Milliken, Colorado Weather Forecast Discussion

561
FXUS65 KBOU 231022
AFDBOU
Area Forecast Discussion
National Weather Service Denver/Boulder CO
422 AM 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 /06Z TAFS through 06Z Wednesday/...
Issued at 1133 PM MDT Mon Sep 22 2025
Ceilings are a bit messy at the moment, with various amounts of
low and mid-level cloud cover near the terminals. Though nighttime
microphysics satellite imagery is mostly blocked by higher clouds,
there does appear to be some filling in of the mid-level cloud
cover and guidance is in fairly good agreement that MVFR to IFR
cigs will quickly redevelop overnight into Tuesday morning. Fairly
likely that IFR conditions will continue most of Tuesday into
Tuesday evening from both rain/visby impacts and the lower
ceilings. Some uncertainty with the precipitation shield tomorrow,
but can`t rule out some brief RA to +RA though the potential for
TS is low.
Meanwhile, winds are also a little squirrelly tonight, with some
weak WSWerly flow. Again, we should see a shift to the north here
during the overnight hours which will more than likely continue
through the rest of the TAF period.
There should be steady improvement Tuesday evening as rain
diminishes and the stratus deck slowly rises... but MVFR
conditions are still likely through most of Tuesday night into
Wednesday AM.
&&
.BOU WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory until 6 PM MDT this evening for COZ033-
034.
&&
$$
UPDATE...Hiris
DISCUSSION...Ideker/Barjenbruch
AVIATION...Hiris

NWS BOU Office Area Forecast Discussion