348
FXUS65 KBOU 280610
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
1210 AM 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 Sunday/...
Issued at 1105 PM MDT Sun Sep 27 2026

Looking at water vapor on satellite, the upper level trough we`ve
been talking about for the past few days is finally moving onto
the West Coast. Ahead of this deep trough, southwesterly upper
level winds are funneling in moisture from Post-Tropical Cyclone
Odalys into our area. At lower levels overnight, south/southeast
surface winds will continue to increase moisture at the surface.
This increase in moisture combined with upper level lift ahead of
the approaching trough will bring a round of showers and storms
Monday afternoon and evening. Showers are forecast to move into
our area from the southwest, progressing northeast through the
evening. This activity is expected to be primarily rain showers,
dropping anywhere from a few hundredths to around half an inch of
rain. However, we could see a few storms develop in the later
afternoon and evening, which could produce some locally heavy
rainfall exceeding an inch.

Rain chances will continue on and off through at least Thursday
morning as that upper trough approaches and then sits just to the
southeast of our area. A cold front will move through early
Tuesday, bringing slightly cooler temperatures and the potential
for surface convergence leading to a band of some heavier precip
sometime Tuesday morning. QG forcing from the upper low and
PWs are forecast to peak on Wednesday. This is the day we`re most
concerned about. There`s still significant discrepancies on where
the axis of heavier precip will form (which is expected), with
models in pretty good agreement that there will be a swath of
area that sees 3+ inches with this event. We will likely be right
at the northern edge of this swath, meaning it could stay just to
our south and east or portions of our area could see very heavy
rainfall and flash flooding. Right now, Denver is sitting at a
medium chance (~40%) of seeing at least an inch of rain on
Wednesday, with chances increasing as we go southeast. Lincoln
county is the most likely area in our CWA to see the higher
rainfall totals, with a 60% to 70% chance of at least an inch on
Wednesday.

The forecast precipitable water values for Tuesday, Wednesday,
and potentially Thursday are VERY high. Models are showing
widespread PWs of 200% to 250% of normal, with potential for it
to reach 300% of normal. With this much moisture, any shower or storm
that can get going will have the potential to produce very heavy
rainfall and flash flooding. So, even if we get missed with the
main swath of heavy rain and don`t receive the widespread heavy
rainfall across our area, there still will be a very real threat
for localized flash flooding for any area that ends up with a
storm.

As for mountain snow chances, temperatures still look very
marginal, which does not bode well for snowfall totals. Areas
above 12,000ft will likely see at least a round or two of snow
flurries in the Tuesday through Thursday time period (best chance
Wed and Thu mornings), leading to a dusting of snow for those
areas above 12,000ft. But chances for much more than that are low
at this time. Rain chances will start to decrease on Thursday as
the upper level system moves off to the east. Temperatures Friday
through the weekend will warm closer to normal.

&&

.AVIATION /06Z TAFS through 06Z Tuesday/...
Issued at 1206 AM MDT Mon Sep 28 2026

An elevated drainage wind has been in progress at KAPA and KDEN over
the last several hours. Recent observations in Douglas and Elbert
counties show numerous locations with gusts up to 25 kts. There is a
TEMPO at KAPA from 7z to 11z to account for the possibility of these
stronger gusts making it into the terminal. Regardless, we expect
wind gusts up to about 20 kts at KAPA and KDEN through the overnight
and into the day on Monday. KBJC should remain light and VRB.

A cyclone is expected to form later this morning. Models have been
in good agreement that this will be centered north of the three
sites. KBJC winds could lean more northerly as a result, but are
overall expected to remain light and VRB until any showers arrive
this afternoon. Several waves of showers are likely- starting at
20z and then lasting through the end of the TAF period- at all
three sites, but KDEN and KAPA will be most favored. TEMPO groups
are in place at KBJC and KDEN with prevailing -SHRA at KAPA from
20z to 24z including the potential for lower ceilings at times.
This represents the period of highest confidence for these showers
as of now. We have VCSH and/or PROB30 for the potential for
showers and lower ceilings continuing at KDEN through the rest of
the TAF period. KAPA is likely to continue to see the most
numerous showers through Tuesday morning and beyond where there is
a TEMPO for Monday evening.

Ceilings start to lower by the late afternoon, especially by 00z
Tuesday (Monday evening) and beyond. For now, there is BKN060 at
KDEN starting at 6z Tuesday. These ceilings will certainly need
to be adjusted in future updates. There is good model agreement
that we will likely see low stratus and reduced visibility at all
three sites starting around 12z Tuesday or even a few hours before
then.


&&

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

&&

$$

DISCUSSION...AP
AVIATION...MV

NWS BOU Office Area Forecast Discussion