159
FXUS65 KBOU 041118
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
418 AM MST Thu Dec 4 2025

.KEY MESSAGES...

- Areas of dense fog north of Denver through mid morning.

- Heavy, wind driven snow event likely (>70% chance) for the
  mountains Friday night through Saturday night, with difficult
  travel conditions.

- Stronger winds at times into early next week.

- Milder temperatures ahead, with above normal temperatures by
  early next week.

&&

.UPDATE...
Issued at 335 AM MST Thu Dec 4 2025

Areas of very dense fog were observed on camera networks this
morning as far south as Commerce City. It stretched north all the
way to around Fort Collins and northeast through Fort Morgan. We
see very little movement through the next few hours, although it`s
nudging ever so close to DIA. With snow on the ground, it will
take a few hours to burn off after sunrise. Hopeful to see
improvement by 9 am, with the Dense Fog Advisory slated to expire
at 10 am. Until then, travelers will encounter some very dense
fog in spots (I-25, I-76, and US85 north of Denver, and Hwy 34),
with visibility reduced to just a couple hundred feet.

Otherwise we`re set up for mostly sunny skies with some moderation
in temperatures. Snow cover and morning fog will limit warmup in
spots, but only minor adjustments to forecast highs considering
the previous shifts had taken this into account.

Our attention will then turn to mountain snow, with potential for
a significant wind driven snow event for at least the first half
of the weekend. One batch of lighter snow is expected to arrive
toward midnight tonight in the northern mountains. While the
airmass is relatively stable with poor lapse rates of only 4-5C/km
and moisture is shallow, orographics will be increasing. The
orographic snow model suggest a 4-9 inch forecast for the northern
Gore and Park Ranges, so we`ve issued a Winter Weather Advisory
for that from midnight tonight through Friday. The rest of the
mountains will see much lighter snowfall, with the I-70 mountain
corridor only see scattered 1-2 inch accumulations as those
locations would be on the southern extent of the mid level
moisture plume.

That changes Friday night through Saturday when another powerful
Pacific jet stream, this one laden with abundant moisture and
improved lapse rates, reaches the forecast area. We are forecast
to get into the left exit region of the upper level jet, and
strong orographics combined with deep moisture and near neutral
lapse rates will mean efficient orographic precipitation.
Considering the above mentioned parameters, we`ve issued a Winter
Storm Watch for all of the northern mountains for Friday night
through early Sunday morning. The heaviest snow will likely fall
late Friday evening through Saturday, with a gradual decrease
thereafter. These types of storm systems typically produce high
impacts and occasional road closures, so we`ll start messaging
this one early (thus the Winter Storm Watch).

Finally, we`ll see bouts of gusty winds in this forecast cycle.
Forecast Sangster data off various models suggested the strongest
winds Friday into Saturday, and then again Tuesday. Cross sections
don`t indicate a favorable stability setup for high winds, but
something to watch given impressive gradients to work with.

UPDATE Issued at 855 PM MST Wed Dec 3 2025

Stratus and very light snow/flurries continue to linger over the
western plains, the lower foothills and the plains south of I-70.
Areas of fog have developed across the northern half of the plains
as well. As some of the models indicate areas of fog and low
clouds for some of the plains, will update the grids in keep this
going longer. this evening.

&&

.SHORT TERM /Through Thursday/...
Issued at 314 PM MST Wed Dec 3 2025

Upper level trough over the region slowly continues to shift
east- southeast through tonight. Weak northeast upslope low level
flow ends and winds turn more westerly. Snow continues to end from
north to south, ending mid evening over the southern foothills
and Palmer Divide. Web cameras show roads are mostly wet across
the Denver metro area. As temperatures fall towards sunset, road
conditions may worsen. Best chance for slippery roads will be
south of Denver along I-25 and westward into the foothills. As far
as the Winter Weather Advisories go, already expired the north
ones where snow has ended. May be able to expire the Boulder and
Denver areas early, will see how the snow plays out. Plan on
keeping the Park County advisory going into the evening hours.

The easterly flow across the continental divide is producing
downslope conditions across Summit and Grand Counties, scattered
snow showers will remain possible through about sunset and
additional snowfall expected to be light and less than an inch.

Drier air spreads across the area tonight with mostly clear skies
expected for much of the area come Thursday morning. Some clouds
will persist in the mountains. Patchy fog will be possible with
the best chance for fog being north of Denver in Weld and Morgan
Counties.

Mostly sunny skies will lead to a warmer day Thursday. Fresh snow
on the ground likely keeps highs below normal with readings in
the 30s to lower 40s. Surface pressure lowers through the day over
northeast Colorado, increasing the pressure gradient across the
continental divide. Expect gusty west to northwest winds to
develop across the Front Range mountains and higher foothills
through the day Thursday with gusts to 40 mph possible.

&&

.LONG TERM /Thursday night through Wednesday/...
Issued at 314 PM MST Wed Dec 3 2025

The general pattern hasn`t changed much for the long term period,
with a gradual warming trend still expected into next week.
Multiple waves of mountain snow, becoming heavy at times, will
continue under increasing northwesterly flow aloft, while the
lower elevations will largely remain dry. Be prepared for winter
travel conditions if heading to the high country this weekend.

Orographic snowfall will return to the mountains beginning
Thursday night, as moisture and flow aloft start to increase with
a 150 kt upper-level jet approaching from the Pacific Northwest.
Cross- sections show a mountain wave signature in place, which
will bring gusty winds to the upper lee side of the Front Range
Mountains overnight (and potential for a pretty sunrise on Friday
morning!). Only light accumulations are anticipated overnight for
the northern mountains, though QPF was slightly increased for
this forecast package as ensembles show a slight uptick for the
Medicine Bow Range south to the Indian Peaks.

Mountain orographics will hold on to light snow across the higher
elevations through Friday (up to 5 inches for the Park Range, 1-3
inches for the Medicine Bow south to the Indian Peaks), before
synoptic forcings increase with the arrival of the aforementioned
jet, reinforcing mountain snowfall. The left exit region will
setup over the forecast area late Friday night, and the greatest
QG ascent is expected between 6Z-18Z (11PM-11AM) Saturday. With
steepening mid- level lapse rates and a hint of frontogenesis
expected, we could see some periods of heavy snowfall (1
inch/hour) with this system, especially for the Park Range and
Rabbit Ears Pass, where hazardous travel will be likely during the
aforementioned time frame, with snow covered roads and reduced
visibilities expected. Snowfall accumulations approaching two feet
will be possible between Saturday and Sunday in the Park Range,
with lesser totals expected for the Central Mountains, where the
I-70 corridor could see between 3-9 inches from Georgetown to Vail
Pass. The Medicine Bow south to the Indian Peaks will settle in
the middle, with 6 to 11 inches possible for elevations above
9,000 feet. Be prepared for winter driving conditions when
traveling into any of the high country this weekend.

While the plains are expected to largely remain dry, we did increase
PoPs for portions of the lower elevations on Saturday, where both
ensembles and deterministic guidance are now hinting at influence
from the left exit region of the jet aloft bringing some very
light QPF (~.01 to .05 inches) to areas along the Wyoming border,
but would like to see what the hi-res solutions grasp onto before
upping QPF any farther. The main story for the lower elevations
will be with temperatures gradually warming through the weekend,
reaching the 50s to kick off the week.

&&

.AVIATION /12Z TAFS through 12Z Friday/...
Issued at 418 AM MST Thu Dec 4 2025

Rare fog event has unfolded at KDEN with visibility less than
1/4SM with southwest winds and no Denver cyclone! RVRs are down
to around 1000-1500 ft. We don`t see much change til at least
13Z-14Z per satellite imagery, although there is some slow erosion
now noted from the southwest. Whatever the case, the fog is
literally creeping at this time so we`ll keep things down til 14Z
and then show some improvement. Short term models like the HRRR
were showing significant improvement by 15Z, but considering the
slow motion it could easily be an hour in either direction
(14Z-16Z).

KAPA should stay clear, while KBJC not entirely in the clear as
fog lurks just to the east (so about a 20-30% chance of impact
there).

Regarding winds, light southwest flow is expected til about 18Z.
Then wind direction signals become mixed. Usually with snow on the
ground we end up with an afternoon wind out of the southeast, but
pressure gradients would suggest west/northwest. For now, we`ll
just go with VRB since wind speeds should remain less than 8 kts
and low confidence in any one direction prevailing. Light drainage
winds from the south/southwest will resume rather quickly by 01Z
Friday. VFR conditions will persist once the stratus/fog threat
end.

&&

.BOU WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory from midnight tonight to 5 PM MST Friday
for COZ031.

Winter Storm Watch from Friday afternoon through Sunday morning
for COZ031-033-034.

Dense Fog Advisory until 10 AM MST this morning for COZ038-043-
044.

&&

$$

UPDATE...20
SHORT TERM...12
LONG TERM.....9
AVIATION...20