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FXUS65 KBOU 280023
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
623 PM MDT Sun Sep 27 2026

.KEY MESSAGES...

- Warm today, but isolated to scattered showers and storms with
  gusty winds this afternoon and evening.

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

&&

.DISCUSSION /Through Sunday/...
Issued at 302 PM MDT Sun Sep 27 2026

High-based/virga showers will be possible over the I-25 corridor
late this afternoon (4pm-9pm) as southwesterly flow aloft begins to
advect Pacific moisture into our mid levels. Given the large surface
dewpoint depressions, gusty outflow winds up to 45 mph will be
possible with the shower activity. A few showers or an isolated
storm may linger over the northeast Colorado plains late tonight
with gusty winds being the main threat.

Tomorrow will be the start of an active weather week with daily
precipitation chances through Thursday evening. An upper level
trough will move over Southern California Monday, strengthening
southwest flow aloft. Precipitable water (PW) values will reach 200%
of climatology as early as tomorrow afternoon. While we do not
expect the heaviest rainfall to occur tomorrow, scattered showers
will develop over the mountains in the morning spreading over the
Denver metro by the afternoon. Showers tomorrow afternoon will be
light and more stratiform-like in nature due to limited instability,
although pockets of brief heavy rain and a few lightning strikes
cannot be totally ruled out. Light showers will linger into the
evening hours with similar shower activity expected to continue
Tuesday.

A cold front and thick cloud cover on Tuesday will keep temperatures
quite fall-like, with highs only reaching the low 70s at low
elevations and 50s in the mountains. Shower coverage should become
more widespread Tuesday evening, and this is where uncertainty
enters the picture as the highly anticipated upper level trough
reaches the Four Corners. Model guidance shows quite a bit of
spread in solutions, indicating that uncertainty still remains.
However, there has been a slight westward shift of the highest QPF
amounts, particularly with Euro deterministic and ensemble
solutions. The GFS/GEFS continues to be on the drier side for our
CWA, but the latest cycles has shifted to include more of
southeast Colorado under the QPF footprint. As the previous AFD
stated, the largest difference in model solutions seems to be tied
to the location of the surface low and its relationship to latent
heat release from shower development. The model solutions that
bring intense rainfall closer to the Denver metro develop the
surface low over eastern Colorado, with drier solutions delaying
the development of the low over west-central Kansas. Additionally,
there is better consensus that the timing of the heaviest
rainfall and at least some localized flash flooding concerns would
be Wednesday as the trough phases with the remnants of Hurricane
Polo and moisture becomes wrapped around the new surface low.
Confidence in more substantial rain amounts for southeast
Colorado, and potentially parts of our southeastern CWA (i.e.
Lincoln and Elbert counties) is also increasing.

While mountain snow chances with this system remain low (<30%), it
is worth mentioning that a westward shift in the bulk of the
moisture (such as what the latest Euro guidance has suggested) would
increase the potential for some dustings or light snow accumulations
if temperatures are able to cool down enough Wednesday afternoon
through Thursday morning, mainly above 11,000 ft. Looking ahead to
the weekend, long-range ensembles are in better agreement that
drier weather will return as ridging prevails over the western
United States. Temperatures look to remain near or below
seasonable averages over the next ten days.

&&

.AVIATION /00Z TAFS through 00Z Tuesday/...
Issued at 552 PM MDT Sun Sep 27 2026

Current radar and observations show showers and a few
thunderstorms have begun to move off the higher terrain and across
the urban corridor. A westerly outflow just passed across KBJC and
is en route to KDEN/KAPA, though should see more of a WNW to NW
wind turn at these locations in the next 20 to 30 mins, with gusts
between 25-30 kts. Gusty and variable winds will be possible for
the next few hours (until ~3Z) from any additional outflows that
develop from showers/storms. South-southeasterly winds are
expected to settle back in once outflows/showers subside (~4Z) at
KDEN/KAPA, with winds becoming light and variable for the
overnight at KBJC.

For the overnight hours, expecting enhanced southerly winds at
KDEN/KAPA to become gusty between 7Z-9Z late tonight. Hi-res show
potential for a cyclone to develop in the morning somewhere north
of the TAF sites. This always brings some uncertainty with wind
direction depending where it forms, but there seems to be fairly
decent agreement that winds will remain southerly at KDEN/KAPA and
increase in magnitude through the morning, with KBJC holding onto
a northerly direction for the afternoon. There have been a few
adjustments made to the timing of precipitation chances with this
package. A PROB30 is now in effect from 19Z to 21Z and a TEMPO
from then until the end of the TAF period across all TAF sites for
rain showers and ceilings as low as 5,000 feet AGL. Rain showers
are expected to persist through the overnight hours Monday night,
with low stratus and reduced visibility likely by early Tuesday
morning.

&&

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

&&

$$

DISCUSSION...AA
AVIATION...9