997
FXUS63 KARX 052356
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
656 PM CDT Tue Aug 5 2025

.KEY MESSAGES...

- Shower and storm chances (20-50%) for late Wednesday and into
  Thursday. Additional probabilities (40-60%) for showers and
  storms for this weekend.

- Gradually warming and becoming more humid this week with.
  Friday is currently expected to be the warmest day with heat
  indices in the middle to upper 90s for most.

&&

.DISCUSSION...
Issued at 1259 PM CDT Tue Aug 5 2025

Today - Friday: Gradual Warming, Mid-Week Shower and Storm Chances

GOES-19 water vapor imagery and 05.15z RAP 500mb heights depict weak
high pressure pushing east of the area this morning which has been
responsible for the subsidence keeping our smoke in place over the
past several days. As this treks eastward, quasi-zonal flow in the
mid-levels will return with some embedded shortwave
perturbations. The first of these shortwave perturbations may
be even as early as today with some of the recent CAMs runs
trying to initiate some weak shower activity. However,
instability profiles are really not overly impressive for today
with values of MLCAPE in the 05.15z RAP for later this afternoon
being only around 250-500 J/kg and there being a fairly dry
sub-cloud layer. As a result, not overly confident in any
precipitation chances (5-15% chance) for this afternoon west of
the Mississippi River.

Shower and storm chances increase some as we head into
Wednesday/Thursday as deterministic guidance (NAM/RAP/GFS) display
some more robust mid-level shortwave perturbations that move
through the area coupled with increasing 850mb moisture
transport. Overall highest confidence given the current state of
model trends puts the nose of the 850mb transport axis into the
local area overnight Wednesday and into Thursday. With
dewpoints increasing into the upper 60s to near 70 during this
period, instability profiles become slightly more conducive for
convective development albeit with fairly notable capping issues
as shown in the 05.15z RAP. However, with the recent NBM only
have medium probabilities (40-70%) for measurable precipitation
during overnight Wednesday and into Thursday, have kept
precipitation chances consistent with the NBM 30 to 50% for now
given the uncertainty on how much capping will be present.

As far as temperatures are concerned, the quasi-zonal flow pattern
will trend more northwesterly as some upper-level ridging attempts
to sneak into the region later into the work week. This will in turn
allow temperatures to gradually warm through the 80s this week and
will culminate in Friday likely being the warmest day with
median high temperatures across the area in the NBM ranging
from the upper 80s to lower 90s. With dewpoints increasing into
the upper 60s and 70s, heat indices will increase with values in
the middle to upper 90s on Friday.

This Weekend: Increasing Chances for Showers and Storms

As we head into the weekend, guidance generally agrees on starting
to pivot a broader synoptic trough through the Northern Plains for
the weekend which will have implications for both temperatures and
convective trends. With the presence of a surface boundary during
the weekend in the Upper Midwest, expecting convection to
develop on it with the main question being the exact location
and timing. Overall, with the larger synoptic trough to the
northwest, some shear may be present. However it remains
difficult to discern how and if this shear is overlapped with an
unstable airmass further south of the trough into the local
area and if this airmass is uncapped. AI and machine learning
severe outlook guidance (CSU- GEFS based/Pangu/FengWu) does
highlight portions of our region with a 15% probability for
Saturday so will be at least a period worth monitoring over the
coming days to see how it trends. Regardless, certainly appears
to be the highest confidence period for any precipitation over
the next 7 days as the grand ensemble (GEFS/EC/Canadian
ensemble) has fairly high (70-90% chance) probabilities for
measurable precipitation this weekend.

&&

.AVIATION /00Z TAFS THROUGH 00Z THURSDAY/...
Issued at 628 PM CDT Tue Aug 5 2025

While VFR SCT-FEW cloud deck decreases with decreasing diurnal
heating, a high BKN-OVC deck seen progressing east through
central Minnesota at 06.00Z TAF issuance.

High confidence (80-90%) for VFR through the forecast period.
Eastern extent of precipitation chances progressing east through
Minnesota Wednesday morning is the primary potential buster.
While precipitation may graze locally western peripheral
counties in northeast Iowa and southeast Minnesota accompanying
MVFR ceilings expected to be lag behind into central Minnesota.

Subsequent potential aviation impacts accompany storm chances
overnight Wednesday into Thursday morning. Expect storms and
impacts primarily north of Interstate 90 in southeast Minnesota
and central Wisconsin. Southeastern extent of storms remains in
question causing low confidence for level of impacts, i.e.,
MVFR/IFR. Precipitation impacts linger in central Wisconsin
through Thursday.

&&

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

&&

$$

DISCUSSION...Naylor
AVIATION...JAR