020
FXUS65 KBOU 161124
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
524 AM MDT Wed Sep 16 2026

.KEY MESSAGES...

- Unsettled pattern rest of this week. Thursday, Friday, and
  Saturday all look fairly active. Locally heavy rainfall threat
  appears to be greatest on Thursday, but still a fair amount of
  uncertainty.

- Temperatures warm today, then settle back closer to or slightly
  below seasonal normals through this weekend.

- Trend toward drier weather by early next week.

&&

.DISCUSSION /Through Tuesday/...
Issued at 306 AM MDT Wed Sep 16 2026

Satellite shows a plume of monsoonal moisture streaming
north/northeast across the Desert Southwest and Great Basin. It`s
been a rather active night for showers and thunderstorms over
western Colorado and Utah, extending southward through New Mexico,
portions of Arizona, and into the Gulf of California. It`s that
far southern end in the Gulf of California where convection is
taking on some tropical cyclone characteristics, and may very well
be the main player how our convection and locally heavy rain
threat unfolds Thursday.

For today, models are in pretty good agreement that we`ll see flat
ridging in between a digging trough down the West Coast and a
dominating 600 dm high over the southeast U.S.  This means we`ll
take on a more southerly flow, with warming temperatures and a
return of weak instability. Precipitable water (PW) values will
start to increase, but not until late in the day and especially
tonight as we see greater moisture advection from the south. This
means most showers and storms will likely be confined to the high
country for most of the afternoon, with chances gradually
increasing on the adjacent plains through this evening as we
continue to moisten and see mid level theta-e advection. However,
even in the I-25 Corridor, there`s just a 30-40% chance tonight,
with decreasing odds farther east across the plains where the
airmass will still be under the influence of the flat upper
ridge. Given the increase in PW, a couple of these showers and
storms could produce brief heavy rain.

Thursday is still tricky, as the disturbance in the Gulf of
California lifts northward and into Colorado. Ingredients will be
favorable for locally heavy rain, as PW grows to over 200% of
normal, and above daily observed maximums (outside of
climatology). In fact, tomorrow`s PW will be near the daily
maximums typically seen in the middle of August. While impressive
as that is, warm cloud depths are not tremendously deep given lack
of significant low level moisture, with models now trending to
about 3,500-4,000 feet...still modest. In addition, storm motion
is expected to be quite fast at 25-30 mph on average. Also, the
mid level cloud deck is likely to impact diurnal heating and
destabilization. Finally, and perhaps most importantly, we`re
unsure of timing of that disturbance arriving from the Gulf of
California. It appears the best lift arrives toward 18Z, which
would be a bit early for more numerous heavy rainers. However,
just an hour or two slower could make a big difference. Even if
the ingredients would all come together, it would likely take
training echoes to cause more significant hydrology concerns
considering storm motions. Right now, that greater threat in our
area is shaping up for the northern border area and adjacent
plains where WPC Excessive Rainfall Outlook is into the Slight
Risk category.

Friday and Saturday are shaping up to be quite active as well.
While PW values drop slightly, they will still be near 150% of
normal. Also, we`ll have upper level support with potential to be
in the right entrance region of speed maxes. Those would be
ejecting ahead of the upper level trough lifting slowly east
across the northern Great Basin and Northern/Central Rockies.
Instability may also have a chance to build slightly given a bit
more sunshine and cooling aloft.

Sunday is now shaping up to be the transition day as there`s
modest agreement that the trough axis passes by. Then somewhat
drier weather is still shaping up for early next week. However,
there are trends toward weak troughiness/shortwaves arriving from
the west or southwest so we still can`t rule out a few storms
around. There`s decent agreement that we`ll continue to see cooler
temperatures for this period, with highs averaging near or just
below climatology (sinking into the upper 70s for the lower
elevations). Also, it`s that time of year where any showers will
be capable of putting down a dusting of snow for our highest
mountain peaks.

&&

.AVIATION /12Z TAFS through 12Z Thursday/...
Issued at 523 AM MDT Wed Sep 16 2026

Light west/southwest or VRB winds will start this TAF period.
Winds are expected to gradually transition to a light E-NE around
10 kts 17Z-20Z and hold most of this afternoon.

Main concern will then shift to storm chances. Mid and upper
level monsoonal moisture will increase through late this
afternoon and overnight, 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 and upper level clouds and slight
ridging aloft as to how much we destabilize in the lower
elevations. Thus, we`ll maintain PROB30 for -TSRA/-SHRA mainly
after 23Z-00Z...a rather late chance but that chance could very
well continue through the entire night with a further increase in
moisture and upper level support. Low levels remain dry enough for
minimal threat of cigs below 6000 feet AGL (at least outside of
showers).

&&

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

&&

$$

DISCUSSION...20
AVIATION...20

NWS BOU Office Area Forecast Discussion