319
FXUS63 KARX 261912
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
212 PM CDT Sat Apr 26 2025

.KEY MESSAGES...

- Precipitation chances overnight into Sunday morning.

- Storm chances overnight Sunday into Monday morning, some
  strong to severe storms possible Monday morning with wind and
  hail as the primary concerns.

- Severe storm confidence increasing for Monday with all hazards
  (tornadoes, hail, & winds) possible.

&&

.DISCUSSION...
Issued at 137 PM CDT Sat Apr 26 2025

Tonight Through Sunday:

Initial precipitation chances shift from west to east tonight into
early Sunday morning as an increased low level theta e lobe pushes
east through the Upper Mississippi River Valley with higher low
level isentropic upglide shifting northeast through the forecast
area through Sunday morning. The responsible weakening ridge running
open upper level wave quickly exits northeast, bringing higher
precipitation potential with it.

Little confidence in how PoPs linger area wide through Sunday as a
Rocky Mountain Low pressure takes shape through the Central to
Northern Plains, continuing the strong low level theta e advection
while a final surge of dry air follows the easterly exiting high
pressure. Have gone with short term model consensus based on high
resolution soundings and mostly tapered PoPs drier during the
day Sunday but again, little confidence. The tightening pressure
gradient from the lingering high and impending low provides
higher confidence in south-southeast winds increasing through
the day with gusts nearing 25kts primarily west of the
Mississippi River Valley.

Sunday Night Into Monday Morning:

As has been the main forecast detail, precipitation and storm
chances increase overnight Sunday through Monday. Initial storm
chances late Sunday night into early Monday morning from
advection of the increased low level theta e lobe with
accompanying isentropic upglide within the tightening warm
sector associated with a deepening Rocky Mountain Low shifting
east-northeast through the Central to Northern Plains. The
responsible synoptic energy from a closed low churning over the
California coast on GOES water vapor imagery loops early this
afternoon expected to rejoin the parent trough overnight,
elongating synoptic forcing and low level moisture transport
from the Southern to the Northern Plains through Monday.

Storms expected to be elevated through the Monday morning on the
nose of a low level jet pointed towards the Upper Mississippi River
Valley. Some strong to severe storms can`t be ruled out during this
time although not overly confident as higher shear will lie farther
southwest within the warm sector of the phasing extratropical
cyclone. Hail and wind would be the main concern as storms will be
moving relatively quickly due to the 50kt+ low level jet.
Although limited members, longer term high resolution
reflectivity also suggests a strong to severe line of storms
along our south with scattered strong-severe storms along our
north.

Monday Severe Storm Confidence:

The severe threat increases through Monday as the extratropical
cyclone undergoes further deepening and phasing along the
aforementioned elongated synoptic trough. A narrow band of
60-90% joint probabilities of SBCAPE/MUCAPE (GEFS&GEPS/EPS)
above 500 J/kg, SBCIN/MUCIN (GEFS&GEPS/EPS) above -25 J/kg, and
0-500mb Bulk Wind Shear (EPS&GEPS&EPS) of more than 30kts
extends from Kansas City, MO through Minneapolis, MN by Monday
evening. Initial 100% probabilities in these aforementioned
parameters are seen farther west towards the Missouri River
Valley from southwest Minnesota into northwest Iowa Monday
afternoon. Confidence remains more than moderate (50-70%) for
severe storms Monday as multiple machine learning forecast
models have suggested over the last few runs.

Monday Severe Storm Impacts:

Can expect severe supercellular/linear storms initially farther
west towards the Missouri River Valley progressing east towards
the Mississippi River Valley through Monday. While an initial
supercell can`t be ruled out locally, eventually can expect a
quasi-linear mode as storms progress towards the forecast area
with a motion of 40+ mph potentially bowing as it progresses.
Given that QLCS/linear storm mode requires less shear than
supercellular storms, spin ups will be likely across this line.
Outside of the lowest 1km which has massive amounts of shear
(30kts) and helicity (250 m2/s2), longer term deterministic
model hodographs also have 40kts (20 m/s) of mostly
unidirectional shear above 1km. Added together, the 60+ kts of
0-3km shear suggested in the NAM and GFS would easily meet the
30kts perpendicular bulk shear nudger for mesovortices. Same
deterministic sources suggest a southwest to northeast (+/-
30deg) oriented shear vector, suggesting a northwest-southeast
perpendicular orientation for the updraft- downdraft
convergence zone. With forecast soundings showing sufficient
hail residence zones, rapid storm motions, and aforementioned
shear: all signs point to all hazards being possible.

Same can be seen in multiple machine learning models, painting
an area along and west of the Mississippi River Valley with
45% to 60% chance for severe storms.

&&

.AVIATION /18Z TAFS THROUGH 18Z SUNDAY/...
Issued at 1232 PM CDT Sat Apr 26 2025

Moderate to high confidence in VFR through the 26.18Z TAF
period. Difficulty lies with very limited, near zero, potential
for MVFR ceilings accompanying -SHRA overnight and early Monday
morning. Have kept PROB30 for -SHRA at both TAF sites and VFR
ceilings.

Aviation impacts expected Sunday night through Monday as storm
chances increase with potential for severe storms through much
of Monday.

&&

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

&&

$$

DISCUSSION...JAR
AVIATION...JAR