398
FXUS63 KARX 132343
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
643 PM CDT Tue May 13 2025

.KEY MESSAGES...

- Isolated showers and storms possible (15-30%) this afternoon
  and again Wednesday, mainly east of the Mississippi River.

- Strong to severe weather continues to appear possible
  Thursday. Confidence in exact location and timing remains low,
  but large hail and damaging winds winds appear possible.

&&

.DISCUSSION...
Issued at 149 PM CDT Tue May 13 2025

Isolated Showers and Storms This Afternoon and Wednesday

With the departing area of low pressure currently over Tennessee,
evident in water vapor imagery, various shortwaves get flung
northwestward into the Great Lakes region today and Wednesday. While
not overly impressive with regards to amplitude/magnitude, these
shortwaves combined with increasing low level moisture and diurnal
heating should prove enough for isolated shower and storm
development during the afternoons, mainly east of the Mississippi
River, but areas west may still see some precipitation. Anything
that does develop is expected to remain weak as skinny CAPE profiles
and weak 0-6km Bulk Shear generally 10kts or less continue to be
depicted in the model solutions. Expecting a broad area of 15-30%
probabilities of precipitation each afternoon.

Strong to Severe Weather Thursday

Unfortunately, not much light has been shed on Thursday`s forecast
today. While consensus remains surrounding an upper level shortwave
trough currently over the western United States ejecting into the
Great Plains and taking on a negative tilt while cyclogenesis occurs
at the surface in response, global deterministic models
(GFS/ECMWF/Canadian) and their ensemble counterparts continue
to vary in their solutions regarding said trough and associated
surface cyclogenesis.

Synoptically, Thursday`s environment looks to be favorable for
strong to severe weather across at least a portion of our area.
Wisconsin will be situated in the left exit region of a 250hPa jet
streak with strengthening winds in the vertical, cooling
temperatures aloft allowing for steepening mid-level lapse rates of
7.5-8 C/km, and surface moisture advection increasing dewpoints into
the low to mid 60s. The 13.00z LREF suggests CAPE values of around
2000 J/kg and 0-6km bulk wind shear of 40-50kts, which would support
strong updrafts. However, the orientation of said shear to the
surface cold front will ultimately determine the storm mode that is
realized with this event. The 13.00z LREF suggests a parallel
orientation to the cold front indicative of upscale growth and and a
linear convective mode. However, the 13.12z operational NAM remains
optimistic of a more perpendicular shear vector indicative of a
discrete storm mode. Given ample CAPE, strong shear, and a low WBZ
height, large hail appears to be a favored hazard with this event.
Similarly, with the addition of dry mid to upper level air allowing
for DCAPE in excess of 1000 J/kg, damaging winds are also a
favorable hazard.

One of the limiting factors to this event is variations amongst the
models in regard to timing of the surface cold front. The 13.06z
GEFS/GEPS remain the faster solutions, bringing the surface cold
front through the area earlier in the day, generally by early
afternoon whereas the 13.06z EPS brings the cold front through the
area later in the afternoon and into the evening hours. The
implications being that the 13.06z EPS allows for the majority of
our area to see strong to severe weather whereas the 13.06z
GEFS/GEPS suggests areas east of the Mississippi River would be
favored. These variations have plagued the forecast over the past
several runs.

Another limiting factor is the possibility of a 700-900hPa warm
nose in place, capping the surface environment from the mid and
upper level environment. If this cap is realized, little in the
way of convection would develop despite the favorable synoptic
environment. Recent model soundings have attempted to erode this
feature, though would like to see a more stable trend one way
or the other.

All said, large hail and damaging winds appear the most likely
hazards with storms that develop Thursday. The risk of tornadoes is
dependent on the shear orientation and ultimate storm mode that is
realized, but it is difficult to rule them out completely at
the moment.

&&

.AVIATION /00Z TAFS THROUGH 00Z WEDNESDAY/...
Issued at 640 PM CDT Tue May 13 2025

Isolated showers and storms gradually diminish in coverage this
evening with VFR conditions overnight into the morning on
Wednesday. Scattered 3-5-kft-based showers and storms once
again develop by early to mid-afternoon on Wednesday, primarily
east of the Mississippi River, though the threat for a stray
storm could extend as far west as RST in the afternoon. Winds
remain from the southeast around 5kts tonight, increasing to
around 10 kts for Wednesday.

&&

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

&&

$$

DISCUSSION...Falkinham
AVIATION...Skow