345
FXUS65 KBOU 240015
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
615 PM MDT Sat Aug 23 2025

.KEY MESSAGES...

- Cooler, wetter pattern through the upcoming week.

- Showers and thunderstorms each day with locally heavy rain, but
  each day comes with several forecast challenges.

- Most active days should end up Sunday and Monday, with locally
  heavy rain in/near the mountains, I-25 Corridor, and Palmer
  Divide.

&&

.DISCUSSION /Through Saturday/...
Issued at 255 PM MDT Sat Aug 23 2025

Another day, and just as much uncertainty in the forecast
regarding convective evolution. We started today with some low
topped, warm core showers west of I-25 that produced some decent
rainfall amounts - peaking around 0.60 inch in the immediate
Lafayette area. Pretty impressive considering the low radar
returns, and indicative of just how efficient these warm rain
processes can be. Those morning showers have since dissipated but
have given way to more typical diurnal convection over the
mountains and foothills. We anticipate these showers and storms to
continue through late afternoon, while also attempting to move
into the I-25 Corridor and Plains. That`s where the uncertainty
enters the forecast - yet again. Looking at ACARS soundings,
there`s a solid cap in place near 650 mb. It appears it would have
to warm into the 83-85F range to break the cap, and we`ve just
recently popped to 82F along the I-25 Corridor (as of 2:50 pm).
There`s still partial sunshine and a lack of widespread anvil
blowoffs so there is a chance (30-40%) we could still hit those
temps and break the cap before too long. IF we do, then stronger
and even a couple severe storms would be possible into the lower
elevations given MLCAPE around 1000-1500 J/kg, and relatively
strong bulk shear greater than 40 knots. Also, it was interesting
to note a couple CAMs still had a stronger storm or two dropping
south from Wyoming and into the northeast plains later this
evening. For later tonight, we expect skies to clear but that
could lead to patchy fog development late tonight and early
Sunday.

Sunday still appears to be shaping up as a more active weather
day, although uncertainty lingers. Low level moisture will hold
strong along/east of the Front Range. However, we will be
relatively dry aloft to start the day. But that should change as
mid/upper level moisture is advertised to deepen from the west
and southwest as the upper level jet stream shifts back eastward
into the Central Plains. MLCAPE is forecast to be 800-1400 J/kg
and still sufficient shear for a few stronger/severe storms.
However, we will still be battling a capping inversion, so again
it`s a conditional risk of stronger storms. We do think we`ll have
higher coverage over the mountains during the afternoon and then
eventually eastward onto the plains, however. Some of these storms
will also be capable of producing heavy rain as precipitable
water (PW) values climb to 150% of normal.

Now Monday...what has consistently looked to be the wettest day
and highest concern of heavy rain and any potential flooding
issues has trended slightly drier. That said, we`ll still be near
150-175% of normal pw, but we may also be battling some
stability/capping issues. The trend seems to be more drying/
stabilization from the north, so the focus of heavier rain could
actually end up southwest of Denver or even into southern
Colorado. We`ll continue to monitor the latest trends here.

Tuesday and Wednesday the upper level ridge will build, but the
deep monsoonal moisture plume will hold strong underneath it. We
think the mountains will see high coverage, but we`ve tried to
trim back the highest National Blend of Model (NBM) PoPs from the
lower elevations as it`s difficult to get widespread convective
coverage without any forcing.

We`re still looking at potential shortwaves Thursday and/or Friday
for an uptick in coverage and storm intensity. There are signs
that we`ll start to transition back to a more routine weather
pattern sometime next weekend or early the following week.

&&

.AVIATION /00Z TAFS through 00Z Monday/...
Issued at 535 PM MDT Sat Aug 23 2025

Convection is currently underway around and south of the TAF sites
this evening. While most of the activity has already passed near
the TAF sites, ACARS soundings still show plenty of CAPE to work
with if A. one of the cells currently in northern Larimer County
makes its way over the TAF sites as portrayed in some of the hi-
res guidance, or B. an outflow or boundary initiates enough lift
to overcome the small cap still in place as portrayed as -8 J/kg
of CIN in the latest ACARS sounding. Have kept the PROB30 in place
due to these factors for -tsra potential until 1Z at KAPA/KBJC,
and added on one additional hour for KDEN (through 2Z). Once
convection diminishes, expecting KAPA/KDEN winds to transition to
the south for the overnight hours, with potential for a rogue
shower or storm to pass east of KDEN that could bring a brief wind
shift to the ENE-E between ~6-7Z. KBJC should see light and VRB
winds before turning to the WNW late tonight. Fog will still be a
concern for early Sunday morning as winds are still expected to
take on a more northwesterly turn that could advect it south from
Larimer/Weld Counties, with the greatest impacts likely remaining
north of KAPA.

For the afternoon and evening, there will be another chance for
afternoon convection beginning between the 21-22Z time frame
capable of producing gusty winds and moderate to heavy rainfall as
precipitable water values are expected to surpass 1.2" (~150% of
normal).




&&

.HYDROLOGY...
Issued at 255 PM MDT Sat Aug 23 2025

Main concerns will be for the burn scars Sunday as precipitable
water (PW) values increase to around 150% of normal. Warm cloud
depths increase to near 6,000 feet, so storms will be more
efficient rainers with 1-2" possible from stronger storms in 45
minutes or less. That could also produce some localized concerns
in lower elevations (underpasses/low lying areas), should
stronger storms be able to sustain themselves.

As discussed above, Monday is trending a bit drier and more stable
from the north, but it could still end up being the peak concern
day for hydrology. We will continue to monitor those trends.
Tuesday and Wednesday will have the most hydrologic concerns for
the mountain burn scars given richer moisture/instability profiles
there versus the plains. Thursday and Friday are starting to
shape up as more active days again, but a lot of uncertainty with
timing of any potential shortwaves.

&&

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

&&

$$

DISCUSSION...Barjenbruch
AVIATION...Bonner
HYDROLOGY...Barjenbruch