909
FXUS65 KBOU 190009
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
509 PM MST Thu Dec 18 2025

.KEY MESSAGES...

- Strong downslope winds return Friday to 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.

&&

.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 /00Z TAFS through 00Z Friday/...
Issued at 457 PM MST Thu Dec 18 2025

Models have normal diurnal wind patterns at DIA overnight, with
fairly weak wind speeds. Winds are expected to have a more
downsloping component on Friday, with a chance of stronger
westerly winds gusting up to 35-40 knots by later afternoon. There
will be no ceiling issues.

&&

.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...
High Wind Warning from 9 PM this evening to 8 PM MST Friday for
COZ033>036.

Red Flag Warning from 5 AM Friday to midnight MST Friday night
for COZ215-216.

Red Flag Warning from 10 AM Friday to midnight MST Friday night
for COZ238>243.

High Wind Warning from noon to 8 PM MST Friday for COZ038-039.

Red Flag Warning until 5 PM MST this afternoon for COZ242-244-
248>251.

Fire Weather Watch from Friday morning through Friday evening
for COZ245>247.

&&

$$

DISCUSSION...Hiris/Schlatter
AVIATION...66
FIRE WEATHER...Hiris

NWS BOU Office Area Forecast Discussion