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