723 
FXUS65 KBOU 262338
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
538 PM MDT Sat Sep 26 2026

.KEY MESSAGES...

- Drier and warm Sunday, with highs reaching into the 80s across 
  the plains.

- Decreasing chances (now 10%-30%) for mountain snow above
  12,000ft Monday through Wednesday.
  

- Potential for heavy rain Tuesday into Thursday, but confidence 
  in amounts and locations remains low.

&&

.DISCUSSION /Through Saturday/...
Issued at 300 PM MDT Sat Sep 26 2026

It's a quiet afternoon across Colorado with mostly sunny skies over 
the plains and fair-weather cumulus over the mountains. Rain chances 
for today remain low (<20% chance), although a sprinkle or two 
cannot be ruled out over the mountains as near-average moisture 
still lingers, and mid-level water vapor imagery shows a ripple of 
ascent/very weak shortwave grazing northwest Colorado. Temperatures 
are currently sitting in the 80s at low elevations and a mix of 60s-
70s in the mountains. 

An upper level trough will approach the California coast on Sunday 
afternoon, which will push the upper level ridge over our area to 
the east. We expect flow aloft to become southwesterly, channeling 
tropical moisture and positive vorticity from the remnants of 
Hurricane Odalys into the Four Corners and Colorado's mountains. 
Although the main plume of moisture from this system will not arrive 
to our area until late Sunday night, we expect isolated to scattered 
showers and storms to develop over the high terrain starting Sunday 
afternoon. For the Denver metro and I-25 corridor, high-res model 
guidance has continued to hint at isolated high-based showers making 
it off the terrain, mainly north of Denver. Therefore, we have upped 
PoPs (20%-30% chances) to reflect this trend. These high-based 
showers will be able to produce erratic gusts up to 40 mph. The east 
Colorado plains may remain too be capped for thunderstorm activity 
in the afternoon. However, model soundings show CAPE increasing in 
the evening hours, 30-40KT of bulk shear, and generally curved 
hodographs. If the instability is realized and/or an outflow 
boundary is able to trigger nighttime convective initiation, an 
isolated strong or marginally severe storm cannot be ruled out over 
the east Colorado plains. 

Moisture continues to increase on Monday as the upper level trough 
gets closer. We expect shower coverage to increase primarily over 
the high terrain Monday afternoon, although a few isolated showers 
will be possible over the plains, including the Denver metro. 
Mountain snow chances still remain quite low through Tuesday 
afternoon although a few dustings are possible for the highest peaks 
and ridges, above 12,000ft.

A period of impactful weather is possible Tuesday through Thursday, 
but the exact nature of these impacts remains uncertain. Recent 
trends in deterministic and ensemble guidance generally keep the 
bulk of the moisture from the remnants of what is currently 
Hurricane Polo just to our southeast, over the Colorado-New Mexico 
border. Although agreement has increased regarding the phasing of 
the upper level trough and post-tropical cyclone (Polo) occurring 
south of the Four Corners late Tuesday-Wednesday, uncertainty 
remains when it comes to the speed of the resulting upper-level 
low. Model guidance has also trended towards a later arrival of 
the moisture, with rainfall chances not increasing significantly 
until Wednesday afternoon through Thursday. The latest Extreme 
Forecast Index and Shift-of-Tails (EFI-SOT) hints at higher 
confidence in a highly impactful rainfall event for southeast 
Colorado and northeast New Mexico on Wednesday. Interestingly, the
EFI-SOT for Thursday puts the eastern Colorado plains (including 
parts of our CWA) under a Low EFI-High SOT. This index combination
indicates that most ensemble members fall within climatology, 
while a few greatly exceed the distribution. In summary, we will 
need to keep watching this event as it evolves given that slight 
changes in the speed and track of the trough and post-tropical 
cyclone may greatly impact the footprint of the heaviest 
precipitation. 

Mountain snow chances still remain low (10%-30%) and there is not 
clear signal for significant accumulations or the best timing. Given 
the fairly constant availability of mid-level moisture from 
southwest flow aloft almost every day next week, it is fair to 
assume that elevations (mainly >12,000ft) that become cold enough 
may see daily chances for some flurries up to a dusting. Some GFS 
ensemble members do hint at some light mountain snow accumulations 
with an upper level shortwave trough next weekend, while ECMWF 
ensembles show zonal flow or even ridging and a return to warmer 
temperatures. This will be another system to watch as the week 
progresses. 

&&

.AVIATION /00Z TAFS through 00Z Monday/...
Issued at 525 PM MDT Sat Sep 26 2026

Winds will become south by 02z and then turn more SSW by 05z.
HRRR shows a few weak high based showers between 02z and 04z but
no gusty winds expected so have not mentioned for this evening.

On Sun, winds will become SE by 18z and then increase thru the
aftn with gusts up to 25 mph by 21z. Have mentioned a prob30
for higher based showers beginning at 22z for APA and BJC and 23z
at DIA. Brief gusty outflow winds up to 30 mph will be possible 
thru 03z. 

&&

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

&&

$$

DISCUSSION...AA
AVIATION...RPK