538
FXUS63 KARX 150542
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
1242 AM CDT Thu May 15 2025

.KEY MESSAGES...

- Isolated showers and storms possible this afternoon (20-40%)

- Severe thunderstorms are possible Thursday afternoon-evening.
  Greatest risk along and east of the Mississippi River. Large
  hail (2"+) and tornadoes are the primary hazards, but damaging
  winds are possible.

- A busier weather pattern is expected this weekend into next
  week.

&&

.DISCUSSION...
Issued at 142 PM CDT Wed May 14 2025

Isolated Showers and Storms this Afternoon

Similar to yesterday, various shortwaves move northwestward
into the area today. These shortwaves combined with increasing
moisture via southeasterly flow and increasing instability via
diurnal heating will (and have begun to) allow for isolated to
scattered shower and thunderstorm activity again this afternoon.
Showers and storms will continue into the evening hours before
waning after sunset. Storms are expected to remain weak.

Severe Weather Thursday Afternoon and Evening

Model guidance is in much better agreement with the timing and
location of the surface extratropical cyclone expected to deepen
over the northern Great Plains tonight and Thursday in response to a
500hPa trough ejecting eastward off the Rocky Mountains.

Looking into the kinematics of Thursday, a 60-70kt jet streak
overspreads the Upper Midwest during the day, increasing 0-6km bulk
shear to 40-50kts with the orientation of these shear vectors
nearly orthogonal to the surface cold front. 0-3km shear shows
decent curvature, with the associated SRH values at 200-300
m2/s2. Overall, kinematics are supportive rotating updrafts and
a discrete storm mode. Storm motion will be quick given the
strong cloud layer winds.

From a thermodynamic perspective, "fat" CAPE profiles in excess of
2000 J/kg continue to be depicted across the region along and ahead
of the approaching cold front (above 3000 J/kg looking at the
RAP/HRRR) as surface dewpoints climb into the mid 60s.
700-500hPa lapse rates also remain impressive, forecast to be
7.5-8C/km. Model soundings continue to suggest some drier mid-
level air, less so than previous runs, but DCAPE values around
1000 J/kg and WBZ heights around 10kft remain.

Overall, storms that develop Thursday afternoon are expected to
quickly become severe. Currently thinking storms will initiate
near the Mississippi River around 2-4pm, moving quickly
eastward into eastern Wisconsin by 6-7pm. Given the shear
orientation orthogonal to the surface cold front, supercells are
expected, forming a broken line along the cold front. The
primary hazards are large hail of 2 inches or greater and
tornadoes. Damaging winds are a secondary, but very possible,
threat. Given the fast motion of the storms, flash flooding is
not expected.

Busier Weather Pattern Ahead

The area of low pressure associated with Thursday`s severe
weather will slowly shift eastward as we head into the weekend.
Winds increase substantially in the 925-850hPa layer Thursday
night into Friday around the low, upwards of 50kts. Model
soundings suggest an inversion should be in place across much of
the area overnight, but stronger gusts over 30kts are possible,
especially during the day Friday.

As the low slowly moves eastward Friday into Saturday morning,
additional showers will move through the area. Upper ridging
builds into the area Saturday into Sunday bringing a brief drier
period. Ridging then begins to break down early next week as
another trough developing across the United States, brining
additional precipitation chances into next week.

&&

.AVIATION /06Z TAFS THROUGH 06Z FRIDAY/...
Issued at 1228 AM CDT Thu May 15 2025

Upstream VFR cloud deck seen moving east through the Missouri
River Valley at 15.06Z TAF issuance. Expect VFR conditions to
continue early this morning as MVFR potential increases from
south to north later this morning.

An initial round of showers accompanies incoming MVFR cloud
deck, progressing northeast through southern Minnesota early
this morning. Highest confidence for impacts lies west of the
Mississippi River through northeast Iowa and southeast Minnesota
locally. Confidence for potential impacts at either TAF site
(KRST/KLSE) is low as southern edge of showers and storms is
may graze both. Therefore, will be an immediate concern to
monitor as any southern building along this initial line may
reach KRST and then subsequent KLSE.

Confidence in the subsequent line of storms early this afternoon
increases to ~75%. A northwest to southeast oriented line of
storms expected to develop near the Mississippi River Valley,
quickly strengthening to severe. Similar to morning storms,
confidence in exact location relative to either TAF site waivers
some due to initial development expected between KLSE and KRST:
near KEAU. Furthermore, KLSE and smaller airports in western
Wisconsin may initially be on the southern edge of the quickly
progressing northeast line of strong to severe storms.
Therefore, highest confidence for impacts locally will be at
KBCK, KMDZ, and K82C.

Light southeast winds at 15.06Z TAF issuance expected to
increase through the early morning hours, gusting near 20kts by
daybreak. As the surface front responsible for the storms
progresses expect a clockwise rotation to south winds gusting
nears 30kts. Further rotation through the evening towards the
end of the TAF period brings westerly winds weakening through
the nighttime hours.

&&

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

&&

$$

DISCUSSION...Falkinham
AVIATION...JAR