680
FXUS63 KARX 301741
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
1241 PM CDT Tue Apr 30 2024

.KEY MESSAGES...

- Severe weather possible mainly west of the Mississippi River
  between 6 PM and 9 PM. Severe weather threats include hail up
  1 inch, damaging winds (wind gusts up to 60-65 mph) and
  possibly a tornado or two. The greatest risk of tornadoes look
  to be in northeast Iowa closer to and west of the Interstate
  35 corridor.

- Heavy rain and maybe another round of severe weather on
  Thursday.

&&

.DISCUSSION...
Issued at 251 AM CDT Tue Apr 30 2024

Late This Afternoon and Evening - Severe weather possible mainly
west of the Mississippi River

The 30.00z models are in fairly good agreement that a negatively
tilted 500 mb shortwave trough will move northeast out of the
Central Plains late this afternoon and then moves quickly
northeast through the Upper Mississippi River Valley this
evening. At the surface, a strengthening low pressure area will
move northeast from near Omaha, NE at 30.18z to somewhere
between Grand Forks, ND and the Twin Cities by 01.00z and over
northern Minnesota by 01.06z. As this occurs, the surface warm
front tries to lift northward toward the Interstate 90 corridor
during the late afternoon and evening.

The HREF 30.00Z spread in the 2 meter dew points is rather tight
with the 10th to 90th percentiles only having up to 3F spread (in
the mid and upper 50s) in the warm sector. The 25th to 75th
percentiles in the HREF are showing surface-based CAPES ranging
from 200 to 1000 J/kg across northeast Iowa, southeast
Minnesota, and southwest Wisconsin. These differences are due to
uncertainties on how far north will the surface warm front will
get. This instability weakens rapidly with the loss of diurnal
heating this evening. This adds to the uncertainty on the
eastern extent of the severe weather later in the evening.

Near the surface warm front, the 0-3 km of the hodograph greatly
lengthens as the low level jet (which is from the south and
southwest) strengthens this evening. The 0-1 km shear climbs to
around 30 knots and the deep-layer (0-6 km) shear climbs to
around 50 knots. SPC has a slight risk covering Dodge, Mower,
and western Fillmore counties in southeast Minnesota, and all
but Allamakee and Clayton counties in northeast Iowa. There is a
marginal risk east toward Interstate 94 in Wisconsin. CSU is a
bit further east with their tornado, wind, and hail threats.

Looking at the soundings and CAMs, it looks like the highest threat
for severe weather this evening will be from 6 PM through 9 PM and
mainly west of the Mississippi River. Skinny CAPES in the soundings
suggest that hail will be likely up to CAPES. However, if the CAPES
are slower at falling this evening or the storms becomes
supercellular, the hail might be a bit larger than that. Other
severe weather threats include damaging winds (winds up to 60-65
mph) and possibly a tornado or two. The greatest risk of
tornadoes look to be in northeast Iowa closer to and west of the
Interstate 35 corridor.

Wednesday night into Friday - Heavy rain and maybe another round of
severe weather on Thursday

On Thursday and Thursday night, another strong 500 mb shortwave will
move east out of the Central Plains and move northeast into the
Upper Great Lakes. Strong moisture transport ahead of this wave will
increase precipitable water into the 1 to 1.5 inch range (LREF
10th to 90th percentiles). With elevated most unstable CAPE up
to 750 J/kg, the MCS might be able to produce some marginal
severe hail and strong to locally severe hail in the vicinity of
the surface warm front. Due to this potential, SPC`s Day 3
(Thursday/Thursday night) has a marginal risk up to Interstate
94. Rainfall amounts look to be mainly in the 1 to 2 inch range
with locally higher amounts possible.

&&

.AVIATION /18Z TAFS THROUGH 18Z WEDNESDAY/...
Issued at 1236 PM CDT Tue Apr 30 2024

VFR conditions will prevail through much of the daytime as winds
trend SE ahead of a cold front. Some shower/isolated storm
potential could occur by 22-00Z at KRST with higher chances for
bands/clusters of showers/storms to move west to east through
the evening. Erratic, gusty winds and IFR/MVFR conditions could
occur with any storms. Skies will gradually clear in the wake of
the storms with winds shifting westerly, possibly gusting up to
25 to 35 kts at times, depending on mixing depth.

&&

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

&&

$$

DISCUSSION...Boyne
AVIATION...KAA/JM