257
FXUS65 KBOU 231153
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
553 AM MDT Sat Aug 23 2025

.KEY MESSAGES...

- Cooler, wetter pattern through the next several days.

- A few showers or thunderstorms this afternoon, and again on
  Sunday. Locally heavy rain possible... but every day comes with
  several forecast challenges.

- More widespread showers and storms Monday, some with heavy rain
  and localized flooding potential.

- Active weather remains through much of next week, with scattered
  showers and storms each day - most numerous over the mountains.

&&

.UPDATE...
Issued at 401 AM MDT Sat Aug 23 2025

One of the more uniquely challenging forecasts for northeastern
Colorado, especially for late August.

There`s been a gradual development of stratus across the Front
Range and I-25 corridor over the past couple of hours. Cloud bases
are widely varying with the lowest stratus near DEN (with 4SM
OVC002 just prior to the 04z ob), while the more prominent
stratus west of I-25 has been a little higher. Guidance has
struggled with the evolution of the low cloud cover since at least
this time yesterday, and there is still significant differences
across the 00/06z suite of guidance regarding how long stratus
hangs around this morning. As a result, there`s a wide range of
temperature possibilities... and one could find support for highs
in the upper 60s to mid 80s across the Denver metro. On the other
hand, there`s fairly good agreement that we`ll stay a little too
stable across the plains for any meaningful rain, with better
chances across the higher terrain of the Front Range.

Guidance again is all over the place with the stratus potential
tonight into Sunday morning. PoPs tick up a bit Sunday afternoon,
but will be dependent on sufficient surface
heating/destabilization.

Beyond this weekend, the overall forecast hasn`t changed
significantly. Cooler temperatures should prevail will well above
normal moisture content... leading to daily chances of showers and
thunderstorms. It still looks like the best potential for heavy
rainfall would come on Monday. Didn`t see a need to deviate too
far from the NBM outside of some minor PoP adjustments.

&&

.DISCUSSION /Through Friday/...
Issued at 233 PM MDT Fri Aug 22 2025

Satellite and radar show one shortwave and area of convection
moving across the Front Range early this afternoon, a bit too
early to support more widespread and stronger storms. In addition,
the cold front that moved through earlier today packed more of a
cooling punch, limiting instability. That said, there`s still
forcing that will push east across the plains into the evening
hours. It appears the main threat of any severe storms would be
shifting far southeast where stronger surface heating occurred.
That`s essentially along and south of the frontal boundary
extending across southeast Elbert, Lincoln, and southeast
Washington Counties. Meanwhile, precipitable water values are
still climbing as dewpoints have pushed into the upper 50s and
lower 60s over most of the plains. So even though other showers
and storms won`t be as intense, they will still be capable of
producing locally heavy rain. They just appear smaller and thus
won`t last as long in any one given area.

For overnight and into Saturday morning, the amount of
instability weakens, but we`ll still have a threat of locally
heavy rain from any showers/storms as the lower atmosphere becomes
nearly moist adiabatic in the low levels and warm cloud depths
deepen. The CAMs are all over the place and very little agreement
(some with stronger storms this evening, others with heavier
showers/storms overnight, and still others like the HRRR with
heavy rain on the plains Saturday morning). At this point,
there`s very low confidence in any one solution. As a result,
we`ll keep a rare but needed extended period of rain chances in
the forecast overnight through Saturday morning.

By Saturday afternoon, the airmass is actually expected to
stabilize on the plains and I-25 Corridor with a capping
inversion and some drying/stabilization aloft. The expected
morning stratus/showers will limit warming and thus the cap should
be able to hold strong through the afternoon and limit any
redevelopment. The mountains should fare better, but probably only
along a relatively narrow sweet spot of moisture and instability
along the Front Range mountains and Park County. Meanwhile, it
stays drier over much of Grand and Jackson Counties. Some of the
showers/storms will still have a reasonable chance to push onto
the adjacent plains late in the day or evening.

Sunday appears to become more active again as the low level
airmass is expected to recover and destabilize. There`s still
quite a bit of uncertainty here, but all ingredients will be in
place for stronger/a few severe storms and locally heavy rainfall.
MLCAPE should grow to 800-1500 J/kg, with sufficient bulk shear.
We`ll have more on the potential for heavy rain and flooding in
the Hydrology section of this discussion.

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 as deep as 8,000 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. It still appears the heavier rain threat should be
shifting slowly south/southwest of our forecast area through
Tuesday and Wednesday, but there is still some uncertainty
depending on how progressive the trough/ridge pattern is. It`s not
impossible the heavy rain producing storm threat doesn`t increase
again Thursday or Friday, but that could very well depend on a
shortwave or two riding across the top of a flattening ridge.

Temperatures will remain well below normal through the entire week
ahead. If more widespread precipitation and stratus does occur,
either Monday or Tuesday could end up with highs staying near/below
70F on the plains and I-25 Corridor. We are essentially a lock
for 70s or cooler those two days, and then only upper 70s to lower
80s at most in the lower elevations on all the other days in the
week ahead.

&&

.AVIATION /12Z TAFS through 12Z Sunday/...
Issued at 545 AM MDT Sat Aug 23 2025

Main impacts will be from stratus this morning. Satellite shows
the edge of the main stratus deck just west of I-25, from just
west of APA through FNL/CYS, while a second patch of stratus
extends from near DEN/BKF and off to the east. Little confidence
in how this evolves over the next few hours, though guidance
generally keeps at least IFR/MVFR cigs through 16-18z at the
terminals. Stratus should scatter out quickly through the late
morning/early afternoon with a return to VFR likely. Some
potential for SHRA/TSRA later this afternoon depending on if
there`s sufficient heating to erode a weak cap. Winds today
should generally remain out of the east to southeast.

Tonight features another complicated forecast with poor model
agreement. At least some potential for stratus especially towards
early Sunday morning, but not confident enough to introduce any
lower cigs at this time. The potential for a Denver Cyclone also
complicates things a bit. Stratus will eventually impact TSRA
chances at the terminals beyond the TAF period during the day
Sunday.

&&

.HYDROLOGY...
Issued at 233 PM MDT Fri Aug 22 2025

For tonight and Saturday morning, locally moderate to heavy rain
will be possible even from the weaker convection on the plains and
I-25 Corridor. That is mostly supported by nearly moist adiabatic
lapse rates in a weakly forced upslope. For the most part, this
is a relatively shallow layer, however there is some chance
(30%) of deeper convection over the northeast plains that could
produce storms with 1-2"/hr rain rates as warm cloud depths grow
to ~5,000 feet. The potential for locally heavy rain Saturday
afternoon should be confined to a relatively narrow corridor from
the Front Range mountains into Park County.

Sunday will feature a chance of stronger storms and locally heavy
rainfall as well, and a little greater threat as long as
convective temperatures can be met. Precipitable water values
start to grow again after a decrease Saturday, and by Monday reach
150-200% of normal. Monday remains our main target day for
potential flash flooding day, as discussed above. There are still
questions regarding if sufficient instability will remain, but it
wouldn`t take much considering the other environmental
characteristics favorable for heavy rain - like warm cloud depths
potentially reaching 8,000 ft.

The main threat of locally heavy rain likely pushes back into the
mountains Tuesday and Wednesday, but still a small threat on the
plains.

&&

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

&&

$$

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