596 
FXUS65 KBOU 231802
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
1202 PM MDT Tue Sep 23 2025

.KEY MESSAGES... 

- First winter storm for the mountains, with several inches of
  slushy snow possible at the high mountain passes. Travel impacts
  possible through this evening. 

- Wet weather across most of the plains. Over an inch of rain
  possible, especially along and north of I-70. Much cooler with
  highs in the 50s.

- Turning warmer and drier again for the rest of the week ahead. 

&&

.UPDATE...
Issued at 353 AM MDT Tue Sep 23 2025

Upper level low continues to spin away across northwestern
Colorado, and that will be the main focus of the next 24 hours.
Based on water vapor satellite, the H5 circulation appears to be
centered over eastern Rio Blanco county in northwestern Colorado 
and has steadily shifted southward over the past few hours. Based 
on my very subjective analysis (the "eye test"), many of the lower
resolution models have not initialized this upper trough 
particularly well... at least compared to the past several cycles 
of the RAP/HRRR.

As we continue into the morning hours, the closed low should
eventually transition back into an open trough, and will drift 
off to the south and east. Moist upslope should deepen through the
day with some strengthening of the mid/upper level northeasterly 
flow. Forecast soundings/cross sections show upslope depths close 
to 300mb. Models have slowly trended towards some stronger 
(20-30kt) upslope later this morning into the afternoon, though 
that's far from a certainty.

The exact track of the shortwave/500mb circulation will be
critical for the evolution of the precipitation shield later
today. As the trough stalls a bit and becomes strongly positively
tilted (the parent trough stretches into the Upper Great Lakes!),
there is more than likely going to be a frustratingly slow 
moving, sharp edge to the stratiform precipitation shield. 
Overnight HRRR/RAP runs have wobbled back and forth a bit, but 
have been consistent enough that I have fairly high confidence in 
the PoPs/QPF. The main area of uncertainty will be along the 
southern gradient - generally across the southern Denver metro 
into the Palmer Divide. Further north, all of the signals I see 
favor a corridor from roughly Boulder to Fort Collins eastward 
towards Greeley/Fort Morgan/Akron for the heaviest QPF. Given the 
slow rather persistent rain, I wouldn't be surprised to see a few 
locations end up with 1.5-3" of precipitation... which is broadly
supported by the 00z suite of CAMs and HRRR/RAP.

Meanwhile... mountain snow is still looking like a safe bet. Given
the depth of upslope and favorable synoptic scale ascent, there
should be good QPF across the higher peaks along with some
spillover west of the divide. Although peak precipitation rates
are during the day, profiles still look like they'd support snow
above 9,500-10,500ft. Can't rule out some accumulation near/below
9,000ft during periods of heavier precipitation from diabatic
cooling effects... but the majority of impacts will be across the
highest elevations. The snow grids (like QPF) were nudged 
upwards, mainly across the northern Front Range mountains.

Given the much quieter weather beyond Wednesday morning, there's
little else to discuss. A warmer/drier trend is likely with highs
near or above normal.

&&

.DISCUSSION /Through Monday/...
Issued at 329 PM MDT Mon Sep 22 2025

Current radar shows scattered showers and storms across the high
terrain and northern plains, as moisture is increasing ahead of an
incoming upper level shortwave. Looking at surface observations,
our cold front just entered northern Colorado, with recorded wind
gusts up to 35 mph. Temperatures are about 10-15 degrees cooler  
behind the front, so expect a cool down as it continues to travel 
south, with breezy northerly winds. 

Short and long range guidance are now in better agreement of the
upper level shortwave becoming a closed low and tracking down to
western Colorado tonight - although there are still some 
discrepancies on exact track and intensity of the low despite 
being just 24-30 hours out. ECMWF still favors a slightly more 
westerly track than the GFS and the short range solutions like NAM
and HRRR, which would affect where highest precipitation amounts 
will fall this evening and overnight. For now, leaning towards the
shortwave having a slightly more progressive track, with PoPs and
QPF in our grids favoring more precipitation for northern Front 
Range mountains and plains. 

For tonight, expect widespread showers and storms to continue, 
with most areas receiving measurable precipitation by Tuesday 
morning. Snow is likely above 10,000 ft., with minimal 
accumulation this evening. In addition, there could be a few 
strong storms this evening, especially along the eastern plains 
where instability has been able to build.

By early Tuesday morning, most guidance has 700-mb winds 
increasing as the mid level low shifts east across Central
Colorado, with temperatures reaching down between -1 to +1 deg C 
(ECMWF continues to be the warmer solution). With favorable 
upsloping, snow levels could lower to 9000 ft. if the colder 
solutions verify. However, snow levels will also depend on 
precipitation rate, where heavier precipitation could cause levels
to dip as low as 8500 ft. With the increased forecasted QPF 
amounts (0.75-1.5" across the high terrain) and this being the 
first measurable snowfall event of the season, have opted in for a
Winter Weather Advisory for the mountains, with most impacts 
occurring above 9500 ft. Exact snowfall amounts are still 
uncertain, but expect 3-8" on the high mountain passes, with up to
a foot of snow or more on our highest peaks. The Tuesday morning 
commute will be affected, with slippery road conditions expected 
through the afternoon. 

For the plains, widespread stratiform rain will continue 
throughout the day Tuesday, with guidance agreeing on 0.5-1.5" of 
QPF, before diminishing by late evening. With the expected cloud 
cover, temperatures should stay in the 50s for highs (or even
lower 50s along the I-25 Corridor). There could also be localized
fog wherever rain persists.

Much drier weather is expected starting Wednesday, as our most
recent storm system exits into the Midwest. An upper level ridge,
and a blocking one at that, will then dominate our weather through
most of the week ahead. This will support a return of sunshine and
warmer temperatures. The next chance of precipitation will likely
hold off until late in the weekend or early next week, and even
that looks like a low chance - only if the blocking ridge weakens
and an upper level low can drift northeast from the Desert
Southwest. Overall, some pretty nice autumn weather Wednesday 
through most of this coming weekend.

&&

.AVIATION /18Z TAFS through 18Z Wednesday/...
Issued at 1200 PM MDT Tue Sep 23 2025

MVFR to IFR conditions expected through Wednesday morning. 
Widespread showers will persist for all TAF sites through this 
evening, with W/NW winds gusting up to 25 kts for DEN and APA at 
times. Short range guidance indicate lighter N/NE between 00-02Z, 
but there is a chance that the stronger NW winds prevail. 

Showers should end late this evening, but there could be some
lingering precipitation through 06Z (07Z for APA). After that, the
stratus deck will slowly erode Wednesday morning, with generally
light W/NW winds.


&&

.BOU WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory until 6 PM MDT this evening for COZ033-
034.

&&

$$

UPDATE...Hiris
DISCUSSION...Ideker/Barjenbruch
AVIATION...Ideker