963
FXUS65 KBOU 041111
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
511 AM MDT Thu Sep 4 2025

.KEY MESSAGES...

- Isolated showers and weak thunderstorms for late day Thursday.

- Near critical fire weather conditions are expected across the
  plains on Thursday due to warm, dry, and breezy weather.

- Better coverage of storms and above normal temperatures expected
  this weekend.

&&

.UPDATE...
Issued at 324 AM MDT Thu Sep 4 2025

A few changes for today and tonight. The main thing is to
emphasize a little stronger cold front passage with a bit more
wind, but the main impact will be advecting in some of the smoke
that`s over the northern plains. There`s a big swath of smoke at
the surface from Missouri to Alberta, in addition to the dense
smoke aloft. With north winds spreading down the east side of the
mountains, I don`t see how we`ll miss this. There are areas with
visibilities between 4 and 10 miles, though it`s not that dense
everywhere. We`re adding smoke to the forecast along and behind
the front, in the northeast this afternoon, and then dropping
across our area this evening. The HRRR says there will be cleaner
air behind the front. Looking at surface obs and where the current
fire/smoke is, I`m skeptical. We`ll certainly have smoke aloft,
will be be entraining low level air from far enough east that it`s
not smoky? In any event, we`ll have a lot of smoke aloft through
Friday and probably Saturday before the flow becomes more
westerly. PoPs were lowered a little through Friday, it`s dry and
then stable behind the front. Mountain showers could drift east,
but probably not very far.

&&

.DISCUSSION /Through Wednesday/...
Issued at 142 PM MDT Wed Sep 3 2025

Current surface observations show widespread wind gusts up to 35
mph across the plains as a dry cold front moved through the area
earlier this afternoon. Satellite imagery has a cumulus field
building over the higher terrain, but most of the forecast area
should remain dry this afternoon/evening as we are under
subsident flow. The only exception would be the foothills, where
upslope flow and steep lapse rates of ~9 dg C/Km would allow weak
high- based showers/storms to develop this afternoon. With a
strong temperature inversion this morning inhibiting instability
building over the area, and SPC mesoanalysis having less than 500
J/kg of SBCAPE across the high terrain and plains for the rest of
the day, the severe threat is low (1000 J/kg,
the main threat will be wind gusts up to 45 mph with the
strongest storms.

Friday will be the coolest day of the forecast period as
temperatures are expected to be in the low 70s. An upper level
ridge will build over the western United States. However, we will
enter an active pattern through the weekend due to multiple
embedded shortwaves within the ridge. Along with the forcing,
moisture will be advected into the region due to a tropical system
in the Pacific. There are inconsistencies in the track of the low,
with the ECMWF indicating a more southerly track while the GFS has
been trending towards a more northerly track. For this reason, the
exact coverage of showers/storms is still uncertain. Most likely,
we will have daily afternoon convection across the higher terrain
that could spill onto the adjacent plains.

Looking forward into next week, ensemble models are in agreement
of slightly above normal temperatures across the forecast area,
with some model runs indicating the possibility of highs in the
low 90s.

&&

.AVIATION /12Z TAFS through 12Z Friday/...
Issued at 509 AM MDT Thu Sep 4 2025

VFR through tonight, though increasing smoke may create slant
visibility issues after 22z. A significant smoke layer aloft is
expected over northeastern Colorado after that time. KDEN/KAPA
will have southwest winds at 10-20 knots becoming north around 10
knots 15z-17z. KBJC will have lighter winds. A cold front will
move through Denver 01z-02z with north winds gusting to 25 knots
and more low level smoke behind it. Isolated showers and
thunderstorms are also expected this evening.

&&

.BOU WATCHES/WARNINGS/ADVISORIES...
None.

&&

$$

UPDATE...Gimmestad
DISCUSSION...Ideker
AVIATION...Gimmestad

NWS BOU Office Area Forecast Discussion