456
FXUS65 KBOU 171153
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
453 AM MST Wed Dec 17 2025

.KEY MESSAGES...

- Strong downslope winds to impact the mountains, foothills, and
  I-25 corridor this afternoon and evening, with potential for
  widespread gusts 60-80 mph and localized gusts up to 90 mph
  expected, peaking early/mid afternoon. Northwest winds gusting
  50-60 mph will extend into the plains this evening.

- Critical fire weather conditions will be present this afternoon
  across all of the I-25 corridor and into portions of the plains.

- Banded snowfall and strong winds will lead to treacherous travel
  conditions in the mountains this afternoon and evening.

- Gusty winds and periods of high fire danger are expected to
  continue through Friday and potentially into Saturday, with
  questions remaining regarding the spatial extent of the
  strongest winds.

- Mountain snowfall looks to return for Saturday.

&&

.UPDATE...
Issued at 305 AM MST Wed Dec 17 2025

Guidance remains steadfast and quite consistent with regard to our
incoming wind event, so underlying changes to the forecast have
been on the minor side early this morning. The lack of a defined
critical layer will not prevent favorable shear profiles, a low-
level stable layer, and strong synoptic forcing from propagating
strong downslope winds into the urban corridor this afternoon. The
strongest gusts will develop right around noon along the base of
our central foothills (especially Boulder/Jefferson Counties),
surging eastward towards I-25 through approximately 4pm with only
slight weakening as they do so. Peak gusts between 75-90 mph
remain likely (80% chance) for the most wind-prone locations
mentioned above. The latest guidance is slightly more bullish with
the eastward progression of the winds, suggesting potential for
wind gusts exceeding our 58-mph threshold in Denver, Adams and
southern Weld Counties, which may necessitate an eastward
expansion of the High Wind Warning if the trend holds. It`s worth
noting that this surge should be rather shortlived, lasting 2-5
hours (towards the upper end of that range closer to the base of
the foothills), with a more notable retreat back into the
foothills and mountains by this evening. As such, have trimmed
back the end time for our urban corridor warnings to 7pm.

Thursday`s focus will shift to the more rural plains, where we see
growing potential for a short period of strong northwest winds
gusting 55-65 mph for our northeasternmost counties. The core of
the jet will be directly overhead Thursday morning, with at least
a portion of model guidance indicating hefty 700mb flow exceeding
60-70 kts near Sedgwick and Phillips Counties, which would have
little trouble mixing down to the surface. It`s still possible the
axis of stronger winds remains just east of here, but opted to
include these two counties in a High Wind Watch for the time
being. Blowing dust may be an additional concern for much of the
plains this evening through Thursday, given very limited moisture
in recent weeks.

&&

.DISCUSSION /Through Tuesday/...
Issued at 350 PM MST Tue Dec 16 2025

With breezy winds gusting up to 40-50 mph and relative humidity
values ranging between 17-25%, elevated fire weather conditions are
expected to continue this afternoon. However, winds should gradually
weaken around sunset, which will decrease the fire weather threat.
This evening will be relatively quiet compared to a very active
weather day tomorrow. We have multiple highlights out in the near
future, so let`s dive in:

High Winds (through Wednesday afternoon): Confidence has
continued to increase in strong winds impacting the mountains,
foothills, and I-25 corridor Wednesday morning and afternoon.
Upper level flow will increase and flatten as a shortwave to our
north treks across the northern United States. Cross-barrier flow
will reach 60-75 kts directly from the west, and there will be
very favorable positive omega values, promoting deep subsidence
ahead of the front Wednesday evening. High-reslution Skew- T
soundings along the base of the foothills have now shown a deep
temperature inversion, with reverse shear above the ridgetop (~70
kts at 700-650 mb weakening to 20-30 kts at 400 mb). Despite no
indication of a wave-induced critical layer, these ingredients are
favorable for downsloping winds reaching to the base of the
foothills and possibly adjacent plains. High resolution guidance
has continued to favor this scenario, with gusts up to 90 mph at
high elevations throughout the day, and gusts up to 60-80 mph
reaching just west of the I-25 corridor. West of I-25, the
strongest winds will last about 3-4 hours early Wednesday
afternoon.

Red Flag Conditions (See Fire Weather Discussion for more details):
Across the plains, relative humidity values will likely range
between 17-30%, however there is some uncertainty with exactly how
low we will get. Despite marginal relative humidity values, strong
winds will promote rapid fire spread, should a fire occur. With the
arrival of the front Wednesday evening, increase in dewpoints will
help mitigate fire weather concerns.

Winter Weather Advisory: With decent QG fields showing ascent under
the left exit region of the jet, and areas of strong frontogenesis,
banded snow is possible for our northern mountains. Short range
guidance has continued to indicate that the heaviest snow will occur
Wednesday evening, with accumulations between 4-10" likely. In
addition, with winds gusting up to 80-90 mph at times, blowing snow
will drastically reduce visibilities, particularly where banded snow
falls. For this reason, have opted in to issue a Winter Weather
Advisory from 11 AM to 11 PM Wednesday.

A cold front is forecast to enter northern Colorado around 5PM
Wednesday, moving southeast through the area in the evening. Behind
the front, winds will switch to more of a bora event versus a
chinook event, with winds turning from the northwest. The strong
subsidence behind the front could be enough to bring the higher mid
level winds down to the surface in the northern plains, with
occasional gusts around 50 to 60 mph from 5PM to midnight. However,
due to the isolated nature of the gusts and the uncertainty in
whether they will reach the 58 mph High Wind Warning threshold, we
held off on upgrading the High Wind Watch to a Warning at this time.

Overnight Wednesday into Thursday a mid level low is forecast to
develop over southern Canada and move south into the Dakotas,
towards Iowa by Thursday afternoon. This will put a decent trough to
our east/northeast Thursday morning. Strong subsidence and strong
winds on the backside of the trough will cause additional wind
concerns for the plains Thursday morning and early afternoon. The
positioning of this trough and the associated wind max will impact
whether our eastern counties see the stronger wind gusts or not.
Right now, models vary on where they`re putting the 700mb jet (70 to
90kts). Some keep it off to our east, while other push it into our
northeastern counties in the late morning. The closer the 700mb jet
is to our area, the higher the winds will be. Confidence is too low
at this time in gusts reaching 58+, so we will hold off on issuing a
High Wind Watch for the plains Thursday morning.

We`re monitoring the potential for another mountain wave and strong
wind event on Friday. Models are showing cross barrier flow around
70 to 85 kts along the front range and gusts around 90 mph in the
higher elevations. It`s difficult to get an exact idea of how far
east the extreme gusts will progress this far out in time, but we
are confident that the mountains and lower foothills will see very
breezy conditions again on Friday. We will hold off on issuing any
wind products this far out, but the higher elevations (if not more
of our area) will need a Watch as we get closer. The other concern
on Friday will be fire weather. Strong winds and dry conditions will
lead to significant fire weather concerns (more on this in the fire
weather discussion below).

The weather should calm down for most of the area this weekend.
Moisture will increase in the mountains late Friday into Saturday.
This moisture combined with weak upslope flow, will lead to light
snow showers in both the Park range and northern Front range on and
off this weekend. Accumulations look to stay on the lighter side ,
with accumulations of only a few inches for the highest elevations.

&&

.AVIATION /12Z TAFS through 12Z Thursday/...
Issued at 443 AM MST Wed Dec 17 2025

VFR conditions expected to prevail for all terminals through the
TAF period (75% chance). The one period to watch for a possibility
of CIGS ~040-070 is after ~04-06Z this evening as a cold front
descends south through the Denver metro, but confidence is rather
low when it comes to extent of any moisture with this.

For winds... anticipate continued drainage flow for KDEN/KAPA, and
potentially even KBJC through ~16-17Z, after which we`ll see some
potential for incursions of westerly winds with a few gusts
exceeding 25 kts (40% chance for KAPA/KDEN). West winds should
become steadier for KBJC during this timeframe. These incursions
for the former terminals will become more likely after 18Z in
particular.

A strong mountains wave is then forecast to push off the higher
terrain into the Denver area terminals between 19Z-23Z, reaching
KDEN towards the end of that time period. With notable model
consistency overnight, have increased peak gust potential with
this surge (~45-50 kts for KDEN, and 60-70 kts for KBJC), which
should last around 2-5 hours (shorter for KDEN/KAPA, closer to 5
hrs for KBJC).

Some moderate weakening is then expected for early evening, before
a FROPA brings a shift to NW winds and potential for renewed
strengthening. Timing of arrival is still somewhat uncertain, and
seems to be a little delayed with the latest runs. Nonetheless,
expect gusts 30-40 kts for a few hours as it pushes through the
area.

&&

.FIRE WEATHER...
Issued at 350 PM MST Tue Dec 16 2025

Downslope winds will increase through the day and become strong by
the early afternoon, where they are then expected to spread east
of the foothills and into the urban corridor. Gusts up to 80-90
mph are very likely for the mountains and foothills above 9,000
feet. For the lower foothills and western urban corridor, gusts up
to 65-75 mph are possible, but will be shortlived (lasting about
3-4 hours in the early afternoon). Areas adjacent to the
foothills/urban corridor could see wind gusts up to 30-40 mph at
times. Despite marginal relative humidity values (17-30% range),
strong winds will promote rapid fire growth. As of now, primary
concern will be between 1 PM to 5 PM, when the stronger winds are
expected to spread east to lower elevations. By Wednesday evening,
a cold front will arrive, which will increase dewpoint
temperatures across the plains and help mitigate fire weather
concerns. Although wind gusts up to 60 mph are possible behind the
front, particularly in the northern plains.

Pockets of elevated fire weather will be possible Thursday.
Relative humidity values will drop into the teens to lower
twenties in the afternoon. Winds will be breezy in the morning
for our northeastern counties. However, winds will begin to weaken
at the same time relative humidities drop. So, there should just
be a brief window of fire weather concerns late Thursday
morning/early afternoon.

Strong winds will return to the mountains and foothills on Friday.
Minimum RH values will drop into the lower teens to mid twenties
east of the mountains. High winds and dry conditions will lead to
near-critical to critical fire weather conditions Friday afternoon.

&&

.BOU WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory from 11 AM this morning to 11 PM MST
this evening for COZ031-033.

High Wind Warning from 8 AM this morning to 5 AM MST Thursday
for COZ033>036.

Red Flag Warning from 11 AM this morning to 6 PM MST this
evening for COZ238>243-245-246.

High Wind Warning from 11 AM this morning to 7 PM MST this
evening for COZ038-039.

High Wind Warning from 5 PM this afternoon to midnight MST
tonight for COZ042-044-048.

High Wind Watch from late tonight through Thursday afternoon for
COZ050-051.

&&

$$

UPDATE...BRQ
DISCUSSION...AP/MAI
AVIATION...BRQ
FIRE WEATHER...AP/MAI

NWS BOU Office Area Forecast Discussion