132
FXUS63 KARX 020540
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
1240 AM CDT Tue Sep 2 2025

.KEY MESSAGES...

- Periodic rain chances throughout the remainder of the work
  week. Highest chances for rain will be Tuesday evening and
  into Wednesday (60-80% chance) as well as on Thursday night
  into Friday morning (40-60% chance).

- Cooler than normal temperatures expected from Wednesday and
  into the weekend. Highs will likely remain in the middle 50s
  to upper 60s for most with overnight lows in the middle 30s to
  upper 40s.

&&

.DISCUSSION...
Issued at 126 PM CDT Mon Sep 1 2025

Rest of Today - Wednesday: Shower/Storm Chances Tuesday & Wednesday

Weak flow aloft today with surface high pressure situated to our
southeast will keep skies mostly clear for much of today along with
keeping temperatures fairly seasonable with highs in the middle to
upper 70s. Cannot rule out a shower or two (under 15% chance) near
the I-35 corridor in southeast MN later this afternoon as shown in a
couple of the CAMs with a corridor of weak instability. However,
with sounding profiles in the low-levels in the recent RAP/HRRR
being very dry at KRST, this seems unlikely to occur.

As we head into Tuesday and Tuesday night, two key features will be
important to watch as a broad upper-level low pressure system begins
to push into the area. The first of these will move through the
region late Tuesday morning and into the early afternoon in the form
of a pre-frontal shortwave. Overall depending on when convection
manifests with this wave, cannot rule out some stronger wind gusts
with any collapsing cores with relatively steeper (7.5-8 C/km) low-
level lapse rates and DCAPE values approaching 600-800 J/kg,
primarily in southeast MN. However, given skinny instability
profiles with MLCAPE values under 1000 J/kg, any updrafts will
likely be weak in addition to having a tendency to fall apart
quickly due to the lack of 0-6km shear.

By the overnight hours on Tuesday, a surface cold front associated
with a descending upper-level low pressure center will begin to move
into the area increasing precipitation chances dramatically as it
approaches the local area. Overall, the grand ensemble
(GEFS/EC/Canadian ensemble) has very high probabilities (70-90%) for
measurable rainfall across the area as this boundary passes through.
However, with instability being fairly meager (under 500 J/kg of
MLCAPE in the 01.09z RAP) and precipitable waters being around 1",
really not expecting this rainfall to be overly efficient. As a
result, probabilities in the grand ensemble (GEFS/EC/Canadian
ensemble) for rainfall amounts over 0.25" are quite low (10-30%)
before precipitation exits Wednesday afternoon.

Wednesday - This Weekend: Unseasonably Cool Temperatures

As the aforementioned cold front exits the region, a north to
northwesterly flow regime takes over ushering in cold air advection
into the area. Consequently, expecting high temperatures to be
noticeably cooler starting on Wednesday with median high
temperatures in the NBM in the middle to upper 60s, around 10-15
degrees below normal for this time of year. Thursday will likely be
the coolest day of the forecast period as the upper-level low
positions just to our north with 850mb temperatures in the 01.12z
GFS/EC of around 0C to -2C across the local area. As a result, the
inter-quartile range for low temperatures on Thursday morning range
from the middle 30s to upper 40s across the area. Some frost cannot
be ruled out in the coldest spots however this will be dictated by
how cloud trends manifest. Daytime high temperatures on Thursday may
even challenge daily cold record highs at some locations with the
inter-quartile range for high temperatures in the NBM ranging from
around 50 degrees to the middle 60s on Thursday. Important to note
that the record cold high temperatures at both La Crosse and
Rochester is 60 degrees on Thursday and there is certainly some
potential these could be challenged.

By overnight Thursday and into Friday, a secondary reinforcing
shortwave and surface frontal passage will bring additional
precipitation chances into the area during this timeframe in
addition to keeping northwesterly flow in place and perpetuating
this cooler than normal pattern into the weekend. The grand ensemble
(GEFS/EC/Canadian ensemble) overall has respectable probabilities
(40-70%) for measurable precipitation with this secondary frontal
passage into Friday morning. Regardless, continued northwesterly
flow will likely keep us cooler than normal into the weekend with
forecast confidence beginning to degrade by the end of the weekend
and into early next week. Realistically two schools of thought are
in play as far as the overall synoptic pattern from Sunday and
onward. The first being that upper-level ridging builds into the
area allowing our temperatures to moderate from Sunday and onward as
reflected in the 01.12z deterministic GFS. The second would favor
additional pieces of shortwave energy to move into the region on
Sunday and thus reinforce the northwesterly flow pattern, keeping us
cooler into early next week. Overall, the grand ensemble
(GEFS/EC/Canadian) mean cluster analysis would suggest a slow
warming trend into early next week but again this will be dependent
on how the synoptic pattern ultimately resolves.

&&

.AVIATION /06Z TAFS THROUGH 06Z WEDNESDAY/...
Issued at 1239 AM CDT Tue Sep 2 2025

Conditions are very favorable for river valley fog by sunrise.
Starting to see the WI and Mississippi river tributaries fogging
per 0515Z GOES satellite fog imagery and surface observations.
NWS office webcam is showing the Miss valley near KLSE starting
to build toward stratus. With the conditions really favorable
and southeast winds at 5 mph, felt the inclusion of TEMPO
IFR/LIFR was prudent for the morning. Even if the fog is not on
the airport proper with IFR observations, VFR pilots would be
challenged on any approach into KLSE.

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. Decided to add 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
included it in the 06Z TAF. This will shift east across the area
overnight affecting KLSE beginning around 05-06Z. This will
most likely be included in the 12Z TAF issuance.

Higher probabilities for lower MVFR and IFR ceilings build in
Wednesday.

&&

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

&&

$$

DISCUSSION...Naylor
AVIATION...Baumgardt