230
FXUS63 KARX 311052
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
552 AM CDT Sun Aug 31 2025

.KEY MESSAGES...

- Besides low precipitation chances mostly Monday evening along
  southwestern peripheral counties of Floyd/Mitchell Counties in
  northeast Iowa and northern peripheral county of Taylor County
  in central Wisconsin, dry through Tuesday morning.

- Persistent temperatures through Tuesday night with daytime
  highs in the mid 70s and overnight lows in the mid 50s.

- Widespread precipitation and storm chances Tuesday night
  through Wednesday morning. Chance for severe storms remains
  low.

&&

.DISCUSSION...
Issued at 140 AM CDT Sun Aug 31 2025

Dry & Slightly Cooler Than Normal Today:

A relatively persistent forecast expected for the start of the new
week as the Upper Mississippi River Valley remains situated between
a synoptic axis of dilatation upstream from the Canadian
Rockies through the southeast CONUS and an area of cyclonic
rotation downstream over the northeast CONUS. Areas of
reflectivity accompanying this axis of dilatation are expected
to mostly remain southwest of the local forecast area through
today as the pattern is reinforced by anticyclonic synoptic flow
sagging southeast through southern Canada.

May see some scattered storms sag south into northern
peripheral counties in central Wisconsin later today due to
added moisture from Lake Superior. Although, the passing high
and east winds will drastically hinder any storm activity,
advecting dry low level air towards any storms that form over
northern Wisconsin. Very limited confidence and impacts leave
PoPs out of local forecast through today.

Slightly Below Normal Temps Through Tuesday Night:

Lack of synoptic forcing will keep a plume of decreased low
level theta e from New England to near the Missouri River
Valley through Monday, providing daytime highs in the mid 70s
locally and highest precipitation chances southwest, in closer
proximity to the aforementioned synoptic axis of dilatation and
low level theta e boundary. While confidence is quite high, a
2-3 degree 25th-75th percentile spread in NBM MaxTs through
Tuesday, variability in behavior of synoptic cyclonic flow
rejoining parent flow over the northeast CONUS Monday through
Tuesday may cause slight temperature forecast changes. Besides
affecting strength and location of a subsequently digging trough
through the central CONUS, main impact would be slightly cooler
temperatures Tuesday morning evident in 1-2 degree decreasing
trend in LREF members from the 30.12Z to 30.00Z runs. Most
noticeable difference locally would be felt in central
Wisconsin, in closer proximity to the residual, cooler airmass.
Regardless of the minimal confidence with convective machine
learning models (Pangu/Fengwu) 500mb isoheight plots only tied
into this solution in one (29.00Z) of the last few runs, overall
higher precipitation/storm impacts expected to remain well to
the west of the local forecast area. Expect LREF members to
continue increasing agreement on exact location of lowering
heights, subsequently increasing confidence in location of
potential convective impacts and local temperature forecasts
through Tuesday.

Storm Chances Tuesday Through Wednesday:

An eventual synoptic upper level trough, currently distinguished
by meager cyclonic mid level flow over the Beaufort Sea on early
morning GOES West Alaska sector(s) water vapor imagery loops,
dives south through the Central Plains into midweek (LREF probs
500mb heights). Resultant precipitation and storm chances
increase locally Tuesday night through Wednesday morning as the
attendant frontal boundary tightens. Current forecast confidence
in storms is minimal due to transient nature of the amplifying
upper level trough and overnight timing limiting diurnally
influenced instability. Lack of confidence most evident in
individual LREF member soundings plumes and accompanying
hodographs covering 2/3 of the unit circle. Similar to early
week temperatures, will also be dependent upon rejoining of
lingering cyclonic flow over the eastern CONUS to the diving mid
level trough. Accompanying machine learning (day 4 @ 30.00Z)
and SPC severe storm categorical outlook (day 3) keeps
probabilities and confidence below marginal thresholds.

Precipitation chances expected to linger through Wednesday as a
stronger solution to the low in LREF members brings 30-50%
probabilities for 0.5" of QPF in Wisconsin, a more widespread
10% to 20% increase over previous runs.

Colder Midweek Into The Weekend:

Higher confidence impacts will be cooler air Wednesday through
the weekend due to the ushering in of a Canadian polar airmass
through the Upper Mississippi River Valley. Coldest temperatures
expected Wednesday night through Thursday with overnight lows
near 40 and daytime highs in the 50s. Confidence in impacts is
low however due to a 10 degree spread in 25th to 75th LREF and
NBM minimum temperatures. Coldest mean solution seen in the GEFS
with the 40 degree isotherm enveloping the local forecast area
early Thursday morning while the EPS keeps area wide temperatures
near 50. While the coldest solutions will be quite capable of
causing frost, increased low level CAA would sustain low level
winds, mixing the lowest layers, and abating widespread frost
concerns. As for cold records, while overnight lows are expected
to remain warmer than previous records in the low to mid 30s,
daytime cold highs could be in jeopardy on Thursday. NBM
probabilities suggest a 30% chance for daytime highs below 58
degrees at La Crosse, WI and 50% chance for cooler than 60
degree daytime highs at Rochester, MN. Similar LREF
probabilities are 65% and 80% for 1:00PM CDT Thursday,
respectively.

&&

.AVIATION /12Z TAFS THROUGH 12Z MONDAY/...
Issued at 554 AM CDT Sun Aug 31 2025

The mainly valley fog should dissipate around 31.14z.

Like this morning, there will be a deep layer (up to 450 mb),
low temperature and dew point departures, and a nocturnal
inversion. This will result in valley fog. However, there
remains uncertainty on whether KLSE will see it or not. For now,
just introduced a MVFR fog.

&&

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

&&

$$

DISCUSSION...JAR
AVIATION...Boyne