229
FXUS63 KARX 230246
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
946 PM CDT Wed May 22 2024

.KEY MESSAGES...

- Limited Scattered & Isolated Rain & Storm Chances Today

- Storm Chances Return Late Thursday Through Friday With Strong
  to Severe Storms Possible

- Storm & Rain Chances Return Sunday Night Into Monday

&&

.DISCUSSION...
Issued at 244 PM CDT Wed May 22 2024

Storm/Precipitation Chances Today/Tonight:

A closed upper level area of lower heights continues lifting north
through the Canadian province of Ontario early this afternoon.
A meridional (north-south) appendage has been deepening and
phasing across the central to southern Mississippi River Valley
with the southern periphery collocated with a 100 kt
subtropical jet streak, resulting in severe weather near the
confluence of Ohio and Mississippi Rivers.

Locally, minute pulses within the body of the shortwave appendage
have been providing high based scattered precipitation chances.
Commercial aviation based ACARS soundings line up with high
resolution forecast models concerning a meager low level
temperature inversion causing the higher cloud bases. Dry air
within this nose and minute pulses within the west-northwest
flow limit overall confidence and spatial continuity in
precipitation chances. Erosion of the inhibition layer will
allow tapping into slight instability and ability to overcome
this drier air. The colder airmass in place has lowered the
freezing level near 8k ft, resulting in pea size hail reports
with the strongest storms. Have continued with scattered and
isolated chances similar to previous forecasts.

Thursday:

Otherwise, drier forecast on tap through much of Thursday with
precipitation and storm chances returning Thursday night, lasting
through much of Friday. Highest temperatures within return flow and
increased low level WAA remains to the west, keeping storm chances
and higher temperatures from the 80s into the low 90s well to the
west.

Storm Chances Return Thursday Night Through Friday:

The trough responsible for storm and precipitation chances late
Thursday night through Friday currently seen on upper level
GOES water vapor imagery closed over the Pacific Northwest.
Upstream of this feature, increased upper level winds are expected
to initially dig an appendage of this closed low across the
Rocky Mountain West before phasing and lifting through the
Central Plains into the Upper Mississippi River Valley. The
narrow filament of increased low level theta e lobe advects
through late Thursday night through Friday morning, challenging
initial storm chances given lack of diurnal heating.

Storm chances increase through Friday as the surface low lifts
through central Minnesota into southern Manitoba with deepening
appendages swinging around the main low. Long term global ensemble
(EPS/GEFS) loses confidence in surface low location as the low lifts
north, challenging local confidence. The occlusion of the
surface low will provide a narrow appendage of potential
instability through Friday shown in an initial rounded mound of
50-70% chances for 500 J/kg of SBCAPE, becoming a filament of
20-50% SBCAPE probabilities along Mississippi River Valley into
Wabasha and Buffalo Counties.

Given the diurnal timing, severe storm chances push into the
Northern Plains on Day 2 (Thursday) and along and east of the
Mississippi River Valley on Day 3 (Friday) with highest
probabilities into central Wisconsin. Primary threat would be
potential for hail but currently, but the forecast window pushes
the end of the high resolution guidance. Therefore, HREF
max/min/mean instability, only till Friday at 12Z, shows 1000
J/kg, <500 J/kg, and 0 J/kg; respectively. Much disagreement on
shear, highest collocated with the occlusion passing over early
Friday afternoon, but very diverse solutions between models.

Otherwise, higher impact will be potential rainfall amounts.
Have slightly backed off from most recent National Blend
forecast model that included 6 hour QPF values near 1" Friday
18Z to Saturday 00Z. Should be mentioned lack of members at that
time after the 54 hour forecast window. Upcoming forecasts will
increase high resolution member inclusion, increasing
confidence in amounts. Otherwise, slight flip flop between long
term global deterministic and ensembles, intramodelly. The ECMWF
with a slower progression paints widespread 0.5" or higher
while the GFS remains transient, limiting overall spatial
impacts.

This Weekend:

A slight reprieve in precipitation chances expected through
Saturday, only to become more widespread again through Sunday into
Monday (Memorial Day). North to south separation of a phasing upper
level trough limits local confidence. Highest confidence lies to the
south where synoptic forcing best aligns for a lifting, deepening
surface low from the Central Plains into the Mississippi River
Valley. Will be subsequent feature to iron out in coming forecasts
given the longer forecast window.

&&

.AVIATION /06Z TAFS THROUGH 06Z FRIDAY/...
Issued at 942 PM CDT Wed May 22 2024

High pressure will build across the area tonight and remain
in control of the weather into Thursday. West winds will shift
to south and southeast on Thursday. Wind speeds will be 5 to 15
knots.

&&

.HYDROLOGY...
Issued at 300 AM CDT Wed May 22 2024

MRMS rain estimates and station reports showed a swath of 2 to 4
inches of rain since yesterday from parts of northeast Iowa
through central Wisconsin, resulting in rises on some rivers,
including the Turkey, Kickapoo, Black, Upper Iowa, and Yellow
Rivers. Minor flooding is expected along parts of the Turkey
River, and is possible along sections of a few Wisconsin
tributaries over the next few days.

&&

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

&&

$$

DISCUSSION...JAR
AVIATION...Boyne/JAR
HYDROLOGY...JM