014
FXUS65 KBOU 031145
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
445 AM MST Wed Dec 3 2025

.KEY MESSAGES...

- First winter storm of the season is well underway. Highest
  impacts for this morning`s commute, greatest along the I-25
  Corridor and Foothills.

- Snow diminishes this afternoon, but some impacts lingering into
  the early evening mainly for the Palmer Divide and Foothills

- Milder to end the week, but strengthening winds and increased
  potential for heavier and prolonged mountain snowfall into the
  weekend.

&&

.UPDATE...
Issued at 332 AM MST Wed Dec 3 2025

Heaviest snow is now pushing back toward the foothills, as local
wind profilers show east/northeast upslope had deepened to 13k ft
MSL. Upslope component was only about 12-15 kts on average though,
pretty weak for an upslope event. Nonetheless, there has been some
locally heavy snowfall along the frontal push we observed just
after 1 am, combining with a neutral stability profile. Radar
returns were now generally stronger west of I-25, and that should
be the overall pattern through at least early afternoon with
continued upslope being the main forcing mechanism (there is only
weak QG forcing in place and the best F-gen sinks to our south
early this morning). Snowfall rates will still be impressive
in/near the lower foothills, as near neutral lapse rates makes for
efficient precipitation production on the lower slopes.

With regard to snow totals, the snow started a little earlier
leading to heavier amounts overnight. However, the heaviest
precipitation is just about over as the upslope component and
weak QG forcing is forecast to continue weakening through the
day. At the same time, we have duration on our side as the weak
upslope is forecast to stick around a bit longer than earlier
advertised. Therefore, we see a slight bump in forecast totals,
but mainly in/near the lower foothills. As previously discussed,
we would not be surprised for the lower slopes (narrow 10 mile
wide corridor) from Coal Creek Canyon to Genesee, southward to
between Ken Caryl and Conifer and then down to Palmer Lake picking
up 10+" or even a foot of snow. Meanwhile, totals for most of the
Denver/Boulder metro area still on track to receive 3-6" (less
east, more west), with locally higher amounts possible very close
to the base of the foothills (e.g. Rocky Flats, Golden,
Larkspur). Totals taper off rather quickly to the east/northeast
as some earlier arrival of dry air/weaker forcing will limit
totals there.

Snow will taper off north to south this afternoon, and should be
all but done in Fort Collins before noon, but not until late
afternoon for Denver, with the Palmer Divide possibly seeing a
little snow even linger til 8 or 9 pm. It should also be noted
with just slight daytime destabilization, lapse rates are
sufficient to allow shallow convection and thus some redevelopment
of a few brief heavier showers this afternoon on the back edge of
the more widespread snowfall.

With regard to impacts, the main impacts so far have been
associated with the heavier snowfall periods as road temperatures
remained relatively warm, and air temperatures have not fallen
that much yet (holding around 30F as of 3 am). However, Cheyenne
has recently dropped to the mid 20s and cold advection aloft with
further snowfall will support cooling of the column. Thus, we
would expect mostly snow covered or slushy roads (and even some
potential for ice given currently wet roads in some spots). Thus,
the morning commute looks particularly slick and hazardous for
much of the I-25 Corridor and foothills. With some decrease in
snow intensity and some solar insolation, we think any main travel
corridors would be melting off by late morning/afternoon except
in the foothills where snowfall rates will likely remain
elevated. Then look for a potential refreeze for the evening
commute, with the highest threat in/near the foothills and Palmer
Divide where temperatures will drop into the upper teens and
lower 20s before the end of the pm commute.

In summary, the forecast and Winter Weather Advisories are all on
track. We did bump up deterministic totals another inch or two
in/near the base of the foothills per latest guidance and
conceptual model, while nudging down farther east. Light snow
could also linger just a bit longer with a weak upslope component
lingering into the early evening hours.

Finally, skies will be clearing toward midnight. That will allow
for a cold night across the area especially with areas of fresh
snow cover. Areas of fog have been added in valley locations
including the South Platte River Valley - potentially as far south
as Commerce City.

&&

.SHORT TERM /Through Wednesday/...
Issued at 326 PM MST Tue Dec 2 2025

Zonal flow is in place across the forecast area today, ahead of a
weak, positively tilted shortwave swinging across the
intermountain West this afternoon. That wave is expected to shear
out a bit as it tracks towards the NV/UT border by Wednesday
morning... eventually reaching northern Arizona by 00z Thursday.

From a pattern standpoint, one would think that this storm track
would not produce a favorable setup for a Front Range snow
event, given how far away the primary shortwave (and its
associated QG ascent) are from our CWA. However, there`s still
enough going for this storm that we`ve got a meaningful snowfall
to discuss. Let`s dive in.

A cold front is expected to push into the region during the
early overnight hours. There`s also much better near surface
moisture behind the front, with mid 20s to low 30s dew points
noted across northern Wyoming and Montana/South Dakota this
afternoon. As the mid-level low circulation drifts into western
Colorado, upslope flow should start to deepen, with generally
10-20kt magnitudes through 700mb. Guidance has broadly trended
towards developing precipitation a little earlier in the overnight
period tonight, which has led to a modest increase in snow amounts
prior to daybreak Wednesday. Some high resolution guidance
including the past several cycles of the HRRR have been quite
aggressive in developing snow prior to 12z... which is one of the
few remaining questions marks with this storm.

Snow should be fairly widespread across the region by daybreak
Wednesday, which should continue into the morning commute. With
BUFKIT soundings suggesting most of the lift intersecting with the
dendritic growth zone, efficient snow ratios appear likely during
the majority of the morning commute across most of the foothills
and Denver metro/I-25 corridor. There is some uncertainty with
regard to how far north/east the snow spreads towards places like
Fort Collins/Greeley/DIA/Limon, with much higher confidence closer
to the base of the foothills. Snow should gradually taper off
from north to south through the day, with most locations seeing
the end of accumulating snowfall near/before 00z.

Model guidance is in remarkably good agreement today, with trends
today generally increasing QPF by 0.1-0.2" across the Denver
metro. Our deterministic forecast is very close to the ECMWF
ensemble mean through 12z Thursday, with a gradient of 0.25-0.6"
across the Denver metro into the foothills. With the University of
Utah`s SLR algorithm suggesting ratios of around 12-14.5:1 this
would translate to around 3-7" of snow across most of the metro,
with higher totals likely in the lower foothills. A few HRRR runs,
along with some NSSL/GSL MPAS-core guidance has also hinted at a
ribbon of enhanced (0.7-1") QPF in the lower foothills where
localized totals of up to a foot appear feasible. Unsurprisingly,
our snowfall totals were nudged up an inch or two with this
forecast package.

Translating this to impacts... not a whole lot has changed to our
thought process over the last 12-24 hours. With snowfall rates
maximized during the early half of the morning commute, travel
issues appear likely across the region... especially since this is
the first winter driving experience for many this year. Travel
impacts are expected to linger through the day into the Wednesday
evening commute, especially if snow does hold on a little longer
than expected. The main change of note today was to add in the
northern foothills/I-25 corridor and South Park into the advisory
given the recent trends to the snowfall forecast.

&&

.LONG TERM /Wednesday night through Tuesday/...
Issued at 326 PM MST Tue Dec 2 2025

These new winter-feeling vibes will continue through the long term
forecast period (at least across the mountains!) as persistent
northwesterly flow returns behind the exiting shortwave Thursday
night. While the brunt of the snowfall will be behind us by Thursday
evening, light snow will remain, gradually diminishing from north to
south across the forecast area through Thursday night as upslope
flow comes to an end. The evening commute could see some snarls,
with the areas of most concern for slick conditons to generally be
along and south of I-76. Cold temperatures are expected overnight as
outgoing moisture allows for clearing skies. Mountain valleys will
be cold, dropping below zero, while the plains and foothills will
range from the single digits in our typical colder spots, to the mid
to upper teens along the foothills and adjacent plains.

Dry conditions are expected on Thursday, with initiation of a
gradual warming trend that will continue through the extended
forecast period. Colorado will be on the eastern side of a broad
ridge that will be situated over the western CONUS for the
foreseeable future. Northwesterly flow is notorious for allowing
moisture and shortwaves to drop southeastward out of the Pacific
Northwest and into the forecast area, and this setup looks no
different. We are expecting multiple waves of additional snowfall
for the high country through the weekend, with the northern
mountains generally more favored in this northwest regime.

By Thursday night, a weak shortwave is expected to move across the
Rockies that will bring some of that aforementioned moisture from
the Pacific into the mountains. Northwesterly flow will gradually
increase through Friday keeping orographic snowfall going through
the day. Light accumulations are expected, mainly for the highest
elevations of the Front Range Mountains, with the greatest
amounts expected for the Park Range through Friday. The lower
elevations are expected to remain dry.

For the weekend, ensembles are in wide agreement that a more notable
snowfall is slated to slide into the mountains with a more potent
shortwave and upper-level jet moving into the forecast area. This
will be our greatest shot at seeing moisture across the lower
elevations, albeit low chances, with ensemble members holding at
around 30% showing any QPF east of I-25. Winds will increase across
the higher elevations as well as the plains, with tightening
pressure gradients and lee troughing centered over the Texas
panhandle. This is still a ways out and will need to lock in more
details as we get closer, but this more active pattern is certainly
starting to feel more like winter is finally here.

&&

.AVIATION /12Z TAFS through 12Z Thursday/...
Issued at 445 AM MST Wed Dec 3 2025

Light snow and IFR conditions will persist through at least 18Z,
only to slowly diminish from north to south and a gradual
improvement to MVFR conditions 20Z-01Z. A few flurries could
linger past 01Z, mainly at KAPA where the snow will be last to
diminish as the storm system moves away and drier air arrives from
the north. Total accumulations are most likely to be ~3" at KDEN,
and 4-6" at KBJC and KAPA. Skies will clear after about 06Z
Thursday. However, there will be a chance of fog development
(about 30% at KDEN) as the southerly drainage winds will be
initially light. The highest threat would be just north and
northwest of KDEN and in the South Platte River Valley, so for now
will mention an extended period of VCFG starting about 06Z. By
12Z-15Z, sufficient drainage wind should take main fog threat even
farther north and away from the Denver area TAF sites. KBJC and
KAPA have a much lower threat to start with (10%).

Winds should stay mostly N/NE at 10 kts or less through 22Z,
before becoming light and variable or more likely southeasterly by
01Z-03Z. An eventual turn to the south toward 06Z will help rid
the TAF sites of lingering low clouds, but will have to monitor
that fog threat at KDEN.

&&

.BOU WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory until 6 PM MST this evening for COZ035-
036-039>041.

Winter Weather Advisory until 8 PM MST this evening for COZ037.

Winter Weather Advisory until 2 PM MST this afternoon for COZ038.

&&

$$

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

NWS BOU Office Area Forecast Discussion