494
FXUS65 KBOU 191154
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
454 AM MST Fri Dec 19 2025

.KEY MESSAGES...

- Today...strong downslope winds will continue across our
  mountains, foothills and parts of the urban corridor, with
  widespread critical fire weather conditions anticipated under a
  dry and (record) warm air mass. Wind gusts exceeding 100 mph are
  possible from the foothills to the adjacent plains.

- Particularly Dangerous Situation Red Flag Warning for the lower
  foothills of Larimer, Boulder, and Jefferson Counties, and the
  CO-93 corridor for high winds and low relative humidity.

- Light to moderate snow returns to the mountains Saturday, with
  deteriorating road conditions expected along and north of I-70
  through the mountains. A winter weather advisory is in effect
  from midnight tonight through midnight Sunday.

&&

.UPDATE...
Issued at 308 AM MST Fri Dec 19 2025

A few updates were made to the forecast for the next 24-48 hours.
First are the winds and expected red flag conditions, including
areas highlighting a Particularly Dangerous Situation. Winds are
already howling in the foothills early this morning, with gusts
in the 55-70 mph range where we have the high wind warning. We
continue to analyze available high resolution model data to get a
sense of how strong the winds could be, and just as important,
how far east of the foothills the mountain wave sets up. In terms
of magnitude, there is good agreement that the foothills of
Larimer, Boulder, and Jefferson Counties (as well as Clear
Creek/Gilpin and extreme NE Park County) will see the highest
winds today, with gusts in the 80-105 mph range. In fact the HREF
has 80-99% probabilities of exceeding 80 mph every hour from now
through about 2 PM in those areas above 7,000 ft. The highest
probabilities are in western Boulder County south to the northeast
corner of Park County, where the mountain wave should be
positioned this morning. Wind speeds in general associated with
the mountain wave and plain old brute force winds at 700 mb are
expected to decrease after 3 PM, but gusts 50-80 mph are likely
to continue across the foothills through 8 PM. All highlights for
the foothills remain in place unchanged.

There continues to be considerable uncertainty in how far east the
mountain wave sets up. The latest high res models have been
consistent with keeping the 80+ mph gusts along and west of
CO-93. The wind gradient is going to be tremendous as just a few
miles east of the maximum wind speeds near the base of the
foothills will see considerably less wind this afternoon, and 3-5
miles east of the mountain wave will probably even see east winds
10-15 mph, consistent with the development of a rotor. There is
roughly a 25 percent chance the mountain wave does not make it to
CO-93 and remains above 6,500 ft elevation, based on your flavor
of CAM. But overall, the ingredients are in place such that the
mountain wave should reach the CO-93 corridor around midday but
not get beyond there until later this evening. North of Fort
Collins along the Wyoming border, which does not need a mountain
wave to produce high winds, should see gusts 60-75 mph this
afternoon, especially north of Wellington. Fort Collins itself is
not likely to see much wind this afternoon, but could see 30-40
mph gusts after 6 PM like much of the rest of the urban corridor.
A strong jet is moving in from the north this evening and when it
does, an influx of subsidence via the right exit region of the jet
should cause the mountain wave to shift east and broaden out.
Wave amplification is probably pretty weak by this evening given
the increase in wind shear via the jet, and the loss of the
inversion above ridgetop. However, perhaps enough subsidence is
the reason CAMs bring strong winds east across the I-25 corridor
and much of metro Denver after 6 PM. From 6 PM through midnight,
most areas across the I-25 corridor (especially east of I-25) are
likely to see their strong winds of the day, after having light
winds this morning throughout the afternoon. Gusts 35-60 mph are
expected this evening all the way out to DIA. The forecast was
refined with the higher confidence in the magnitude and timing of
the evening`s winds.

In terms of the fire weather threats, nothing much has changed
since yesterday`s forecast. RH is still expected to drop below 15%
where the winds are highest from CO-93 westward into the foothills
up to 8,000 ft. These areas from Larimer to Jefferson to Boulder
counties continue be a concern for rapid fire growth from this
morning through the evening hours. It continues to be a
Particularly Dangerous Situation. We implore all to not do
anything that could spark a fire in these areas. Elsewhere, the
RFWs remain in place especially when the winds increase late
afternoon/evening.

Finally, after winds and low RH improve, there is a good amount
of Pacific moisture coming into Colorado tonight through Saturday.
Snowfall amounts were updated this morning for up to a foot of
snow in isolated areas (but more like 4-10 inches outside of the
summit of the Park Range), and given moderate snowfall rates and
winds 40-60 mph Saturday morning, feel impacts will be great
enough to warrant a winter weather advisory from midnight tonight
through midnight Sunday (24 hours). Travel conditions are expected
to deteriorate across the mountains, especially across the
mountain passes, by Saturday morning. Travel conditions should
improve significantly by Sunday morning.

&&

.DISCUSSION /Through Thursday/...
Issued at 307 PM MST Thu Dec 18 2025

Unfortunately we are not done with the high winds yet. As the
strong 500 mb trough moves further east into the Great Lakes
Friday morning, flow across Colorado flattens to almost due
west. The pressure gradient between the ridge to the south and
the long wave trough to the north means zonal flow along will
increase tonight into Friday morning. Cross-barrier flow near
70kts (mean wind direction at ridge top is around 280) should be
in place early Friday morning, which is also when winds in the
higher elevations of the Front Range will increase. We have a high
wind warning in place for the mountains and foothills above 6,000
ft starting 9 PM tonight. Sustained winds 40-60 mph with gusts
approaching 100 mph are expected from the latter half of the night
into early Friday. The windiest spots on the tops of peaks/ridges
will probably exceed 100 mph by Friday morning. There is some
lingering uncertainty regarding how quickly winds will develop and
begin to advance down into the lower foothills. It`s possible
stronger winds don`t develop until closer to daybreak for
locations below 7,000ft MSL... as hinted by a few of the daytime
CAMs.

Attention then turns to the fire weather conditions driven by
strong downslope winds throughout the day Friday. Initially, high
resolution model soundings show a fair amount of wind shear at 600
mb and above. Though lapse rates remain weak at ridge top, the
forward wind shear should prevent a mountain wave from forming
across the lower elevations, at least initially. The inversion
aloft is close to 600 mb Friday morning. These factors seems to
the reason why all high res models keep the high winds above 6,000
to 6,500 feet until early afternoon. At that time winds between
700-500 mb reach 70-80 kts, there are hints of an inversion, and
winds decrease with height. Most but not all high res models bring
those high winds down the east slopes to the CO-93 corridor, from
Lyons to western Boulder and then Jefferson County to Golden.

Friday afternoon looks like a textbook steady state, narrow
mountain wave event with gusts up to and exceeding 100 mph, and
more than likely those gusts will make it to the CO-93 corridor
by the late morning or early afternoon hours. For now, we expect
that the winds should weaken rapidly east of the mountain wave
such that eastern Boulder, Larimer, and Jefferson Counties may not
see much wind at all. The exception will be the wind prone areas
north of Fort Collins along the Wyoming border, where gusts to 75
mph are possible (e.g. north of Wellington). High resolution
guidance offers a variety of solutions in this timeframe, and the
blend of the HRRR/RRFS/REFS/RGEM/HiRes CAIC WRF is reasonably
close to our deterministic forecast.

Another thing to note is that after sunset the high res models
push the mountain wave eastward. This is likely in response to the
right exit region of the jet approaching our area around that
time. While wind speeds should be significantly lower when this
occurs, it could result in a few hours of gusts 35-60 mph across
areas that didn`t see much wind throughout the day, including all
of the I-25 urban corridor and metro Denver. There is surprisingly
more confidence in this period compared to the "steady state"
portion of the event during the day.

The concern is the combination of the dry air, critical fuels, and
high winds. Record warmth is expected, plus bone dry downslope
winds, and the result are RHs dropping below 15 percent. RH will
start off low in the foothills which is why we`ve hoisted an RFW
starting at 12Z, which is highly unusual. Once the winds increase
midday, red flag conditions are expected to spread east. The RFW
for the I-25 corridor eastward includes all of metro Denver, Weld
County, and the Palmer Divide through midnight. That`s the other
unusual aspect of this RFW, the fact that RH recovery is going to
be poor well into Friday evening. It is possible (and perhaps
likely) that a majority of the Red Flag conditions in the Denver
area hold off until near/after sunset and continue into the early
overnight. Finally, uncertainty remains in the extent of the
strong winds across E. Adams/Arapahoe/Elbert and Lincoln Counties,
thus we are keeping a fire weather watch in effect for those
areas through Friday evening.

In the mountains, Pacific moisture works its way into Colorado
with the zonal flow. Snow chances increase first across the
northern mountains Friday evening. Wind direction is expected to
be right around 280 and lapse rates above ridge top will allow for
snow to continue into Saturday evening. Sometime early Sunday
lapse rates and west winds both weaken, though there is still
moisture in the westerly flow through much of Sunday. So while it
may still be snowing above 9,000 ft in the mountains on Sunday,
snowrates are expected to be much lighter than on Saturday. The
moisture plume from Saturday into Sunday stays mainly across the
northern mountains, which is also where we expect to see travel
impacts (US 40 and CO-14), but Saturday morning through midday a
few inches of snow looks likely (>80%) for the I-70 corridor as
well so probably some slick/slushy spots from Georgetown to Vail
Pass as well. Total snow amounts should range from 6-12 inches
across the northern mountains, and 2-6 inches for the I-70
corridor. A winter weather advisory may be needed for the
mountains on Saturday, but will punt to the midnight shift given
the significant focus on the fire weather conditions tomorrow.

Elsewhere, despite the strong zonal flow, a shallow cold front is
expected to move across the plains of Colorado throughout the day.
Because the front is shallow and there is west flow over the
Rockies, the front will wash out quickly especially across the
I-25 corridor. However, behind the front and as it is in the
process of washing out, there is a period of shallow north-
easterly upslope flow later Saturday morning into the afternoon.
Precipitation chances will remain low and any accumulation will be
light. Wet Bulb temperatures should be 34 degF or warmer so
P-type will probably be rain across the I-25 corridor and eastern
plains below 6,500 ft. For now, PoPs are 15-30 percent across the
plains Saturday late morning through the afternoon. Temperatures
look similar both Saturday and Sunday with highs in the 50s across
the plains. The mountain valleys will be in the upper 30s
Saturday, warming to the mid 40s on Sunday afternoon with more
sunshine.

Next week will be depressing for snow lovers, but about as good as
you can imagine for those that don`t like winter.  A developing
trough off the California coast builds an impressive ridge aloft
across Colorado and the Central Plains of the U.S. Monday afternoon
the ridge builds significantly overhead, then slides east Tuesday
and Wednesday.  Those three days will be exceptionally warm, with
highs across the plains and I-25 corridor in the mid-upper 60s.  It
may hit the 70s across the I-25 corridor on Monday.  It should be
dry across the area Monday through late Wednesday.  700 mb temps
warm to between +2 and +4 degC (which is about 2 standard deviations
above climatology), meaning temperatures across the mountain valleys
and east slope foothills will be in the 50s.

After the spring like warmth Wednesday, A strong Pacific storm
moves onshore, and has an impressive moisture plume that moves
northward across Colorado ahead of it starting late Wednesday or
early Thursday. 7 days out predictability is naturally quite low,
but an ensemble analysis clearly has a signal of increasing snow
chances for the mountains, especially the central mountains but
with the flow out of the SSW, at this point not getting too
excited for a good mountain snow. Additionally, ensemble mean 500
mb plots stall the main trough along the coast, meaning synoptic
forcing will be weak across Colorado, and precipitation production
will rely mostly on upslope and lapse rates. Unfortunately for
the northern mountains and plains, SSW flow is a challenge for
precipitation production so despite an influx of Pacific
moisture, it sure looks warm and dry Christmas Eve and Christmas
for now.

&&

.AVIATION /12Z TAFS through 12Z Saturday/...
Issued at 454 AM MST Fri Dec 19 2025

VFR through Saturday. Low confidence in general with the winds,
which will create impacts during the afternoon hours. Starting
with DEN, winds should be SE at 10 kts or less until 17Z, but
could shift to WSW for several hours after that. High res models
have been all over the place with wind direction at DEN through
about 20Z. After 20Z, confidence in the west direction increases
but speeds could be erratic with the mountain wave along the base
of the foothills to the west. Bursts of west winds often occur
downstream of the main mountain wave, and they can last well under
an hour, but return a short while later. In between the bursts
often there are much lighter winds, even southerly, are possible.
All that said, a TEMPO group for those stronger bursts seems
prudent. Sometime after 04 or 06Z, winds should become much more
steady and strong out of the west, gusting to 35 kts. These winds
may last 6-8 hours before weakening sometime around 12Z to under
10 kts. APA will be similar to DEN but overall will remain SE at
10 kts until a burst of wind. The steady and stronger west winds
look to pick up around 05Z, gusting just over 30 kts through
10-12Z.

BJC is much closer to the mountain wave so will have even more
erratic winds as bursts of west wind move over the airfield off
and on through 01Z. Gusts are expected around 45kts with those
bursts. After 03/04Z winds will become more steady and strong out
of the WNW, gusting to 50 kts or so. These winds should last
through about 10Z but gradually weaken between 05Z and 10Z. The
erratic nature of the winds through this afternoon resulted in two
TEMPO groups.

&&

.FIRE WEATHER...
Issued at 247 PM MST Thu Dec 18 2025

A dry airmass, combined with a potentially significant downslope
wind event, will lead to a large area of critical fire weather
conditions beginning as early as Friday morning. The current
airmass is quite dry across the I-25 corridor and most of the high
country, with dew points generally in the -10 to +5F range this
afternoon, and only modest moistening of the airmass is expected
by tomorrow. Overnight humidity recovery of 30-50 percent is
expected for most of the Front Range with daytime humidity falling
in the 10-20 percent range (lower in the Denver metro).

The greatest uncertainty at this time is how downslope winds
evolve in the Foothills into the I-25 corridor during the daytime
hours. There is still some notable differences across guidance
regarding the amount of overlap between the strongest winds and
driest air in the foothills. Similarly, there is fairly low
confidence in how far east the mountain wave advances east during
the day. A blend of the last several cycles of the HRRR and RRFS
(and its ensemble) would suggest that a majority of the wind stays
in the foothills below 7500ft into the immediately adjacent
plains (Highway 93 corridor, for example). Guidance does
eventually push some gusts - generally in the 35-55 mph range -
into the I-25 corridor by late afternoon and evening.

Combined with what`s been a very dry past 60 days (70 mph winds and low
enough humidity overlaps.

&&

.BOU WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory from midnight tonight to midnight MST
Saturday night for COZ031-033-034.

High Wind Warning until 8 PM MST this evening for COZ033>036.

Red Flag Warning until midnight MST tonight for COZ215-216.

Red Flag Warning from 10 AM this morning to midnight MST tonight
for COZ238>243.

High Wind Warning from noon today to 8 PM MST this evening for
COZ038-039.

&&

$$

UPDATE...Schlatter
DISCUSSION...Hiris/Schlatter
AVIATION...Schlatter
FIRE WEATHER...Hiris

NWS BOU Office Area Forecast Discussion