Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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FXUS65 KBOU 060213
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
713 PM MST Mon Dec 5 2022

.UPDATE...
Issued at 700 PM MST Mon Dec 5 2022

Radar composites and webcams show snow becoming steadier and more
widespread over the high country early this evening, as a band of
pronounced frontogenesis and healthy moisture slowly shifts
eastward across northwestern Colorado. It`s this elongated band
that will produce the bulk of the snow expected in the coming days
in the high country, and snowfall rates between 1-2" per hour will
be possible for a short period tonight in the mountains
surrounding Jackson County. The Park Range, Medicine Bow Range,
Rocky Mountain NP and Indian Peak Wilderness continue to look like
the big winners with this wave of precipitation, and the latest
HREF Localized Probability-Matched Mean shows potential for
higher than forecast QPF focused over our far northern mountains.
Have blended some of this heavier QPF into the forecast for those
areas to bump up snowfall amounts slightly. Overall though, the
below discussion covers the forecast details excellently, and no
significant changes are needed. Still on track for some minor
accumulations tonight over and adjacent to the Cheyenne Ridge and
northern plains, but downslope flow on the lee of the Front Range
will keep the majority of the urban corridor snow-free save for a
few flakes possible here and there. Will continue the current
suite of winter weather headlines as is.

&&

.SHORT TERM...(This evening through Tuesday)
Issued at 155 PM MST Mon Dec 5 2022

A 120 kt jet out of the west-southwest is pretty much overhead
this afternoon, and will slowly slide south through Tuesday
evening. An associated trough remains well upstream of Colorado.
In fact, between this afternoon and Tuesday evening the 500 mb
trough moves southward along the California Coast, but doesn`t
move very far east. Ahead of the trough in WSW flow, pretty decent
Pacific moisture is being advected into Colorado. Because of the
persistent WSW upslope flow, at least light snow is expected
across the mountain over the next 24 hours, slowly ending from
north to south by late Tuesday into early Wednesday. Thus, will
keep the winter weather advisory for the northern Mountains in
place. We expect another 4-10" through early Wednesday in those
areas. Travelers should expect slick roads across the highest
passes over the Continental Divide, as well as the Rabbit Ears
area, especially after dark this evening.

You might be wondering about the I-70 corridor. So far, webcams
have shown wet roads and light snow throughout today. However, the
700 mb front makes a run across the mountains around midnight
tonight. There is a strong band of frontogenesis associated with
the front, especially from midnight to 6 or 7 AM. Frontogenesis
bands are always hard to nail down, but the best chance for banded
moderate to heavy snow would be across the I-70 corridor during
the early morning hours of Tuesday. The better orographics and
lapse rates overspread Summit County and the Mosquito Range around
that time as well, with light to at times moderate snow possible
during the daylight hours as well. With expected totals in the
4-11" range and at least a few hours of moderate to heavy snow
early Tuesday morning, we have opted to hoist a winter weather
advisory for zone 34 (Summit County, Continental Divide south of
RMNP, and the Mosquito Range) from midnight Tuesday to midnight
Wednesday. The worst travel conditions on I-70 from near the
Tunnel to Vail pass should be Tuesday morning, though slick
conditions are possible given the expected snow through Tuesday
evening. A lot will depend on the formation and evolution of the
frontal snow band.

Because of the westerly jet overhead and downslope flow east of
the Divide, there won`t be much precipitation below 8500 feet
elevation on the east slopes. An inch or so is possible in South
Park through Tuesday. Across the plains, a decent jet- induced
precipitation band (mostly snow) sets up across southern Wyoming
after midnight tonight. A part of the band could be across
northern Colorado as well, thus we have chance PoPs in place
through the morning hours Tuesday. Trace to an inch of snow is
possible north of a Loveland to Holyoke line. South of that area
and below 7,000 feet elevation in the foothills, just
flurries/sprinkles are possible through Tuesday.

Temperatures tonight across the plains and I-25 urban corridor
will be near seasonal normals, with lows in the mid 20s across the
I-25 corridor, and teens elsewhere. Highs Tuesday should be in
the mid to upper 40s below 7,000 ft, with 30s in the mountain
valleys and ample cloud cover along and west of the Divide.

.LONG TERM...(Tuesday night through Monday)
Issued at 155 PM MST Mon Dec 5 2022

Tuesday night, the trough will continue to advance. There will be
a brisk, though decreasing, westerly to southwesterly flow aloft.
Light snowfall will persist in the mountains, also decreasing and
with little accumulation through Wednesday morning.

Wednesday, the trough will continue to advance and flow aloft
will continue to decrease, and will be more blocked orographically
as it turns more southwesterly. This should mean a lull in
snowfall early Wednesday. However, there will be gradually
increasing synoptic ascent ahead of the trough. QG fields suggest
the best forcing late Wednesday and early Thursday. There will
also be decent moisture at that time, models show precipitable
water generally 150 to 200 percent of normal, and a weak front
late Wednesday. These factors should support another 1 to 3 inches
generally above 9000 feet, and 3 to 6 inches across the southern
Front Range mountains. We have also added some light snowfall
across much of the plains in this forecast package as well, late
Wednesday and early Thursday, though precipitation will be limited
by downsloping winds especially near the foothills and Palmer
Divide.

Thursday, light snow showers probably persist in the morning then
there will be subsidence, clearing and drying behind the trough
axis, around afternoon. It also looks to get breezy in the
afternoon but humidity should still be high following the recent
precipitation and front.

Friday through the weekend, models agree generally on a more dry
pattern with a couple more shortwaves of weaker synoptic forcing
and less wind and orographic forcing, but disagree on the finer
details such as timing and intensity. Expect scattered light snow
showers mostly confined to the mountains and near-normal
temperatures.

&&

.AVIATION...(For the 00Z TAFS through 00Z Tuesday evening)
Issued at 431 PM MST Mon Dec 5 2022

VFR conds through 00Z Wed, with a chance of ILS conds after that
(bases 050-070). SE winds this afternoon will becoming SW drainage
by this evening, although models in general agreement in showing a
temporary spurt of NW winds between 08-13Z approx.

Anticipate a brief return to drainage after sunrise before
becoming increasing SE or E later on Tuesday. An isolated high-
based shower is possible starting around 00Z Wed, though
confidence is low.


&&

.BOU WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory until 6 AM MST Tuesday for COZ031-033.

Winter Weather Advisory from midnight tonight to midnight MST
Tuesday night for COZ034.

&&

$$

UPDATE...Rodriguez
SHORT TERM...Schlatter
LONG TERM...EJD
AVIATION...Rodriguez


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