591
FXUS65 KBOU 171938
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
138 PM MDT Thu Sep 17 2026

.KEY MESSAGES...

- Active weather pattern through Saturday. Locally heavy rainfall
  will be the greatest storm threat, although a couple severe
  storms possible Saturday.

- Near or just above normal temperatures settling below normal for
  Sunday and Monday.

- Trend toward drier weather by early next week.

&&

.DISCUSSION /Through Thursday/...
Issued at 138 PM MDT Thu Sep 17 2026

A very moist south-southwest flow aloft will continue through
Friday between an upper level high over Texas and an upper level
low to the northwest. Latest ACARS soundings show precipitable
water valuesof 1.2 inches, about 220 percent of normal. The
soundings also show a long skinny CAPE profile of about 500 J/kg,
which favors heavy rainfall. Storms willmove somewhat quickly,
which will limit the heavy rain and flooding threat. Best chance
for thunderstorms through this evening will be east of I-25 where
it`s been mostly sunny the past few hours and temperatures have
climbed into the upper 70s to lower 80s. To the west of I-25, it`s
been cloudy most of the day with on and off light rain.
Temperatures have been slow to warm with 1:00 PM readings in the
upper 60s to lower 70s. Light rain showers are expected to linger
into the evening, with the radar showing additional showers lined
up to the south-southwest.

The upper level low will slowlytravel eastward across southern
Idaho and Wyoming this weekend. Moisture decreases, but remains
well above normal (140-180 percent of normal) through Sunday. Mid
level temperatures will cool as the upper levellow nears the
region. MLCAPE reaches 600-1200 J/kg both days. Increasing
westerly flow aloft around the upper level low will help produce
good shear Saturday with 25-40 knots of bulk shear (06 km) and
35-50 knots of bulk shear (06 km) Sunday. High temperatures are
expected to be mainly in the 70s with Sunday being a few degrees
cooler, which could end up capping convection. The passage of a
shortwave trough is expected to result in better
shower/thunderstorm coverage on Saturday. Models hint at heavier
rainfall over the northeast plains after 00Z Sunday.

On Monday, the upper level low will weaken as it exits the
region. A new upper level low/trough moves over California and the
Great Basin. In the meantime, an upper level ridge will be
centered over Texas. For Colorado, weak ridging is expected to
prevail, resulting in dry conditions.

For Tuesday through Thursday, an upper-level ridge remains over
the south central and southeast part of the country, while
troughiness will be over the southwest. This pattern will allow
shortwave troughs to lift northward out of Mexico/the eastern
Pacific and move across the region. There isn`t much agreement yet
on the location and timing of the shortwave troughs, but there
should be a day or two with at least scattered showers and
thunderstorms. Temperatures during this time frame will be near
normal with highs in the 70s across northeast Colorado.

&&

.AVIATION /18Z TAFS through 18Z Friday/...
Issued at 1221 PM MDT Thu Sep 17 2026

Showers have started popping up over the foothills this morning,
with several skirting over KBJC. Throughout the afternoon and
evening, we expect the greatest threat for showers/heavy rain to
be over the foothills and northeastern plains, though we can`t
rule out some scattered showers or storms over the metro area.
While there`s not much instability, plenty of moisture has been
brought in from the south, and so, should any showers or storms
move over the TAF sites, locally heavy rainfall would be the main
concern. Still, confidence regarding storms over the TAF sites is
lower than it was yesterday, so we`ve dropped the previous TEMPO
group down to a PROB30 for this afternoon and evening. Most models
keep precipitation in the area through at least 02Z, with
rainfall lingering a little longer in the vicinity of KBJC. As
showers leave the area, winds should be light and variable before
settling into drainage overnight.

The late afternoon/evening tomorrow will be our next best chance
for storms/showers. As such, we`ve introduced a TEMPO starting at
22Z and lasting through the end of the TAF period; that being
said, model guidance currently suggests the bulk of convection to
be later (23Z-03Z), so we`ll likely be extending the TEMPO group
at the 00Z TAF update this evening.

&&

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

&&

$$

DISCUSSION...BM
AVIATION...AGS

NWS BOU Office Area Forecast Discussion