442
FXUS63 KARX 021056
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
556 AM CDT Tue Sep 2 2025

.KEY MESSAGES...

- Some storm chances (30-50%) today along western half of the
  forecast area increase and become widespread overnight
  (60-80%) along passing frontal boundary.

- Storm chances linger through Wednesday morning along frontal
  boundary. Showers with embedded convection linger through
  Wednesday evening.

- Turning cold after Wednesday and through the weekend with
  daytime highs in the 60s and overnight lows from the 30s to
  low 40s.

&&

.DISCUSSION...
Issued at 323 AM CDT Tue Sep 2 2025

Quite a different picture in upper level satellite imagery over the
last 24 hours with the changing synoptic pattern providing storm
chances initially today and overnight.

Meager Storm Chances From Northeast IA through Central WI Today:

Initial, low to moderate (30-50%) storm chances progress east-
southeast through Minnesota later this morning into the afternoon,
evident in scattered returns on early morning radar imagery. While a
meager pool of increased low level moisture, mid 60 degree surface
dewpoint observations over southwestern Minnesota early this
morning, will provide sufficient instability (1000+ J/kg), an
overall lack of forcing means showers and storms will be on the
decrease as they progress east into a lingering cooler, drier
airmass locally. Therefore, a sharp cutoff in these aforementioned
higher precipitation chances exists across the forecast area,
currently expected from northeast Iowa through central Wisconsin.

Increased Storm Chances Tonight Through Wednesday:

Subsequent increase in upper level synoptic forcing from a fold in
the dynamic tropopause, seen in a tightening cyclonic rotation over
northern Manitoba on early morning GOES water vapor imagery loops,
provides increased storm chances within a tightening frontal
boundary overnight through Wednesday morning. While forcing will be
on the increase, a slowing trend in the upper level cyclone abates
higher instability locally as high resolution model soundings show
limited, skinny CAPE profiles with mean HREF amounts from 100-500
J/kg. Therefore, expect storm activity overnight along the frontal
boundary.

Storm chances (50-80%) continue through Wednesday morning as the
frontal boundary continues south-southeast. A subsequent pocket of
strong low level CAA will sufficiently steepen low level lapse rates
to cause showery convection through Wednesday evening. Therefore,
have increased precipitation potential (20-30%) through the evening
hours and into the overnight in collaboration with surrounding
offices. This will require further adjustments in coming forecasts
as lower QPF scenario is causing lower PoP biases.

Storm Chances Overnight Thursday Into Friday:

Subsequent storm chances progress through the Upper Mississippi
River Valley overnight Thursday into Friday as another strong
wave dives southeast through Canada along northwest flow,
joining leftover synoptic energy wobbling around the ongoing
upper level low over Ontario. A relatively stable airmass (EPS)
limits widespread storm chances with heavy rain and strong winds
as the main concerns. Current LREF confidence (0%) for winds of
30+ mph doesn`t initially appear representative of strong wind
potential. However, given the aforementioned stable lower half
of the atmosphere, strength of the strongest winds (125kt GFS)
accompanying the PV fold will struggle penetrating below 500mb.
Will be detail to monitor in coming forecasts.

Cold After Wednesday:

Main forecast impact after Wednesday will be a Canadian Polar
airmass bringing anomalously cold temperatures (-2 to -3
Standardized Model Anomalies in NAEFS & ENS) to the Upper
Mississippi River Valley.

Outside of the coldest solution in the GEFS, high model confidence
(70-90%) in the local forecast area dropping into the 30s in central
Wisconsin to low 40s elsewhere initially overnight Wednesday through
Thursday morning then again overnight Friday and Saturday (60-80%
confidence). While these temperatures support frost production,
nature of low level mixing winds and lingering cloud cover limits
overall, widespread confidence in accompanying impacts.

While an overall widespread, extreme event is unlikely with a ECMWF
extreme forecast index shift of tails of 0, current forecasts are 3-
5 degrees away from record cold highs in both La Crosse and
Rochester Wednesday through Saturday (outlined below).

&&

.AVIATION /12Z TAFS THROUGH 12Z WEDNESDAY/...
Issued at 555 AM CDT Tue Sep 2 2025

River valley fog has set in pretty well at KLSE /LIFR/ with a
good southeast wind advecting it across the airfield. Fog is
also visible via GOES satellite and surface observations
throughout the river valleys this morning. Most of this should
dissipate by 9 am.

Some weak-moderate instability and lift will shift from west to
east across the area this afternoon. So, some SHRA or TSRA may
result mainly north of Interstate 90 with a PROB30 to account
for this east-shifting window at KRST and KLSE.

This evening, a NE-SW oriented cold front will approach from
the northwest with some VFR showers. Some mild but waning
instability may still be around for TSRA near KRST so have
maintained it. However, kept it only SHRA at KLSE overnight. The
front will begin a period of showers with lower ceilings (MVFR)
suggested for WI by late tonight. There are some probabilities
for IFR as well after 06Z Wed at KLSE and in WI...and this will
need to be monitored for trends. &&

.CLIMATE...
Issued at 323 AM CDT Tue Sep 2 2025

Based on current forecast, cold temperature records may be challenged at
La Crosse and Rochester this week. Below are the daily temperature
records that could be broken:

 La Crosse, WI

Date           Record Cold High       Forecast High
-----           -----------            -----------
9/4              60F(2008)                63F
9/5              59F(1896)                63F
9/6              60F(1965)                65F

 Rochester, MN

Date           Record Cold High       Forecast High
-----           -----------            -----------
9/3              60F(1934)                64F
9/4              60F(1994)                62F
9/5              59F(1956)                59F
9/6              58F(1965)                62F

&&

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

&&

$$

DISCUSSION...JAR
AVIATION...Baumgardt
CLIMATE...JAR