National Weather Service Forecast Discussion

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533
FXUS65 KBOU 210621
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
1221 AM MDT Mon Sep 21 2026

.KEY MESSAGES...

- Cooler conditions continue Monday, with more limited shower and
  thunderstorm coverage.

- Gradual warming returns Tuesday, with generally isolated
  precipitation chances each day.


&&

.DISCUSSION /Through Sunday/...
Issued at 1215 AM MDT Mon Sep 21 2026

Current radar imagery shows a few strong thunderstorms lingering
over Elbert and Lincoln counties as well as a couple of weak showers
over the northern Colorado plains. Any storm that is able to
strengthen over the next hour or two will be able to produce large
hail and torrential rainfall. However, we believe the severe
thunderstorm threat to be nearing its end for the night as
mesoanalysis shows an overworked atmosphere and decreasing CAPE.
Areas of patchy fog will likely develop due to the low level
moisture left over from today`s convection and small dewpoint
depressions.

Monday afternoon looks to be fairly quiet as a weak ridge builds
over Colorado and subsidence gives way to mostly sunny skies.
Temperatures will remain slightly cooler than normal, with afternoon
highs mainly in the low 70s at low elevations, and 50s-60s for the
mountains. Model guidance suggests a weak shortwave grazing
southeast Colorado Monday night as well as an increase in wind
speeds at the 700mb level. Given the slightly above normal moisture
already in the environment, few sprinkles over the plains cannot be
ruled out late Monday night into Tuesday morning.

The upper level ridge tracks east and flow aloft turns southwesterly
on Tuesday. At least a few isolated showers seem possible over the
high terrain due to the slight uptick in monsoonal moisture, with
the plains remaining dry. One point of uncertainty for Tuesday is
that model guidance suggests breezy southerly winds over eastern
Colorado/western Kansas which could lead to a richer atmosphere
for storm development, although instability should remain fairly
limited.

A disturbance will track north from New Mexico onto Colorado on
Wednesday, which will bring a better chance for showers and
storms over the eastern Colorado plains in the late afternoon and
evening. The mountains will see an increase in shower activity
Wednesday afternoon as well, but it remains unclear whether any
showers from the foothills would survive and make their way onto
the Denver metro or if low level moisture will be transported west
enough. Moisture will generally remain above normal through the
entire forecast period, and models have been consistent in
bringing precipitable water values that are at least 150% of
normal to the region Wednesday through Friday. However, given the
cooler temperatures expected to be in place, lifting mechanisms
will be needed for meaningful rainfall to come to fruition.

&&

.AVIATION /06Z TAFS through 06Z Tuesday/...
Issued at 1110 PM MDT Sun Sep 20 2026

VFR conditions are favored to prevail for the majority of the TAF
period. We are seeing come patchy CIGS of 050-070, although these
should gradually scatter out in the coming hours. With recent
precipitation, there`s a slim chance of patchy fog/low stratus
early morning, but generally 15% or less, and primarily for KAPA.
At this juncture, will continue to exclude from TAFs.

Current convective activity should continue to remain due east of
the metro, though may clip the 5 mi vicinity of KDEN through
07-08Z. Associated weak outflows look to shift winds briefly to
the NE for the first 1-2 hrs of the TAF period, before lighter and
more VRB flow settles in.

On Monday, expect generally east flow to develop by midday, with a
clockwise veering towards drainage flow in the evening. This
drainage could be robust for KDEN and possible KAPA, with speeds
of 10-15 kts after ~08Z Tue.

&&

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

&&

$$

DISCUSSION...AA
AVIATION...BRQ

NWS BOU Office Area Forecast Discussion