625
FXUS65 KBOU 191139
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
539 AM MDT Sat Sep 19 2026

.KEY MESSAGES...

- Isolated to scattered showers/t-storms return this afternoon to
  the mountains and urban corridor. These will become numerous
  over the plains, where some will be strong to severe through
  this evening, producing locally heavy rain and hail.

- Trending drier Sunday into Monday, although scattered afternoon
  storms may continue Sunday, particularly along and south of
  I-70.

- Cooler temperatures expected Sunday into Monday, before warmer
  temperatures return. Next week will bring a few additional
  opportunities for precipitation, mostly for our mountains, with
  lesser heavy rain potential.

&&

.DISCUSSION /Through Friday/...
Issued at 1243 AM MDT Sat Sep 19 2026

Ongoing showers over the eastern plains early this morning will
steadily dissipate as we near sunrise, while moisture will
continue feeding into the high country this morning where showers
will linger. Despite modestly cooler conditions today (highs will
remain below 80F over the plains), we`ll hold onto moderately
unstably conditions and ample moisture. The latest forecast
soundings indicate bulk shear of 35-50 kts across the majority of
the plains this afternoon, which would support another round of
numerous thunderstorms, some of which will be strong to severe.
Thus far, there isn`t much indication of low stratus this morning,
so don`t foresee convective temperatures not being reached.
Instability will be exceedingly marginal though for areas roughly
along and west of I-25, so the potential for stronger storms and
heavier rainfall will be considerably tapered outside of our rural
plains. Hail and heavy downpours will be the primary threats from
today`s stronger convection.

The bulk of the stronger thunderstorms will move out of the plains
by late evening. However, with the upper-level trough traversing
overhead, we`ll retain potential for overnight showers and weak
thunderstorms. Increased PoPs notably for our northern areas in
particular. Will additionally need to monitor potential for patchy
fog Sunday morning across portions of the lower elevations,
especially the South Platte River Valley.

Sunday stands to be our coolest day of the week with the passage
of the shortwave. If we`re able to clear out morning cloud cover
more efficiently, the environment would support renewed scattered
showers/t-storms in the afternoon and evening. It`s a big if, with
the convective potential being notably lower for the northern half
of our forecast area, and best for the Palmer Divide and areas
downstream into Lincoln County where it would be easiest to erode
any low-level capping. Depending on how quickly the low ejects to
our east, isolated to scattered late-day showers and thunderstorms
could linger into Monday, but any coverage should be notably
reduced relative to previous days.

Starting Tuesday, warmer conditions will return as a southwest
flow pattern takes hold aloft. An embedded shortwave late Tuesday
into Wednesday favors an increase in precipitation potential
namely for our mountains, with lesser coverage/more uncertainty
for the lower elevations. Regardless, moisture profiles won`t be
as robust as those currently being observed, so accumulations
should be on the lighter side if we see any.

&&

.AVIATION /12Z TAFS through 12Z Sunday/...
Issued at 526 AM MDT Sat Sep 19 2026

As expected, winds have transitioned to light drainage flow at the
terminals, which has helped limit stratus development this
morning, along with some cloud cover aloft to mitigate radiational
fog. Still, some patchy stratus is evident over the Palmer
Divide, S/SE of KAPA, however it appears unlikely that this would
expand drastically northward to impact KDEN (though stranger
things have happened...). In any case, much like the prior update,
confidence remains too low for any inclusion in the TAFs. If
stratus and fog does migrate over one or more terminals, it should
be very brief, lifting shortly after sunrise.

Looking ahead at the daytime period, expect a transition to
prevailing NE flow by 18-19Z after light and VRB morning winds.
Confidence is higher than previous days in afternoon convective
potential for KDEN and KAPA, however the potential for stronger
TSRA and associated vis restrictions appeared somewhat bullish in
preceding forecasts for areas west of I-25 in particular (i.e.
KBJC). The current forecast thus continues to reflect this
spatial disparity in confidence and/or coverage with a PROB30 for
KBJC. Even KAPA and KDEN will likely be on the western fringe of
the stronger TSRA potential. Timeframe of impacts remains the
same, although have opted to trim the tail end of the TEMPO groups
(now ending 00Z) to closer align with the rapid egress of
convection to the east which is depicted in the latest guidance.
Subsequent impacts should be primarily confined to the NE and SE
KDEN gates, though isolated -SHRA may linger for an additional 1-2
hrs in the vicinity.

As far as the fog potential for tonight is concerned, still
believe there`s a limited threat for any evening impacts given
more prevalent drainage flow, so have maintained VFR conditions
through ~09Z for the terminals. Thereafter, potential for an
incursion of NE flow appears more distinct, and thus too the
fog/low stratus potential. Have therefore introduced some lower
CIGS for KDEN late in the TAF period, though confidence remains
rather moderate.

&&

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

&&

$$

DISCUSSION...BRQ
AVIATION...BRQ

NWS BOU Office Area Forecast Discussion