258 
FXUS65 KBOU 290058
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
658 PM MDT Mon Sep 28 2026

.KEY MESSAGES...

- Rain chances will prevail Monday afternoon through early
  Thursday. Highest potential for heavy rain will be on 
  Wednesday. Best chance for higher rain amounts will be in our 
  southeastern counties.

- Rain chances decrease late Thursday, leading to warmer and drier
  conditions for the weekend.

&&

.DISCUSSION /Through Monday/...
Issued at 318 PM MDT Mon Sep 28 2026

Current radar imagery shows scattered light showers over the 
southern foothills and Palmer Divide moving NNE over the Denver 
metro, and a few thunderstorms over the east CO plains. This uptick 
in shower activity is a result of the increased moisture over the 
region as the highly anticipated upper-level low pressure over 
central California continued to track east, channeling Pacific 
moisture. Precipitable water (PW) values are already above the 90th 
percentile of climatology across the state, with PWs in the 0.7"-
1.0" range at low elevations. We still expect rainfall amounts to 
remain light due to southwest flow favoring downsloping for most of 
our area and very limited instability. One exception to this would 
be localized heavy rainfall with the thunderstorm activity over the 
east CO plains.

A cold front will push through the Denver metro in the very early 
hours of Tuesday morning, bringing a shift to northerly surface 
winds and a low cloud deck. Given the anomalous moisture that will 
be in the environment, a band of showers may accompany this cold 
front. Otherwise, intermittent shower activity similar to that seen 
today can be expected Tuesday afternoon with mostly light rainfall 
rates. The upper level low pressure system will merge with the 
remnants of Hurricane Polo by Tuesday evening. 

The main period of concern with this rainfall event will begin 
Tuesday night, continue through Wednesday, and end Thursday morning. 
The bulk of the moisture from the remnants of Hurricane Polo will 
arrive Tuesday night, with PWs reaching or exceeding 1.3" over the 
east CO plains. This amount of PW would be impressive even for late 
July, and it is 250%-300% of normal for this time of year. Such high 
amounts of moisture with an airmass of tropical origin in place, and 
the forcing from a potent upper-level cyclone indicate a threat for 
flooding where the swath of the heaviest rainfall amounts. There is 
still some uncertainty regarding the location of the surface low 
pressure that will develop with this system, and therefore, how 
far west or north into the Denver metro area the heavy rainfall 
ends up. The Euro deterministic and ensemble solutions still hint
at totals above 3.0 inches for parts of our southeastern CWA, 
while the GFS is still trending very dry with only about 1.0 inch 
over this area and just a trace for the Denver metro. High-res 
guidance is also split between rainfall amounts, with some 
solutions showing as high as 5.0 inch totals over the Palmer 
Divide, and other showing less than 0.5 inches. Interestingly, the
latest Extreme Forecast Index and Shift- of-Tails (EFI-SOT) now 
hint at higher confidence in a highly impactful rainfall event for
much of central Colorado, with SOT values between 2 and 5. Such a
high SOT implies that a significant amount of ensemble members in
the Euro suite go far beyond the forecast climatology. 

Using an ingredients-based approach, and considering the slight 
westward shift in rainfall amounts, we have issued a Flood Watch for 
the Palmer Divide and adjacent plains, including parts of Elbert and 
Lincoln counties. Generally speaking, rainfall amounts between 1.5 
and 2.5 inches are possible for areas in the southeastern Denver 
metro, with locally higher amounts between 3.0 and 4.0 inches 
possible where the Flood Watch is in effect. While rainfall rates 
should remain on the weaker side, continued moderate rainfall over 
the same areas may lead to flooding of low-lying and poor-drainage 
areas.

In terms of snow, while no significant accumulations are expected, 
dustings may be possible at elevations above 11,000ft. The weather 
system will exit the area Thursday afternoon, and we expect quieter 
weather to arrive for Friday and the weekend.

&&

.AVIATION /00Z TAFS through 00Z Wednesday/...
Issued at 554 PM MDT Mon Sep 28 2026

Today's showers have moved out of the TAF sites and remain
generally north, east, and south of them all at this time, with a
few showers observed on radar over the mountains to their west. 
There remain some uncertainties with this latest TAF package, as 
ample moisture remains in place across the area for further 
showers to be possible pretty much through the TAF period. Water 
vapor imagery also shows a steady plume of tropical moisture 
continues to move into the region, with a high-impact rain event
still looking possible for portions of Colorado late Tuesday night and
Wednesday. 

The main impacts for this TAF package will be with low stratus
bringing periods of MVFR throughout the TAF period. Guidance
continues to show potential for another wave of light showers
later this evening before a cold front brings a northerly wind
shift in the early morning. We should see a brief period of SW to
NW winds before the cold front comes between 13-14Z. Light showers
will be possible with the front, and ceilings are expected to
gradually lower with the northerly flow. While some of the MOS
guidance shows a brief period of IFR developing between 15Z to
18Z, most forecast soundings keep the lowest ceilings hovering
around 3,000 to 4,000 feet AGL, with the best chances for the
lowest ceilings to occur at KAPA/KBJC. Have kept PROB30s in play
through the period, but these may be trimmed back on the next AMD.


&&

.BOU WATCHES/WARNINGS/ADVISORIES...
Flood Watch from late Tuesday night through Thursday morning for 
COZ041-046-047.

&&

$$

DISCUSSION...AA
AVIATION...9