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 (<25% chance) 
Sunday afternoon, although a few sprinkles may be possible over the 
mountains. Temperatures will run warmer than seasonable both days, 
with highs in the low to mid 80s at low elevations and 60s-70s for 
the mountains. 

The pattern becomes much more complex starting Monday. Deterministic 
global model runs and long-range ensemble members still show 
considerable spread regarding the evolution of an upper level trough 
and the remnants of what is currently Hurricane Polo. One of the 
reasons for this level of uncertainty is the disagreement regarding 
the speed of upper level trough and the post-tropical cyclone 
(Polo) as it approaches the New Mexico/Texas-Mexico border. 
Broadly speaking, there are two possible scenarios here. Once 
scenario favors a slower track of the upper-level trough, allowing
it to encounter and phase with the post-tropical cyclone just 
south of the Four Corners. If this occurs, a more meaningful 
portion of the post- tropical cyclone's moisture would be lofted 
northwards into our area and lead to widespread rainfall coverage 
for Tuesday-Wednesday. Another scenario, and what looks most 
likely at this point, is an earlier arrival of the upper level 
trough and phasing with the post- tropical cyclone occurring 
farther east closer to the Texas Panhandle, which would greatly 
decrease rainfall chances for our area as the bulk of the moisture
is pushed over the central Great Plains. Regardless, ensemble 
solutions do hint at anomalous moisture (>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