a sharp switch to northerly
winds after 08z along with scattered light snow showers in the
vicinity of the terminals through Monday morning. A trend toward
dry and stable conditions is expected for Monday morning through
the rest of the day with with an east to southeasterly component
to the winds through Monday afternoon.

&&

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

&&

$$

UPDATE...Fredin
SHORT TERM...Kriederman
LONG TERM...Barjenbruch
AVIATION...Fredin


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