847
FXUS65 KBOU 221137
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
537 AM MDT Fri Aug 22 2025

.KEY MESSAGES...

- Pattern change begins today. Temperatures a little cooler, with
  a few strong to severe storms this afternoon.

- Cooler, wetter pattern likely through most of next week.

- A few scattered strong storms again Sunday. Better chances for
  widespread rainfall... including the potential for heavy rain...
  by Monday.

&&

.UPDATE...
Issued at 433 AM MDT Fri Aug 22 2025

For the first time in quite a while, I`m writing an AFD that (1)
includes more than one day of legitimate precipitation chances,
and (2) does not have any mention of 90+ temperatures across the
Denver metro. There aren`t many changes to the overall pattern
and forecast thoughts described in the previous shift`s
discussion, but there are a few things worth noting this morning.

For today... still expecting a frontal push into the plains later
this morning, though guidance is maybe a touch slower than before.
Assuming we do see upper 50s/near 60F dew points, there should be
enough instability for at least a few stronger storms to develop
across the high country and spread into the lower elevations.
Deep-layer shear should increase as the low-level upslope flow
strengthens later this afternoon, with forecast soundings
suggesting around 30-40kt bulk shear magnitudes later today.
Overnight CAMs are fairly bullish developing a cluster of storms
across Larimer county into the I-25 corridor, and at least a
marginal hail/wind threat seems reasonable.

There`s a bit more uncertainty tonight into Saturday, as the
upslope flow weakens (or at least is temporarily disrupted by
thunderstorms/outflow). HRRR/RAP/ECMWF runs are fairly insistent
on another push of moist upslope developing overnight tonight,
with elevated convection across the plains with more of a
stratus/light rain setup along the base of the foothills. ECMWF
ensembles are similarly bullish on the rainfall potential through
Saturday morning, with 24 hour (18z today through 18z Saturday)
mean QPF near a half inch at DEN. With forecast soundings showing
a very deep stratus layer, it`s not a surprise that there are a
few solutions much cooler for Saturday`s high temperatures. I did
nudge temperatures down close to 5 degrees (mid 70s), but the
ECMWF/HRRR solutions would be a struggle to make it out of the
upper 60s. We`ll let the day shift dive into this a little more.

By Sunday, it does look like we`ll warm back up into the upper
70s/low 80s. If we hold onto the low level moisture, we could see
another round of strong to severe storms during the afternoon
hours. Another day of potentially heavy rain is possible early
next week, with guidance still favoring Monday. The cool/moist
pattern looks to continue through the end of the forecast period.

&&

.DISCUSSION /Through Thursday/...
Issued at 154 PM MDT Thu Aug 21 2025

Satellite imagery shows a band of monsoonal moisture rotating
northeast around the periphery of the upper level high that`s
bringing near record heat to the forecast area today. Meanwhile, a
cold front has pushed through much of eastern Wyoming. A few
showers/storms will continue to develop late this afternoon and
evening over the mountains on the periphery of this moisture, as
well as in the vicinity of the temporarily washed out frontal
boundary. This means the northern tier of plains should also see a
few of these showers/storms into late evening. While a
prefrontal/outflow wind shift or two can be expected, the main
frontal surge will likely occur early Friday morning which starts
a relief to the summer heat and recent dryness.

Surface moisture will increase behind the front on Friday, with
dewpoints expected to push into the mid 50s along the Front Range
I-25 Corridor, and up to the lower 60s over the northeast plains.
Moisture aloft will also be increasing as the upper level ridge
begins to retrograde, allowing the upper level moisture plume to
shift over us as well. Thus, we`ll end up with considerably higher
precipitable water values (near 1.2-1.25" for the I-25 Corridor
and slightly higher east). Instability will also grow in the post-
frontal upslope environment, with MLCAPE expected to reach
1000-1200 J/kg. Meanwhile, bulk shear strengthens to 30-35 kts,
enough to sustain rotating updrafts and a couple severe storms.
Large hail would be the primary threat, but also heavy rainfall.
The deeper moisture combined with modest instability and a deeper
warm cloud depth (4,000-5,000 ft) means storms be capable of
producing locally heavy rainfall of 1-1.5" in 30-45 minutes. Burn
scars would be most susceptible to any flash flooding, but these
rain rates can also produce local minor flooding in urban areas.

For Saturday, the trend over the last 24-36 hours is for less
convective coverage, likely related to brief drying aloft working in
from the northwest. Forecast soundings also suggest some capping
between 650 and 700 mb. However, it`s not certain that cap will hold
into the evening if weak subsidence can reverse signals with any
passing disturbance. Something to watch for since our low levels
will still be moist and airmass modestly unstable (MLCAPE
800-1200 J/kg).

If Saturday is a down day convection-wise, Sunday should become more
active again as we`ll have abundant low level moisture and some
forcing through another frontal surge, upslope, and weak QG
forcing. MLCAPES are advertised to again reach 800-1200 J/kg with
the greatest instability in/near the Front Range and I-25
Corridor. Thus, a couple strong/severe storms would be possible
given sufficient shear in northwest flow aloft. Heavy
rain/localized flooding (mainly burn/urbanized areas) will be a
threat as precipitable water (PW) values climb back to near 1.2"
for the I-25 Corridor.

The greatest risk of heavy rainfall and localized flooding issues is
still expected to occur Monday. There is still good agreement in
the ensembles that this is the day when all heavy rain/flash flood
ingredients come together. Those include; 1) PW increasing to
around 1.25", or 150-200% of normal and near climatological
maximums. 2) moist adiabatic/skinny CAPE profiles, 3) Deep warm
cloud depths potentially to 8,000-9,000 ft, and 4) sufficiently
slow storm motions of 15-25 mph, but just enough shear/upslope to
keep storm cold pools (as weak as they may be) balanced with storm
motion. Something we`ll certainly keep an eye on. At this point,
the heavier rain threat should be shifting slowly south/southwest
of our forecast area through Tuesday and Wednesday, but still
some threat depending on how progressive the trough/ridge pattern
is.

Temperatures will be well below normal through at least the middle
of the week. If more widespread precipitation and stratus does
occur, it`s not impossible that either Monday or Tuesday could end
up with highs staying near/below 70F on the plains and I-25
Corridor. We remain almost a lock for 70s or cooler those two
days, and only near 80F in the lower elevations on the other days
from this weekend through next Thursday.

&&

.AVIATION /12Z TAFS through 12Z Saturday/...
Issued at 535 AM MDT Fri Aug 22 2025

Weak winds this morning should turn to the north or northeast
sometime between 15-18z as a frontal boundary pushes through the
terminals. Scattered convection is likely this afternoon into the
evening, with the best chance of impacts from about 22-03z this
afternoon and evening. Winds should prevail out of the northeast
outside of any convective outflow.

Lower confidence tonight into Saturday morning. Still a good
signal for widespread stratus development after 06z tonight, with
IFR cigs likely at the terminals by the end of the TAF period.
Stratus is expected to persist through the first half of the day
Saturday with continued MVFR/IFR cigs likely. Can`t rule out some
periods of LIFR conditions, especially for DEN/BJC near 12z
Saturday morning.

&&

.HYDROLOGY...
Issued at 154 PM MDT Thu Aug 21 2025

We`ve addressed much of the hydro threats and meteorological
parameters in the Discussion section above, but there will be
some threat of flash flooding for the burn scars on Friday
afternoon/evening as precipitable water values and rainfall
efficiency increases. The burn areas have recovered over the last
few years, and the limited flash flood threat outside of the burn
scars would preclude the necessity of a Flash Flood Watch. The
most prone burn scar would be Alexander Mountain. Urban areas
would also be prone to generally minor flooding of roads and
underpasses.

We`ll monitor the threat for flooding and the latest data for
Saturday and Sunday. The highest threat for flooding is still
shaping up for Monday when more widespread hydrologic highlights
may be required.

&&

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

&&

$$

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