385
FXUS65 KBOU 160539
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
1139 PM MDT Tue Sep 15 2026

.KEY MESSAGES...

- Cooler weather today. Limited shower and storm chances, mostly
  over the higher terrain.

- Temperatures bounce back tomorrow, but then average near or
  even slightly below seasonal normals through next weekend.

- Unsettled pattern rest of this week. Thursday looks like the
  highest opportunity of measurable rain and a locally heavy
  rainfall threat.

&&

.DISCUSSION /Through Tuesday/...
Issued at 256 PM MDT Tue Sep 15 2026

Temperatures across our entire CWA are currently in the 60s and 70s,
with even cooler readings at mountain peaks and ridges, a reminder
that fall is just around the corner. However, the monsoon is not
over just yet, with daily chances for showers and storms persisting
through at least Saturday. Rain chances today remain very low (<20%)
for the Denver metro and east CO plains given the cooler
temperatures and limited instability as we sit in a post-frontal
airmass. The high terrain will have a better chance for showers and
storms late this afternoon and evening, although coverage will be
scattered to isolated in nature.

At the upper levels, a closed low pressure system over northern
California and slight ridging to our northwest will turn flow aloft
south-southwesterly by tomorrow afternoon, which will increase
monsoonal moisture advection. We expect shower coverage to increase
tomorrow afternoon and evening, with widespread shower development
over the mountains as precipitable water (PW) rises to values that
are greater than the 90th percentile of climatology. Temperatures
over the Denver metro will still have a chance to climb into the
80s before cloud cover increases late in the afternoon.

By Thursday morning, a disturbance tracks through Arizona and onto
southern Colorado. As the previous AFD stated, the arrival of this
disturbance will be key in the coverage and intensity of rainfall on
Thursday. An arrival by early afternoon would coincide with peak
instability and moisture, which would imply a better chance for
widespread showers and storms over both mountains and east CO
plains, including the Denver metro. With this scenario, the threat
for localized heavy rainfall and flash flooding would also be
higher. Looking at flash flooding parameters, moisture alone looks
quite impressive, with PW values as high as 5 standard deviations
above normal for the time of year. High-res model guidance also
shows warm cloud layer depths as deep as 4,000-7,000 ft, and storm
motions around 20KT. With these ingredients in mind, at least some
localized flash flooding seems possible, although the spatial
footprint of the heaviest precipitation remains unclear.

For Friday, moisture remains about 100-150% of normal, and a weak
upper level trough begins to track eastwards providing yet another
push for showers and storms. Depending on the evolution of this
trough and another shortwave to the south over the Four Corners,
Saturday could also be an active day, particularly for the high
terrain as moisture remains well above normal through Saturday
evening. Temperatures will remain slightly below normal Thursday
through Tuesday, with afternoon highs in the mid 70s for low
elevations, and 50s-60s for the mountains. Long-range model
ensembles show a series of upper level troughs and disturbances
tracking through the western United States, which introduces quite
a bit of uncertainty next week.

&&

.AVIATION /06Z TAFS through 06Z Thursday/...
Issued at 1139 PM MDT Tue Sep 15 2026

Denver cyclone has become rather elongated as of 06Z and stretches
N-S from just west of KAPA and KDEN northward to around KFNL.
That zone is creeping slowly eastward so expect southerly winds
around 15 kts shifting more W-NW by 08Z at KDEN, while becoming
VRB at KAPA. As the cyclone continues to weaken overnight, we may
see periods of lighter SW winds at times. KBJC should stay more
northwesterly, but all winds are light (<8 kts) after the shear
zone passes by. Airmass is too dry to support any chance of
stratus. Winds are expected to gradually transition to a light
E-NE around 10 kts 17Z-20Z and hold most of Wednesday afternoon.

Mid and upper level moisture will increase through Wednesday and
Wednesday night ahead of an upper level disturbance moving across
the Desert Southwest. This will bring more numerous showers and
storms to the mountains, but there is still uncertainty given the
mid level clouds as to how much we destabilize in the lower
elevations. Thus, we`ll maintain PROB30 for -TSRA/-SHRA mainly
after 00Z...a rather late chance but that chance could very well
continue through Wednesday night with a further increase in
moisture and upper level support.

&&

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

&&

$$

DISCUSSION...AA
AVIATION...20