495
FXUS63 KARX 031107
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
607 AM CDT Wed Sep 3 2025

.KEY MESSAGES...

- A cold front moving through today will begin a period of
  temperatures running 10-15F below normal through about
  Sunday.

- Some frost is possible (40%-65%) in central/northcentral WI
  Thursday morning. Confidence is medium with clear skies still
  in question. A Frost Advisory may be needed along/north I-94
  tonight.

- Showers and isolated thunderstorms will end this morning,
  possibly returning late afternoon very early eve west of La
  Crosse. Confidence is fairly high on rain Thursday night with
  amounts of 0.25-0.50" probable.

&&

.DISCUSSION...
Issued at 345 AM CDT Wed Sep 3 2025

Showers and Isolated Thunderstorms Ending This Morning, Cool

Large scale trough deepening over the region is well underway
with a cold front just entering the forecast area located from
Eau Claire to Mason City at 08Z. A band of post-frontal rain
showers is present as well as some thunderstorm activity in the
warm advection portion of the system (WI). Overall thunderstorm
coverage tonight has been less than expected...probably due to
the weak low-level convergence with 850 mb flow all northwest
per 88D VWPs. The front will continue southeast this morning
with shower and storm chances exiting southwest/central WI and
far northeast IA by late/mid morning. Clouds upstream per GOES
fog product and observations suggest the day will be quite
cloudy based on advection and persistence. Have increased clouds
today in the forecast. When combined with moderate cold
advection on northwest winds, high temperatures will only be a
few degrees from the morning readings.

Some Isolated Showers Possible Late Today West of La Crosse

GOES satellite and Canadian radar showing showers associated with
another shortwave trough in northerly flow over southern Manitoba
approaching the northern MN/ND border. This arrives in the later
afternoon northwest of La Crosse, with steeping 850-700 mb lapse
rates, peaking over central MN at ~8C/km. By the time it arrives
in southeast MN into western WI by 5-6 pm, some stabilizing occurs.
But, there should be scattered showers moving into sern MN and
nern IA in the 5-9 pm window, decreasing in coverage southward.
Forecast has a 20-30% chance north and west of La Crosse to
address this, a bit more into northeast Iowa than previous
forecasts.

Frost Possible (35-65%) Thursday Morning near/north of I-94

The unusually cold air mass sinks over the area and skies appear as
though they could clear out tonight in central WI. NBM low
temperatures are in the 34-37F range in central WI along and
north of I-94 with about a 10-20% chance of freezing suggested
in the NBM probabilistic spreads (highest north of Highway 29).
The 02.18Z ECMWF ensemble suggests a warmer solution with *all*
members 40F and above at Medford, WI Thursday morning. Interestingly,
the GEFS has only 5% of its members *above* 35F for a low?? Quite
a spread in the Grand Ensemble...8F in the 25-75th percentile. Cloud
cover seems to be part of the reason for the spread with various
member ideas on cloud cover, with the EC carrying a bit more
cloud. So, there are questions on how cold it will get Thursday
morning - thus have remained with the deterministic NBM
forecast for now...34F range (KMDZ) to 35-38F in central WI. A
Frost Advisory may be needed, but confidence was not high enough
to issue as of yet.

Rain, Some Thunder Thursday Night

A strong and dynamical system in northwest flow arrives late
Thursday and Thursday night bringing rain to the area. Confidence is
growing in this solution after numerous runs of deterministic
solutions suggesting the rain and now short term CAMS also on
board. Strong tropospheric-deep forcing in response to a
compact potential vorticity anomaly / tropopause fold yields
strong low-level warm advection lift Thursday evening across the
area. Surface cyclogenesis occurs with the low tracking through
about central MN and WI. A narrow, moderate moisture ribbon
feeds into the forcing although instability is limited (MUCAPE <
500 J/Kg). On the nose of that moisture is a strong convergent
low-level jet isentropically upgliding...just a number of good
ingredients coming together for rain and some thunder. It is a
quick hitter unfortunately as the area could use rain...only
lasting 4-6 hours and bringing about 0.25". The latest 03.00Z
Grand Ensemble suggests a higher probability (30-50%) north of
I-90 for a bit more rain /0.50"+/ - a step up from the previous
02.12Z runs. These heavier amounts are likely tied to a
deformation region north of the surface low and a bit longer
duration rain.

Behind this system, winds should gust in the 25 to 35 mph range
into Friday, with temperatures cooling again and making a run
at record cool high temperatures (see Climate section for values).
NBM wind gusts were too low for this period it seemed based on
deterministic sounding guidance and mixing depth...decided to
use a NBM/NBM90th percentile blend.

Continued Cool This Weekend, Warmer Next Week

The longwave trough continues to dominate the Great Lakes
weather with cool temperatures about 10-15F below normal. One
last shortwave trough shifts through Saturday although the
sensible weather (showers) outcome is still of low confidence.
The 02.12Z Grand Ensemble suggests areas north of I-90 have a
20-35% chance of light precipitation from this wave, but a quick
looks at the 03.00Z runs bounce higher to 30-50% north of I-90.
So, Saturday looks like it will trend wetter in future
forecasts. At this time the forecast remains dry. Large scale
ridging begins to build into the High Plains next week and is
fairly well agreed upon on in the 02.12Z Grand Ensemble. This
will bring a return to warmer temperatures, but being downstream
of the ridge axis, still a bit of cooler troughiness overhead.
Highs should return to the 70s for most of the area.

&&

.AVIATION /12Z TAFS THROUGH 12Z THURSDAY/...
Issued at 549 AM CDT Wed Sep 3 2025

Frontal boundary pushing south through the morning, shifting
storm chances south with it. Isolated storm threat could
persist through the evening as bouts of strong cold air
advection causes transient instability. While not widespread,
current area of concern primarily from southeast Minnesota
through central Wisconsin.

MVFR ceilings expected for much of the area through the morning
as strong low level cold air advection saturates lowest levels.

Cold air advection also causes increased northwest winds area
wide this morning through the evening. Subsequent local impacts
expected overnight Thursday into Friday morning with additional
storm chances. Ceiling and strong wind impacts could be felt
through Friday due to more incoming cold air.

&&

.CLIMATE...
Issued at 110 AM CDT Wed Sep 3 2025

There still appears to be some chance at setting a record cool
maximum temperature on Friday (Sep 5) based on the current
forecast.

 La Crosse, WI

Date           Record Cold High       Forecast High
-----           -----------            -----------
9/4              60F(2008)                65F
9/5              59F(1896)                61F
9/6              60F(1965)                64F

 Rochester, MN

Date           Record Cold High       Forecast High
-----           -----------            -----------
9/3              60F(1934)                64F
9/4              60F(1994)                63F
9/5              59F(1956)                58F
9/6              58F(1965)                62F

&&

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

&&

$$

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