021 
FXUS65 KBOU 290507
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
1107 PM MDT Mon Sep 28 2026

.KEY MESSAGES...

- The threat for heavy rainfall will increase late Tuesday,
  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 
  Tuesday 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 /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 
COZ036-041-046-047.

&&

$$

DISCUSSION...AP
AVIATION...9