652
FXUS65 KBOU 252105
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
305 PM MDT Fri Sep 25 2026

.KEY MESSAGES...

- Another round of showers and isolated storms Friday afternoon
  and evening.

- Drier and warmer this weekend before an upper level trough
  brings cooler temperatures and potentially our next chances for
  showers/storms early next week.

- Low chances (30%-40%) for mountain snow above 11,000ft late
  Monday into Tuesday.

&&

.DISCUSSION /Through Friday/...
Issued at 304 PM MDT Fri Sep 25 2026

A shortwave trough moving through Colorado this afternoon is
promoting another round of showers and storms with localized heavy
rain possible. Current soundings show a very moist profile, long-
skinny CAPE, and precipitable water (PW) values 0.7-1.0". With the
the cool temperatures and cloud cover today, instability will be the
main limiting factor. Current visible satellite does show some
clearing over the Denver metro and east Colorado plains, allowing
for some surface heating to occur. Additionally, numerous
thunderstorms have already developed over the foothills. Although
storm motions (15-20 mph) and deep warm cloud layers (4,000-5,000
ft) are not exceedingly favorable for flash flooding, localized
flooding cannot be ruled out, particularly if the heaviest showers
track over the Denver metro or Palmer Divide. Clearing behind the
shortwave late tonight may lead to some patchy fog over the east
Colorado plains for a few hours around, clearing soon after sunrise.
For the mountains, zonal flow aloft and weak downsloping will bring
breezy conditions during the early morning, with gusts up to 40 mph
possible above the treeline, and gusts up to 25 mph for the
foothills.

We expect quiet weather to prevail for Saturday and Sunday due to an
upper level ridge building over Colorado and considerable
subsidence. As such, skies will remain mostly sunny and the chances
for showers over our CWA are near-zero Saturday afternoon. The upper
level ridge will continue to track east on Sunday and flow aloft
will become southwesterly, bringing a slight increase in mid level
moisture. Rain chances will generally remain very low (90th percentile of
climatology) in association with this event Tuesday through
Wednesday. As the previous AFD stated, this system is a
potentially high impact, low confidence event, where a westward
shift in the meeting point of the trough and post-tropical cyclone
would imply heavy rainfall concerns. At this point, it looks like
mountain snow chances are decreasing (30%-40% chance) with this
system. A post-tropical cyclone would imply a moist and warm
airmass, while a farther track and direct hit from the trough
alone would imply a colder but drier airmass.

Looking ahead, ensembles are trending towards a return to zonal
flow, near-average temperatures, and drier air for Thursday and
Friday. The next weather system may arrive late Saturday into Sunday
of next week, with some GFS ensemble members hinting at a couple
inches of accumulating mountain snow. However, great uncertainty
remains given that some ensemble members show no precipitation, and
the ECMWF hints at ridging over our region. This will be another
system to watch as the details become more clear.

&&

.AVIATION /18Z TAFS through 18Z Saturday/...
Issued at 1150 AM MDT Fri Sep 25 2026

Fog and stratus had lifted to a SCT deck (except at KBJC where it
was still BKN as of 1730Z). That, too should gradually lift and
break with further warming. ACARS soundings showed very little
convective inhibition, and a compact shortwave was lifting
east/northeast across the Wyoming/Nebraska border. With those
ingredients, we expect a round of thunderstorms to develop along
the Front Range and move east across the TAF sites 20Z-24Z. High
enough coverage and intensity/moisture to warrant TEMPO with some
visibility restriction and VRB G35kts. There is just a slight
chance that a shower/storm would linger past 00Z.

Then skies will clear overnight, with any storm influenced winds
turning more S-SSW 01Z-04Z. We expect a pretty solid drainage
winds at 8-12 kts and no cyclone, so that would limit any fog
threat to the the South Platte River valley well north and east of
the Denver area TAF sites.

&&

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

&&

$$

DISCUSSION...AA
AVIATION...20

NWS BOU Office Area Forecast Discussion