487 
FXUS65 KBOU 152103
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
303 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 /18Z TAFS through 18Z Wednesday/...
Issued at 1157 AM MDT Tue Sep 15 2026

Low clouds have dissipated recently dissipated and VFR conditions
are expected to prevail for the rest of the TAF period (thru
Wed). ACARS and forecast soundings show a strongly capped 
airmass. The chance a shower survives off the higher terrain and 
reaches DEN and APA is very low, therefore removed the PROB30 for 
showers. Kept the PROB30 for showers at BJC since it is closer to 
the foothills and mountains. Southeasterly winds will prevail this
afternoon and a Denver Cyclone is expected to form. Models show 
the cyclone tracking north of DEN and APA, which would cause 
southeast winds to shift to the southwest this evening. Cross 
sections show a good amount of mid and high cloud cover for 
tonight and Wednesday with ceilings expected to stay above 6000 
feet. Models do not show this, but if the Denver Cyclone tracks 
farther south than expected, northerly winds could pull in enough 
low-level moisture for low clouds towards 12Z Wednesday. A storm 
system moving out of the Desert Southwest will bring scattered to 
numerous showers and thunderstorms to the Central Rockies 
Wednesday afternoon, but the best chance for showers and 
thunderstorms in the Denver area is expected to be after 00Z 
Thursday.

&&

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

&&

$$

DISCUSSION...AA
AVIATION...BM