396
FXUS65 KBOU 021819
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
1119 AM MST Tue Dec 2 2025

.KEY MESSAGES...

- First notable snowfall of the season for the I-25 Urban Corridor
  and foothills is still on track, impacting the Wednesday morning
  commute. Some impacts possibly lasting into the pm commute
  especially for the foothills west and southwest of Denver.

- Light snow showers for the mountains today are a precursor to
  more widespread snow developing tonight.

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

&&

.UPDATE...
Issued at 355 AM MST Tue Dec 2 2025

Snow lovers finally have something to look forward to, versus our
last couple teases of minuscule snow. We have increasing odds of
seeing a modest snow event for the Denver metro area and
foothills, with the latest QPF and snow forecasts actually
creeping up just a bit from yesterday.

Satellite shows our next weather system now diving across the
northern Rockies with a powerful 150+ kt jet max knifing southward
across the Pacific Northwest. Therefore, the trough will continue
to dig southward today, with a piece of energy dropping into Utah
tonight. As a result, we`ll get into at least weak QG lift for
the majority of our forecast area, and modest QG frontogenesis is
also noted. At the surface, a cold front will arrive toward
midnight, shifting winds around to the east/northeast with
deepening (albeit still rather weak 15 kt) upslope through
mountain top by late tonight. While low levels are quite dry, in
the prefrontal environment, we`ll see low level moisture advection
with the front. While not great, it`s much better than the recent
dry advection events we`ve had. Low level lapse rates are also
much better, and close to moist adiabatic at least to start with
late tonight. Thus, any sort of lift through upslope or QG forcing
is expected to be efficient in snow production. We`ve seen
another slight increase in ensemble QPF, with amounts now ranging
mostly from 0.25-0.40" for Denver metro, and higher amounts in the
foothills especially west and southwest of Denver. These weakly
forced upslope events frequently produce the most snow in the
lower foothills, and this one seems no different. The latest 06Z
runs of CAMs seems to be picking up on this on well, with most
showing 0.50" or greater in the foothills immediately west of
Boulder to Genesee/Lookout Mountain to Ken Caryl/Conifer, and
Perry Park.

Next challenge is snow ratios. While the dendritic growth zone
(DGZ) isn`t terribly deep (2500-3000 feet) due to nearly neutral
lapse rates, we will have good vertical motion through that layer.
Thus, we still expect fairly high snow ratios closer to 15:1 on
average. This is always a tricky one though, as even in the same
storm we can fluctuate wildly. Combining the latest QPF and snow
ratios, we have seen a bump in snow totals, especially in the
lower foothills. We`ve split out the southern Front Range
Foothills in the Winter Weather Advisory for the increased totals
to 4-9", and wouldn`t be surprised to see a couple local 10+"
amounts if everything comes together. For Denver metro and Palmer
Divide, we`re looking for 3-5" with locally 6+" amounts right at
the base of the foothills. While there was a slight bump in totals
farther north including Fort Collins and the northern Front Range
Foothills, we`re still just shy of advisory criteria and the bulk
of snow for Fort Collins should be winding down during the
morning commute, but we will still monitor that potential closely
as it wouldn`t take much of an upward trend to get more impacts in
that direction. Meanwhile, the Wednesday morning commute for
Denver, Boulder, Palmer Divide, and I-70 Foothills will certainly
be impacted with slick, slow, and hazard travel conditions. In
fact, the morning commute is likely when the heaviest snow will
fall with snowfall rates averaging at least 0.5" per hour, and up
to 1"/hr in the lower foothills. We still expect snow to taper off
during the afternoon for Denver, so impacts should be lessening
as we head into the pm commute. Snow diminishes last in the
southern foothills, so more significant impacts could last for the
afternoon/evening commute there.

For the rest of this forecast, we`ll see light snow showers
develop in the mountains this morning as a shallow plume of mid
level moisture moves across. It won`t be much but an inch or two
up by Rabbit Ears Pass would support some travel impacts there.
Only scattered/light snow showers today for the I-70 Mountain
Corridor. While snow becomes more widespread in the high country
this evening, we don`t see it being persistent or heavy enough to
warrant an Advisory at this time. Main impacts on Wednesday will
be shifting to the east/northeast upslope favored areas of the
Front Range.

&&

.SHORT TERM /Through Tuesday/...
Issued at 305 PM MST Mon Dec 1 2025

Current satellite water vapor imagery shows dry conditions in place
across Colorado today. Despite sunny skies, observations show
temperatures remain in the 30s across the vast majority of the lower
elevations, with only a few readings climbing into the 40s observed
near the base of the foothills and adjacent plains, and along the
South Platte River Valley. The mountains have warmed into the 20s
where they are expected to remain through the afternoon.

Looking upstream, water vapor imagery also shows our next
approaching system currently moving across the Pacific Northwest and
expected to slide a shortwave southeast across the Intermountain
West through the day Tuesday. As this system approaches, cross-
sections show northerly winds aloft are on track to transition to
the west through tonight, bringing increasing cross barrier flow
along the Front Range Mountains. A mountain wave is likely to
develop sometime overnight tonight, with gusty winds between 25-35
kts expected to spill down the eastern slopes of the Front Range,
with some stronger gusts possible in our typical windy spots in the
foothills of Boulder County, and up to 55 kts along ridgetops. One
upside to the downsloping winds will be the compressional warming
that comes along with them. This will help keep overnight low
temperatures slightly warmer for those who live in the foothills and
along the adjacent plains tonight. While overnight low temperatures
are forecast to be widespread teens across the plains, and single
digits in the mountains and valleys, we can expect at least a ten to
fifteen degree warming effect for the aforementioned areas near the
foothills.

Snow is expected to begin in the northern mountains early Tuesday
morning as upwards ascent begins to increase ahead of the
approaching shortwave. With the latest hi-res guidance trend showing
increased QPF values across the Medicine Bow, Park Range, and Indian
Peaks this afternoon, have bumped up totals through the day Tuesday
for this forecast package. This will lead to slick travel
conditions developing throughout the day, especially for the high
mountain passes. Afternoon temperatures are expected to warm to
near normal values across the plains ahead of the long awaited
widespread snowfall expected to slide eastward onto the plains
Tuesday night. See below for more details.

&&

.LONG TERM /Tuesday night through Monday/...
Issued at 305 PM MST Mon Dec 1 2025

Had to rub my eyes a few times to make sure it wasn`t fiction, but
here we are less than 36 hours from the onset of our next system,
and we`re remaining steadfast in our forecast of a few inches (not
tenths of an inch!) of snow for much of the urban corridor for
the first time this season. So what`s brewing?

A shearing shortwave still looks to detach from it`s parent
trough Tuesday night and descend southward over the Four Corners.
QG fields indicate broad ascent associated with this feature and
overspreading Colorado through Wednesday. Latest guidance has
continued to accelerate the low`s progression, albeit slightly,
thus moving up the onset of wintry precipitation to shortly after
midnight Tuesday night for areas east of the Continental Divide.
Low-level (850-700mb) frontogenesis looks healthy for the morning
hours, and there`s relatively good consensus when it comes to a
period of 6-12 hours of predominantly northeast upslope flow below
700mb centered on the first half of the day. Variability still
exists as far as the duration of any upslope, whether or not we
may see intrusions of more easterly winds (some of the recent CAMs
would support this), and to a lesser extent the speed of the
shortwave as it looks to merge back with the broader flow
pattern, which will undoubtedly lead to additional fine-tuning of
snowfall amounts and distribution. Nevertheless, confidence has
grown with regard to most of the Denver metro, southern foothills
and Palmer Divide receiving at least 2" of snow (75-85% chance),
with the heaviest of the snowfall coinciding with the morning
commute.

With this in mind, have issued Winter Weather Advisories for
these locations in anticipation of the difficult road conditions
that will result, regardless of exact amounts. Highlights may need
expanding into the mountains and/or farther north depending on
the trend in upsloping characteristics over the coming forecast
shifts. With there being greater alignment now among ensemble
systems (the previously lighter GEFS has shifted closed to
ECMWF/Canadian solutions), total QPF of 0.2-0.4" looks reasonable
for a large portion of our urban corridor, foothills and
mountains, with a focus in the aforementioned areas. With snow
ratios anticipated to near 15:1 much of the time, felt
comfortable trending the forecast snowfall totals a little higher,
with areas of 3-6" appearing increasingly likely, and potential
for localized higher totals closer to the base of the foothills.
Snow will look to taper off quickly through the latter half of the
day as surface winds veer to the south, giving way to a mostly
sunny but cool day on Thursday.

Friday and into the weekend, the pattern will be dominated by
strengthening northwest flow aloft on the NE periphery of a high
pressure region over the eastern Pacific. Moisture embedded in
this flow looks to impinge on the high country as early as Friday,
when we have a few orographic snow showers in the forecast for
the higher elevations. Meanwhile, our lowlands should see dry and
milder conditions. The bulk of the moisture should hold off until
the weekend however, associated with one or two weak shortwaves
and a strong upper-level jet. Such a pattern would be conducive to
a more extended period of mountain snowfall which could be heavy
at times, favoring the northern mountains (i.e. Park and Medicine
Bow Ranges) in particular, but impacting most/all of our mountain
zones at some point or another - a pattern chance which is sorely
needed in this region. Wind will be the other aspect to watch
during this timeframe, with breezy conditions likely to become
more widespread, as well as an opportunity for some
downslope/mountain wave enhancement around Saturday. The lower
elevations do still have another window for wintry precipitation
over the weekend, but it`s much less set in stone compared to our
mountains and will depend on the evolution of any embedded
shortwaves that do develop.

&&

.AVIATION /18Z TAFS through 18Z Wednesday/...
Issued at 1119 AM MST Tue Dec 2 2025

VFR conditions expected through this evening before lower ceilings
and reduced visibilities arrive by 06Z/07Z as a disturbance will
bring snow to all TAF sites. However, conditions could deteriorate
as early as 03Z/04Z with scattered snow showers ahead of the main
system. Have added a PROB30 for DEN and APA for that reason. IMC
conditions are expected with the arrival of snow, with ceilings
and visibility dropping to IFR, and LIFR possible for APA and BJC
Wednesday morning. Heaviest snow rates and worst
visiblities/ceilings (up to 0.5"/hr and briefly 1"/hr for APA and
BJC) are likely between 10Z and 14Z, before a gradual improvement
to MVFR conditions through 18Z. After that, snow will slowly
decrease to scattered snow showers through 00Z.

Total snow accumulations are still most likely 2-4" at DEN, and
3-5" at BJC and APA. Temperatures will drop to freezing or below
freezing by 00Z this evening, so expect snow to accumulate on
paved surfaces.

In terms of wind, generally light (<10 kts) N to NE winds for DEN
and APA with a gradual turn to SE this evening. Then, with the
arrival of a cold front, winds will transition back to ENE. There
is a chance (~20%) for downsloping winds to briefly reach DEN
between 18Z and 23Z, where 10-15 kts is possible. Most likely,
those westerly winds will stay south of the airport. For BJC,
westerly winds generally under 15 kts will continue for the next
couple of hours before turning light and VRB this afternoon.



&&

.BOU WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory from midnight tonight to 6 PM MST
Wednesday for COZ036-039>041.

&&

$$

UPDATE...20
SHORT TERM...9
LONG TERM...BRQ
AVIATION...MAI