098
FXUS65 KBOU 291818
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
1218 PM MDT Tue Sep 29 2026

.KEY MESSAGES...

- The threat for heavy rainfall will increase late tonight,
  continuing through early Thursday. The highest chance for the
  larger rain amounts will be in our southeastern counties.
  Uncertainty remains in exact amounts.

- Flood Watch in effect for the southern Front Range Foothills,
  Palmer Divide, and Elbert and Lincoln Counties from late tonight
  through Thursday morning.

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

&&

.DISCUSSION /Through Monday/...
Issued at 1053 PM MDT Mon Sep 28 2026

As of late this evening, the upper level trough is moving through
southern California and Hurricane Polo is nearing the west coast of
Baja California. The trough will slowly makes its way across the
southeast U.S., eventually absorbing the remnants of Polo on
Tuesday. Moisture ahead of and with this system is anomalously high,
leading to rain chances for the next few days. Tonight, a weak cold
front will move through the area bringing northerly winds and more
dense low to mid cloud cover. The northerly winds behind the front
could provide enough weak upslope flow along the Palmer and southern
Foothills for some light showers throughout the day Tuesday.
Temperatures will be slightly cooler behind the front, with temps
dropping into the 60s for much of the plains.

The main concern for this forecast will be the late Tuesday through
mid Thursday time period. The upper level low is forecast to move
just south of the four corners region early Wednesday. At the
surface a low is forecast to develop just ahead of the trough
somewhere in New Mexico. The path of this low is one of the main
differences between models, with some taking it northeast into the
Texas Panhandle and others take it more north into southeastern
Colorado. The location of this low will be very important for our
rain chances. If the low tracks closer to the area, our chances
for more widespread precip will increase as well as the potential
for higher rain totals. If the low tracks more towards the Texas
Panhandle and Oklahoma, the larger precip amounts will likely
miss our area. I will say, higher resolution models are leaning
more towards the surface low moving into southeastern Colorado,
which increases our confidence in the higher precip amounts for
our southern most counties. However, the northward extent of the
heavier rainfall is still very uncertain. Right now, it looks
like that northward extent of heavier precip could sit somewhere
near Denver, which unfortunately means that the metro is one the
area with the highest uncertainty for rain totals.

We are starting to get increasingly concerned about the Palmer
Divide area in terms of the higher rain totals. If the low does end
up moving over SE Colorado, strong northerly winds could provide
decent upslope along the Palmer and southern Foothills, which could
nudge it towards those higher precip amounts. Because of this we
did extend the Flood Watch to include the southern foothills zone.
I did consider adding Arapahoe county and Washington county to
the Watch as well, but we will wait until confidence increases in
the northward extent of the higher rain totals. Areas within the
watch are expected to see rainfall totals around 1.5 to 3 inches,
with the potential for some areas to see 4+ inches. As of the
latest model runs, Denver is sitting at a 50% to 60% chance of
seeing at least 1 inch of rain late Tuesday through Thursday,
which is slightly higher than what we were seeing yesterday.

Snow chances in the higher terrain will peak early Thursday
morning as temperatures finally start to cool more aloft. Amounts
are still forecast to be on the lower end (a couple inches at
most), with snow levels generally around 11,000ft. Rain chances
will start to taper off on Thursday as this system moves off to
the east. Behind the trough, we`ll see weak ridging start to build
over the area this weekend, leading to drier and warmer
conditions. Models are showing a cold front moving through the
area on Saturday, keeping temperatures slightly cooler and closer
to normal for Sunday.

&&

.AVIATION /18Z TAFS through 18Z Wednesday/...
Issued at 1207 PM MDT Tue Sep 29 2026

CIGS will be tricky today as cloud base should be between 045 and
060, but for now FEW or SCT seem to be the most likely coverage at
that height. The chance for sprinkles this afternoon and evening
is under 15 percent so we left any mention out of the TAFs. North
winds 10-13kts will transition to east 7-11 kts this afternoon,
then southeast after 03Z. We are tracking moisture form Polo as it
merges with an approaching Pacific trough. Confidence in the
forecast continues to be low overall, including for aviation
impacts. For now, it looks like rain should hold off until 12Z or
later. BKN CIG 030-045 are expected after 09Z at all TAF sites.
Best chance of rain is after 15Z at APA, 16Z at DEN and BJC, and
rain chances are high throughout tomorrow afternoon and evening.
It`s rare, but we are keeping a prevailing SHRA all afternoon and
evening. Winds should be north to maybe northeast 8-14kt with
gusts to 20kt. We do not expect any TS. CIGs under the heavier
showers may drop below 020, but shouldn`t get any lower than that
and VIS should be 4SM or better even in the heavier showers. Heavy
rain and CIGS dropping below 020 are not likely, but for now will
include with a PROB30 in the TAFs.

Looking ahead rain chances will remain in the forecast through 18Z
Thursday, along with continued MVFR CIGs. We should be able to
transition from prevailing to TEMPO or prevailing after 06Z
Thursday, stay tuned.

&&

.BOU WATCHES/WARNINGS/ADVISORIES...
Flood Watch from midnight MDT tonight through Thursday morning
for COZ036-041-046-047.

&&

$$

DISCUSSION...AP
AVIATION...Schlatter

NWS BOU Office Area Forecast Discussion