460
FXUS63 KARX 261041
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
541 AM CDT Sat Jul 26 2025

.KEY MESSAGES...

- Hot Sunday and Monday. Moderate (40-60%) confidence in heat
  indices of 90-100 degrees for Sunday and 100-110 degrees for
  Monday, warmest south of Interstate 94.

- Storms possible Sunday night into Monday morning. Highest
  confidence (40-60%) lies in central Wisconsin. Much to work
  out as current timing places local forecast area on
  southeastern periphery of higher confidence.

&&

.DISCUSSION...
Issued at 337 AM CDT Sat Jul 26 2025

Limited Storm Potential in NE Iowa and SW Wisconsin Today:

Monitoring convection moving northeast from northwestern Missouri
through southern Iowa on radar early this morning. The
responsible low level jet can be seen on VWPs through Kansas
with upstream anticyclonic curvature become west near eastern
Iowa. As a result, expect a more easterly trajectory as storms
continue through the morning hours than current observations
(26.08Z). A second low level jet (VWPs) points into the Northern
Plains causing a split in upstream flow locally, influencing
anticyclonic flow seen at KARX. Low level moisture transport
will follow these two axes through the morning hours, ushering
in a drier airmass locally, and abating highest precipitation
potential farther south. Depending on exact location of the
surface boundary, could see some storms pop north into far
southwest Wisconsin although will be on a weakening trend should
they attempt points farther north as they interact with more
stable, dry air.

Extreme Heat Potential Sunday & Monday:

The anomalous ridge (RAOBs vs. SPC climatology) over the
southeastern CONUS on early morning water vapor imagery will begin
regressing overnight into Sunday. Resultant composite mean
reanalysis parameters of 850mb Ts (24C+) and 500mb height
anomalies (70m+) for hottest days will be met across the Upper
Mississippi River Valley.

Confidence in excessive heat is highest on Monday as the strongest
return flow today into the Northern Plains zonally advects
east. Current confidence (40-80%) paints 90-100 degree apparent
Ts across much of the forecast area for Sunday. This is a slight
decrease compared to previous forecasts as shorter forecast
window has increased membership in the NBM and brought maximum
surface dewpoint temperatures from near 80 to the mid and upper
70s.

However, dewpoint temperatures remain a determining factor for
Monday`s heat indices with more moist solutions painting the 80
degree isodrosotherm causing heat indices above 100F. NBM
membership is being influenced by a NAM/GFS solution of
widespread low and mid 80s maximum surface dewpoint
temperatures. Therefore, have blended in bias-corrected models
in collaboration with neighboring offices to bring apparent
temperatures closer to overall confidence (40-70%) of 100F to
110F along and south of Interstate 94.

Based on current forecast confidence, heat headline criteria
(Advisory:100-104F) wouldn`t be met for Sunday while both
warnings and advisories would be met on Monday. Given the
longer window and only select counties potentially requiring a
Watch (Warning), have opted not to issue heat headlines at the
current forecast hour but will need to be assessed in coming
forecast.

Storm Chances Sunday Night Into Monday Morning:

Low level moisture ramps up locally within a nocturnal low level
jet sagging southeast from mean synoptic flow of a longer
synoptic trough progressing the Rocky Mountain West. Local
impacts will ultimately depend on contention between this
synoptic trough and the dominating synoptic-alpha anomalous
ridge. While the warm, moist air will provide ample instability,
current post-peak heating timing suggests a capping inversion
challenging storm access. Current confidence keeps progression
of the low level jet northwest of the forecast area, limiting
low level shear while influence of the upper level ridge
challenges mid level shear.

High resolution models have started seeing into this 48-60 hour
forecast window, with the Fv3 (26.00Z) and NAM4km (26.06Z)
suggesting an MCV advecting south through Minnesota and Iowa
into Monday morning. Given previous reasoning, current
confidence does not agree with this scenario from the ridge
exerting its strong shield weakening storms as they push south.
A more plausible solution seen in HRRR (26.06Z) with storms
rounding the ridge periphery. Much to work out still/yet.

&&

.AVIATION /12Z TAFS THROUGH 12Z SUNDAY/...
Issued at 536 AM CDT Sat Jul 26 2025

IFR/LIFR stratus and fog are spreading northward as of issuance
time. Expect these to give way to VFR conditions early this
morning. However, cumulus in the 2000-4000 foot layer is
possible during the day today so cannot rule out a stray MVFR
ceiling being reported. As for TS, this looks to remain in the
far southern portion of our area today, mainly south of an OLZ-
OVS line. Looking ahead to tonight, fog may very well return,
especially in NE IA and SW WI, but confidence is too low to
include in RST/LSE TAFs with this issuance.

&&

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

&&

$$

DISCUSSION...JAR
AVIATION...Ferguson