992
FXUS65 KBOU 212005
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
205 PM MDT Mon Sep 21 2026

.KEY MESSAGES...

- Isolated chances for showers/thunderstorms Tuesday afternoon and
  evening.

- A disturbance moves through Wednesday and Thursday bringing
  increased chances for showers/storms.

- Drier and warmer this weekend, but isolated chances for showers
  and thunderstorms in the evening are possible.

&&

.DISCUSSION /Through Monday/...
Issued at 124 PM MDT Mon Sep 21 2026

We are stuck in NW flow aloft due to ridging centered in Southern
New Mexico and two separate low pressure systems in California
and the mid-west. This is fueling much drier air over our area,
current ACARS sounding and water vapor analysis  very dry air
aloft between 700-400mb, but there is enough low-level moisture to
produce cumulus clouds across the plains and higher terrain.
Lower PWATs exist across the mountains and urban corridor while
PWATs around 0.7-0.8 remain across the Eastern Plains. Thus model
guidance is not excited about precip chances for the rest of today
and this evening. Noticeably the NAMNEST came in as an outlier
this morning displaying the chances for showers moving through the
urban corridor. We think the greatest chance for any isolated
development would be near the Palmer Divide closer to midnight,
but chances remain very low ~10%.

On Tuesday our flow aloft shifts back to the southwest as the
aforementioned low in California drifts toward Utah bringing the
return of better PWATs and moisture aloft. There should be a much
better chance of afternoon/evening showers and thunderstorms
across the higher terrain (30-50% chance), and with 20 kts of
mid-level flow out of the WSW, an isolated shower/storm or two
could make it across the plains (10-20% chance mainly west of
US-85). Instability should remain limited across the area so main
threats would be lightning and brief gusty winds to 40 mph.

Wednesday a weak disturbance moving in from the southwest U.S. is
still on track to bring better chances for showers and
thunderstorms. Most forecast soundings indicate a fairly saturated
upper profile, but uncertainty remains in how moist the lower
levels will become. The EURO indicates PWATs increase to 160-180%
of average in the urban corridor while PWATs near 200% of average
may exist across the Eastern Plains. Extensive cloud cover pretty
much all day long will likely inhibit development of
showers/storms as well, and given limited instability any
showers/storms that get going will not pack much of a punch
especially across the higher terrain and I-25 urban corridor. The
Eastern Plains may see more of a clearing in the morning hours
increasing the odds they see a shower/storm, and since they will
be closer to the better low-level moisture, there could be a few
stronger storms that fire in the late evening hours with storm
chances continuing past midnight. Some NWP guidance has about 1000
J/kg SBCAPE across the northeastern plains Wednesday evening and
through midnight. Temperatures Tuesday and Wednesday should be
near seasonal averages, or a little warmer, with highers uppers
70s to near 80 across the plains and urban corridor, and 60s in
the mountain valleys and eastern foothills.

For Thursday, a weak 500mb trough moving across Colorado
continues the persistent southwest flow with PWATs remaining
elevated through Friday evening. This will likely continue
chances for isolated showers/storms. The greatest chances will be
in the higher terrain and lower in the plains. Ample cloud cover
and potential for upslope enhancement (based on the Euro ensemble
solutions) will mean a much cooler day with highs in the low 70s
but we may need to drop that further if the more ensemble output
favors low clouds and east upslope. In the wake of the trough on
Friday it should be much warmer with highs returning to the low
80s across the plains and 60s in the high country. Convective
activity is likely to mainly impact the mountains and Palmer
Divide, with the best chances (40-60%) along and south of I-70 for
now.

Saturday and Sunday we should dry out as there is high confidence
that ridging is expected to develop overhead once again. However,
isolated shower/storms still cannot be ruled out as PWATs still
remain slightly above average trapped beneath the ridge. Ensemble
guidance indicates slight warming is expected into the upper 70s
and low 80s across much of the area.


&&

.AVIATION /18Z TAFS through 18Z Tuesday/...
Issued at 1129 AM MDT Mon Sep 21 2026

VFR conditions are expected to prevail through the TAF period.

Day cloud phase satellite shows a small patch of cumulus near
DEN/APA, with cloud bases somewhere between 2-4kft AGL. While a
FEW-SCT deck may continue for a couple more hours, guidance
generally lifts/dries out any remaining boundary layer moisture.
Winds should remain light today with a prevailing direction out of
the east.

Winds should slowly turn clockwise this afternoon and evening
before settling on drainage overnight tonight... before weakening
and becoming more variable Tuesday morning. Depending on how much
surface moisture/instability is available in the afternoon, there
could be a few showers or thunderstorms near the terminals by
mid/late afternoon Tuesday. Instability looks weak enough to only
carry a PROB30 for SHRA at this time.

&&

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

&&

$$

DISCUSSION...Schnell
AVIATION...Hiris

NWS BOU Office Area Forecast Discussion