719
FXUS63 KARX 211053
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
553 AM CDT Thu Aug 21 2025

.KEY MESSAGES...

- This morning will have a mix of fog and clouds with some areas
  seeing locally dense fog. This should improve quickly this
  morning with quiet weather to follow today.

- Friday evening has low chances of a strong or marginally
  severe storm but it is very conditional and mainly in
  southeast MN and northeast IA. Scattered showers and a few
  garden variety thunderstorms look like the more likely
  outcome, shifting east across the area during the night.

- Much cooler air will move in Saturday and continue into next
  week with temperatures 10-15 degrees below normal with 60s for
  high temperatures Sunday and Monday looking probable (80%
  chance). Some showers will be around Sunday, higher chances
  (50%) in northcentral WI, decreasing southwestward.

&&

.DISCUSSION...
Issued at 155 AM CDT Thu Aug 21 2025

Showers and Storms Friday - Low Severe Storm Risk

Friday will be the last day of summer warmth and mugginess for a while
as a strong cold front plows through Friday night. Some fog
would be possible early in the day Friday across southwest WI.
Much of the strong QG forcing aloft and low-level moisture
transport convergence is well north of the area for any
organized storm threat. Ahead of the front, weak shortwave
trough energy in westerly flow moves in locally during the day
with clouds and possibly showers and isolated storm chances
leveraging weak elevated instability. This early wave packet,
which is well agreed on, will muddle the forecast as it looks to
hamper surface-based instability growth in the afternoon. There
seems to be indications that MLCAPEs of around 1000 J/Kg could
be present, with skinny CAPE profiles in modest 6.5C/km mid-
level lapse rates by late afternoon northwest of La Crosse. But
confidence in reaching that CAPE is lower due to the pre-frontal
waves. Deep layer bulk shear over 35 kts is to the northwest of
the area, with only a 20% probability of over 30kts from 0-6km
in sern MN.

One larger limiting factor, aside from the instability questions
and lack of wind shear, appears to be the early wave and a
preference toward a more westerly wind component in the low-
level wind field. This tendency ahead of the front reduces
convergence overall, presenting as a broad cyclonic wind field
with any frontal cold advection to the northwest as well /KINL-
KABR/. This is where much of the Cape/Shear overlap is signaling
for storms to be stronger...central MN in the 21.00Z HREF at 7
pm Friday. The increased westerly component in low-levels also
reduces the bulk wind shear with westerly flow aloft. Should
MLCAPEs reach 1000 J/Kg, removing the capping inversion, the
lack of convergence would limit storm coverage and intensity
with severe storm chances looking low. So, overall a low
confidence in severe storms for Friday unless we see better
signals develop. SPC has a marginal risk /level 1/ for severe
storms. Timing would be mainly Friday evening.

A Return To Fall Weather, Sunday Showers

The strong cold front and cold advection moves through the area
early Saturday morning with a cooler breezy day on tap.
Stratocumulus clouds should advect in from the northwest as the day
wears on under cold advection. A sprinkle might be possible. The
cold pool aloft moves over Sunday with increased forcing and
saturation in the lowest levels. It appears it will be a showery day
in northcentral WI (60%), with rain chances decreasing
southwestward per LREF 20.12Z guidance. Latest NMB guidance has
nearly a 0% chance of Sunday showers due to its low QPF-event
(~0.05" or less) bias correction toward 0. Have trended toward
the LREF in the forecast for Sunday as 21.00Z deterministic
model forcing and soundings support the showers suggested in the
LREF.

Temperatures from Saturday through next Wednesday will be some 10-
15F below normals for late August. There are even some hits for
less than 40F Tuesday morning north of I-94 in the 24.12Z 100
member Grand Ensemble. Most records are in the upper 30s to low
40s for low temperatures and near 60F for highs. The 20.12Z
ECMWF Ensemble EFI does suggest an anomalous temperature event
but not record, and much the same for the NAEFS suggesting -2 to
-3 standardized anomalies (over 3 is usually near record).

&&

.AVIATION /12Z TAFS THROUGH 12Z FRIDAY/...
Issued at 553 AM CDT Thu Aug 21 2025

Based on current satellite imagery and surface observations,
there is quite a bit of river valley fog out there with
many locations seeing areas of dense fog. The one river valley
experiencing very little fog is the Mississippi River Valley.
There remains a low chance of some valley fog forming in the
Mississippi, however there is an area of MVFR/IFR stratus moving
from east to west across central and southern Wisconsin that may
ruin any chance at seeing fog along the river. Any fog or low
stratus is expected to diminish or move out of the area by the
mid morning hours, however there remains a chance that the low
stratus deck could linger into the early afternoon. Light
southeast winds shift to southwest winds this evening.

&&

.ARX WATCHES/WARNINGS/ADVISORIES...
WI...None.
MN...None.
IA...None.

&&

$$

DISCUSSION...Baumgardt
AVIATION...Cecava