506
FXUS65 KBOU 170332
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
832 PM MST Tue Dec 16 2025

.KEY MESSAGES...

- Strong downslope winds to impact the mountains, foothills, and
  I-25 corridor Wednesday into Wednesday night, with potential
  for widespread gusts 60-85 mph, strongest near the base of the
  foothills. Breezy conditions will extend into the plains through
  Thursday morning.

- Critical fire weather conditions can be expected Wednesday
  afternoon for the urban corridor.

- Mountains to see deteriorated travel conditions Wednesday
  afternoon/evening due to banded snowfall and wind.

- Another round of gusty winds and high fire danger looks likely
  (75% chance) on Friday.

&&

.UPDATE...
Issued at 832 PM MST Tue Dec 16 2025

Forecast is on track. We see consistent output in the 00Z short
range/mesoscale models regarding high wind event that starts in
the mountains and foothills Wednesday morning, and spreads onto
the nearby adjacent plains west of I-25 Wednesday afternoon. No
changes to existing suite of High Wind and Red Flag Warnings, or
the Winter Weather Advisory for the northern tier of mountains.

&&

.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 /00Z TAFS through 00Z Thursday/...
Issued at 442 PM MST Tue Dec 16 2025

Main concern will be the winds. Right now, winds are starting to
diminish and appears (70-80% confidence) they will just
transition to a normal south/southwest flow overnight. With
increasing pressure gradients, we do anticipate enhanced SSW flow
with gusts to 20-24 kts possible after 10Z-12Z. The difficulty
with the forecast lies mostly with the expected mountain wave and
enhancement Wednesday afternoon. KBJC will see a high probability
of blasting west winds developing by ~20Z with stronger gusts to
50 kts or even more possible after 21/22Z. Meanwhile, at KDEN and
KAPA winds could actually be variable for a while in the lee of
the mountain wave Wednesday. However, we do have higher confidence
(60-70%) for at least gusty winds 30+ kts at times 21Z-01Z, and a
lower confidence (30-40%) chance of gusts to 40 kts or greater.
It appears more widespread gusty winds with gusts 28-35 kts will
spread across KDEN and KAPA by 01Z as stronger subsidence arrives.

VFR conditions will prevail for this period. Only SCT-BKN mid and
high clouds with a passing sprinkle possible 00Z-03Z Thursday.

&&

.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 to 11 PM MST Wednesday for
COZ031-033.

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

Red Flag Warning from 11 AM to 6 PM MST Wednesday for COZ238>243-
245.

High Wind Warning from 11 AM Wednesday to midnight MST Wednesday
night for COZ038-039.

High Wind Watch from Wednesday afternoon through Wednesday
evening for COZ042-044-048.

&&

$$

UPDATE...20
DISCUSSION...AP/MAI
AVIATION...20
FIRE WEATHER...AP/MAI

NWS BOU Office Area Forecast Discussion