597
FXUS65 KBOU 121809
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
1209 PM MDT Wed Aug 12 2026

.KEY MESSAGES...

- One last day of warm temperatures today. Cooler temperatures
  arrive by Thursday and Friday, with a slow warming trend by next
  week.

- A few strong to severe thunderstorms will be possible over the
  northeast CO plains this afternoon, with damaging winds and
  large hail as the primary hazards.

- Monsoon moisture arrives and lasts through the weekend,
  bringing scattered showers and thunderstorms most days. Locally
  heavy rainfall possible. The best chance of rain across the I-25
  corridor is Thursday/Friday.

&&

.DISCUSSION /Through Tuesday/...
Issued at 359 AM MDT Wed Aug 12 2026

Today is the day of the monsoon`s return! However, that does not
automatically mean that every part of our CWA will receive moisture
equally, so let`s break things down. From a large-scale
perspective, the upper level ridge axis mentioned in previous AFDs
is finally shifting eastwards over the central U.S., which favors
Pacific moisture transport due to southwest flow aloft. This
change in the synoptic pattern alone will promote precipitable
water (PW) values that greatly exceed the climatological average
over the entire state of Colorado, starting today and persisting
through at least Saturday. As such, temperatures will also trend
much cooler, with this afternoon being the last day we have
90-degree highs in the forecast until Tuesday.

Rain coverage will begin to increase over the mountains first early
this afternoon as moisture flows in from west to east, and as the
day`s heating destabilizes the high terrain. Residual dry air near
the surface may diminish the efficiency of rainfall production for
the Denver metro and I-25 corridor, at least early in the afternoon.
High-res model guidance does suggest a second round of showers
moving off the foothills early in the evening , which would provide
a better chance of more scattered rainfall coverage over this
area.

The northeast Colorado plains will have the best chance of seeing
heavier rainfall rates today due to the chance for a few strong
thunderstorms late in the afternoon. The SPC has placed our
northeasternmost counties in a Slight (2/5) risk for severe
thunderstorms, with damaging winds and large hail being the primary
hazards. Higher dewpoints being transported by southeasterly
surface winds, around 30KT of 0-6km shear, steep mid-level lapse
rates (~8 C/km), and MLCAPE >2,000 J/kg would definitely support
the development of a few strong thunderstorms over this area.
Additionally, recent satellite imagery and station plots show an
outflow boundary that originated from last night`s convection over
Nebraska heading towards northeast Colorado. Behind this outflow
boundary are 60-degree dewpoints, which could provide an
additional boost for thunderstorm development in this area.

The forecast remains on track for Thursday afternoon/evening being
the peak of moisture transport for this week`s monsoonal event.
Rainfall production will also be most efficient on this day, with
warm cloud layer depths reaching (and at times exceeding) 5,000
ft, and storm motion vectors ~15KT. Because of these parameters,
the anomalous moisture already in place, and soils likely being
primed from the previous day`s rain, Thursday appears to have the
highest potential for flash flooding. It remains difficult to tell
exactly where the strongest showers and thunderstorms will occur.
On one hand, the mountains require less forcing and instability
due to orographic enhancement. On the other hand, east to
southeasterly surface winds may bring more plentiful moisture onto
the eastern Colorado plains, where better wind shear may support
stronger thunderstorms with high rainfall rates.

Friday does not look too different from Thursday, with showers and
thunderstorms becoming numerous in the early afternoon, from the
high terrain to the low elevations. Ensemble model guidance suggests
a slight decrease in available moisture and instability for Saturday
and Sunday, although both days will still have the necessary
ingredients to produce scattered afternoon showers and storms,
given that the current pattern does not change until Monday or
Tuesday. Even then, long-range ensembles continue to suggest
above-average PWs and an active monsoon through at least the end
of next week.

&&

.AVIATION /18Z TAFS through 18Z Thursday/...
Issued at 1150 AM MDT Wed Aug 12 2026

Winds are currently light and VRB due to a cyclone. A little
uncertain with the wind direction for the next few hours as the
cyclone evolves, but expect a general east/northeast wind early
this afternoon. However, there is a chance winds could turn more
southeast instead.

We are still on track for convection to begin across the high
terrain, with outflow winds making a direction change most likely
to the west/northwest. Some guidance has hinted at the I-25
Corridor missing out on the bulk of precipitation this afternoon
(convection becomes more organized east of the airports), so have
changed to a PROB30 instead of a TEMPO to reflect that
uncertainty. Regardless, any nearby convection should bring strong
wind gusts. With the amount of moisture in the atmosphere, some of
the strongest storms will produce heavy rainfall, so have reduced
visibility in the PROB30 group.

Some uncertainty with wind direction behind the convection this
evening, however generally think it will have an easterly
component to it due to outflow winds from storms east of the
terminals. Light winds expected overnight, with the potential for
low clouds coming in by 10Z-12Z. Have kept the TEMPO in there as
is, but some models indicate ceilings lower than 2000 feet.

For tomorrow, another round of showers and thunderstorms are
possible in the afternoon. With better moisture entering the
region, it is more likely that any shower that occurs will produce
heavy rainfall. For this reason, have introduced a PROB30 with 3SM
as the visibility.

&&

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

&&

$$

DISCUSSION...AA
AVIATION...MAI

NWS BOU Office Area Forecast Discussion