Severe Thunderstorm Warning 198, NWS La Crosse, WI

Area Forecast Discussion
National Weather Service La Crosse WI
1255 PM CDT Fri Sep 4 2026

.KEY MESSAGES...

- Shower and storm chances persist through the holiday weekend.

- Hot temperatures linger this afternoon, mainly across
  northeast Iowa and southwest Wisconsin, before trending
  slightly cooler Sunday into Monday.

- A low pressure system is expected to move through the Upper
  Midwest next week, brining the potential for showers and
  storms Tuesday into Wednesday (50-70%).

&&

.DISCUSSION...
Issued at 1254 PM CDT Fri Sep 4 2026

Tonight - Monday: Persistent Shower/Storm Chances, Trending Slightly
Cooler

A quasi-stationary boundary remains draped across the Upper
Mississippi River Valley through the rest of the afternoon and
into tonight, mainly south of I-90. While forcing along the
boundary is fairly weak, some showers and storms have been able
to develop with additional development possible (10-30%) into
this evening. Plenty of instability is present for these storms
given the hot and humid conditions that remain across the region
(2000-3000 J/kg of MUCAPE), so thinking some updrafts could be
on the perkier side. Very little in the way of wind shear is
present across the region though (0-6km bulk shear of 20kts or
less), so storms will struggle to sustain themselves leading to
low potential for severe hazards. However, some small hail and
gusty winds are possible under the stronger updrafts.

Overnight, anticipating areas of fog to develop as low level
ridging begins to build into the region. The soupy conditions
and diminishing winds within the stabilizing atmosphere should
result in conditions primed for fog development. Would expect
fog to be dense at times with the 04.12z HREF suggesting 20-40%
probabilities for visibilities less than 1/4mi. Fog will
dissipate through around 9-10am Saturday morning.

Heading into the holiday weekend, upper level ridging builds
across the central United States allowing for northwest upper
level flow to set up across the Upper Mississippi River Valley.
A couple shortwave impulses are progged to move through the
region within the mean flow, sparking periods of showers and
storms (30-50%), most favorable Saturday night into Sunday and
west of the Mississippi River. Strong to severe storms appear
unlikely though as upper level winds, and by association the
wind shear, should be rather light. Shower/storm coverage may
also end up being rather scattered given low level dry air in
place, resulting in a cap across the region.

High pressure sinking southward across the Great Lakes through
the weekend should also force the boundary that's been over the
region through the better part of the week to southward,
allowing for a slight and brief cooling trend Sunday into
Monday. Temperatures will still be above normal, though only by
5-10 degrees rather than the 10-15 degrees we've seen this past
week. Currently expecting highs to top out in the upper 70s to
low 80s with the 25th-75th percentiles of ensemble guidance
highlighting this as the most favorable range. This trend will
be short-lived though as temperatures begin to warm again
through mid-week.

Tuesday - Wednesday: Low Pressure System and Shower/Storm Chances

Greater potential for showers and storms area-wide comes
Tuesday into Wednesday when the longwave trough currently
situated over the Pacific Northwest ejects eastward and deepens
a surface low across the northern Great Plains. An associated
cold front is expected to move through the Upper Midwest as the
low shifts eastward across southern Canada which will be the
main focus for these shower and storm chances.

Many of the members of the various ensemble suites
(GEFS/ENS/GEPS) do highlight precipitation moving through the
region with this cold front, though there is some variation
regarding timing and location. Currently, rainfall probabilities
in the 04.13z NBM sit around 50-70% across our area, though are
highest north of our area where 1000-500hPa QG convergence is
maximized. There is also the potential influence of dry low
level air across much of the Upper Mississippi River Valley as
suggested by the 04.12z GFS solution which would limit
shower/storm development along the cold front and could result
in lower probabilities. Regardless, some storms that develop
could be on the stronger side given 35-40kts of 0-6km bulk
shear, MLCAPE on the order of 750-1250 J/kg, and AI/ML guidance
highlighting some low potential for strong storms.

&&

.AVIATION /18Z TAFS THROUGH 18Z SATURDAY/...
Issued at 1222 PM CDT Fri Sep 4 2026

Line of TS south of LSE continues to slowly exit the area to the
southeast and will remain outside the 10 mile radius of both TAF
sites. Strong signal for at least IFR stratus/fog to spread
southwest overnight into early Saturday so have retained IFR
mentions at both RST/LSE and, considering we are still a couple
TAF cycles away, have been conservative with timing. VFR
conditions will very likely (80%) return by the end of the
period.

&&

.ARX WATCHES/WARNINGS/ADVISORIES...
WI...Heat Advisory until 7 PM CDT this evening for WIZ061.
MN...None.
IA...Heat Advisory until 7 PM CDT this evening for IAZ029-030.

&&

DISCUSSION...Falkinham
AVIATION...Ferguson

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Special Weather Statement
National Weather Service La Crosse WI
327 PM CDT Fri Sep 4 2026

IAZ011-042100-
Allamakee IA-
327 PM CDT Fri Sep 4 2026

...A STRONG THUNDERSTORM WILL IMPACT SOUTHEASTERN ALLAMAKEE COUNTY
THROUGH 400 PM CDT...

At 327 PM CDT, Doppler radar was tracking a strong thunderstorm near
Waukon, moving east at 30 mph.

HAZARD...Wind gusts up to 50 mph and penny size hail.

SOURCE...Radar indicated.

IMPACT...Gusty winds could knock down tree limbs and blow around
         unsecured objects. Minor hail damage to vegetation is
         possible.

This strong thunderstorm will be near...
  Harpers Ferry around 350 PM CDT.

Other locations impacted by this storm include County Roads A 52 And
X 52, Volney, Waterville, Elon, Maud, Watson, Dalby, Yellow River
State Forest, Thompson Corner, and Waukon Jct.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

If outdoors, consider seeking shelter inside a building.

&&

LAT...LON 4308 9143 4328 9143 4330 9109 4327 9107
      4326 9107 4326 9106 4325 9106 4323 9107
      4320 9112 4315 9115 4313 9118 4308 9118
TIME...MOT...LOC 2027Z 282DEG 28KT 4320 9145

MAX HAIL SIZE...0.75 IN
MAX WIND GUST...50 MPH

Naylor

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