496
FXUS65 KBOU 241133
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
533 AM MDT Sun Aug 24 2025

.KEY MESSAGES...

- Wetter pattern continues through the upcoming week with below
  normal temperatures.

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

- Low confidence in any particular day`s rainfall chances across
  the metro due to poor model consistency. Stronger signal for
  rain across the Front Range mountains today and Monday, and
  again by mid/late week.

&&

.UPDATE...
Issued at 332 AM MDT Sun Aug 24 2025

No significant changes to the forecast tonight, as I continue to
be baffled by short-range/convective allowing guidance.

Main question for today is what happens to the frontal/outflow
surge that`s expected to sink in from WY/NE later this morning.
Starting to see that push organize from roughly CPR to BFF. We`ll
see if we can find the right combination of T/Td and upslope to
minimize capping concerns, but in general the best precipitation
chances will be across the higher elevations during the day. There
are signs for showers/shallow convection to develop beyond 00z
this evening, which may be the safer bet for those wanting
precipitation across the I-25 corridor and adjacent plains.

Sunday night into Monday will feature another edition of "will
there be stratus?" across most of the plains... with important
implications for heavy rainfall potential during the day. Too much
stratus likely keeps most locations a little too cool for any
instability (NAM/3km NAM), while the slightly warmer solutions
(GFS) show skinny CAPE profiles and favorable warm cloud depths
for efficient rainfall out of any storms that develop. Given
recent poor model performance with stratus in the 24-36 hour
window, I`m reluctant to do too much to the previous PoPs/QPF.

The pattern looks like it will stay active through most of the
week with a broad area of well above normal moisture (PWAT
standardized anomalies of roughly 1.5-3 sigma) remains over the
region through at least Thursday or Friday.

&&

.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 /12Z TAFS through 12Z Monday/...
Issued at 525 AM MDT Sun Aug 24 2025

Several challenges with the TAFs through Monday morning. Weak
Denver Cyclone has set up this morning but wind speeds are
generally less than 7kt. Should see a more pronounced turn to the
north through 15z, followed by a stronger push of N/NE by late
morning or early afternoon. Guidance is split close to 50/50 if
this will be cool/moist enough for stratus development. FG/stratus
is already well established in SE WY/SW NE, but unusually low
confidence in how that evolves over the next several hours.

The morning stratus potential will have implications on
instability and resulting TSRA potential this afternoon. Model
soundings have LCLs in the 020-035 range so we may see some MVFR
cigs with any showers or storms that develop.

Better agreement for this evening and tonight that showers will
linger near the terminals, along with lowering cigs. Again still
quite a bit of uncertainty if we drift into IFR/LIFR cigs or if a
slightly higher (MVFR) deck develops instead. Have opted on the
more aggressive side through the end of the TAF period given a
better model consensus during this period.

&&

.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.

&&

$$

UPDATE...Hiris
DISCUSSION...Barjenbruch
AVIATION...Hiris
HYDROLOGY...Barjenbruch