Special Weather Statement, NWS La Crosse, WI

Area Forecast Discussion
National Weather Service La Crosse WI
1052 PM CDT Mon Oct 23 2023

.DISCUSSION...(This evening through Monday)
Issued at 301 PM CDT Mon Oct 23 2023

Key Messages:

 - Thunderstorms develop tonight north of I-90 with a few becoming strong
   to severe. Hail up to 1" in diameter is the primary threat.

 -  The heavy rainfall potential increases after tomorrow
    afternoon with multiple rounds of showers and storms expected
    primarily north of I-90 Tuesday - Friday. Thursday will
    require extra attention regarding the heavy rain threat, but
    precipitation earlier this week will dictate the potential
    impacts seen later this week.

 - Becoming cold Friday and beyond after the passage of a cold
   front.

STORMS DEVELOPING TONIGHT:

A low-level jet once again develops across Iowa and southern
Wisconsin tonight. The differential moisture advection is not as
strong as we saw early this morning, but tonight's surge of moisture
comes with much higher MU CAPE between 1500-2000 J/kg. We expect
elevated storms to develop on the northeastern edge of the
moisture transport and CAPE gradients where the strongest upper-
level divergence will be located along an axis from south-central
Minnesota into north-central Wisconsin. A few strong to marginally
severe storms are possible north of Interstate 90 due to the
enhanced instability aloft and steep mid-level lapse rates. Hail
up to 1" in diameter is the primary threat. Storms move out to
the north east shortly after sunrise tomorrow morning.

WARMER, MORE STORMS TUESDAY:

After showers and storms clear out in the morning, a few dry hours
are likely before another round of storms develops near a stationary
cold front again spanning from south-central Minnesota into north-
central Wisconsin. This system exhibits much weaker instability in
the 500-1000 J/kg range, so the threat for stronger storms is a
bit more limited. There is also a lack of organized forcing
Tuesday afternoon, but continued moisture advection and weak PV
advection in the southwesterly mid-level flow will likely be
enough to initiate convection near the baroclinic zone. A few of
the more perky storms could pose a small hail threat, but
confidence is low for severe weather impacts at this time. Due to
the convective nature of this precipitation and precipitable water
values increasing to 2+ standard deviations above normal (NAEFS),
there's an increasing signal for locally heavy rainfall of 1 to
2.5" Tuesday afternoon-night in the latest run of the HREF.

Confidence is high for temperatures in northeast Iowa and southwest
Wisconsin to exceed 70 degrees tomorrow. This is where clouds could
become scattered in the afternoon allowing for enhanced daytime
heating in addition to the surge of warm air advection into the
region. Further north in southeast Minnesota and western
Wisconsin, high temperatures in the upper 60s to low 70s are
forecast. Higher low-level relative humidities will likely lead
to more cloud coverage in this region and afternoon convection
could also affect the degree of warming experienced leading to
more uncertainty in just how warm temperatures will rise.
Regardless, an unseasonably warm (and humid) day is on tap for
all.

CONTINUED PRECIPITATION CHANCES THROUGH FRIDAY:

Periodic heavy rain chances continue through Friday as additional
convection develops near the stationary boundary described above.
The forcing continues to to be unorganized until Thursday when a
strong shortwave trough approaches in the southwesterly flow aloft.
This added forcing will likely increase the heavy rain threat on
Thursday morning - Thursday night. Impacts of the heavy rainfall
later in the week will be highly dependent on how much rainfall
areas receive earlier this week and how the soils, rivers, and
streams respond. It will be a period that requires heightened
attention nonetheless so continue to monitor the forecast throughout
the week as we refine rainfall amounts and impacts related to this
late-week trough.

CONSIDERABLY COOLER THIS WEEKEND:

All signs point to a drastic pattern change from unseasonably warm
and wet conditions to a prolonged blast of cool air this weekend
into next week. Considerable spread in temperatures, precipitation
timing, and precipitation type makes it hard to nail down the
details of any sensible weather this weekend, but all should prepare
for a major swing in temperatures for Friday and beyond.

&&

.AVIATION...(For the 06Z TAFS through 06Z Tuesday night)
Issued at 1049 PM CDT Mon Oct 23 2023

A low-level jet will move overhead overnight bringing LLWS at LSE
at RST. With this low-level jet, elevated instability will bring
a period of thunderstorms at RST and LSE around or after 6Z
through 12Z and have included a tempo for these chances. Behind
these thunderstorms, MVFR conditions are expected to develop and
prevail through the rest of the TAF period at RST and LSE. There
is a 20% chance for a period of IFR ceilings at LSE and RST early
Tuesday morning. On Tuesday evening, IFR conditions are expected
to develop (50 to 70% chance) at RST/LSE along a stationary
boundary with abundant low- level moisture.

&&

.ARX WATCHES/WARNINGS/ADVISORIES...
WI...NONE.
MN...NONE.
IA...NONE.
&&

DISCUSSION...KAA
AVIATION...JAW

Hazardous Weather Outlook
National Weather Service La Crosse WI
743 PM CDT Mon Oct 23 2023

IAZ008>011-018-019-029-030-MNZ079-086>088-094>096-WIZ017-029-
032>034-041>044-053>055-061-241100-
Mitchell-Howard-Winneshiek-Allamakee-Floyd-Chickasaw-Fayette-
Clayton-Wabasha-Dodge-Olmsted-Winona-Mower-Fillmore-Houston-
Taylor-Clark-Buffalo-Trempealeau-Jackson-La Crosse-Monroe-Juneau-
Adams-Vernon-Crawford-Richland-Grant-
743 PM CDT Mon Oct 23 2023

This Hazardous Weather Outlook is for portions of southeast
Minnesota...northeast Iowa and southwest into central Wisconsin.

.DAY ONE...Tonight

Scattered storms overnight, mostly around and north of Interstate
90. A few could be strong, potentially severe, with a hail threat.

.DAYS TWO THROUGH SEVEN...Tuesday through Sunday

Scattered storms Tuesday with an isolated strong to severe risk.

Periods of rain, heavy at times, through Thursday night with
several inches of accumulation possible. Highest amounts of 3 or
more inches currently favored north of Interstate 90.

Cold weekend into next week with subfreezing low temperatures
expected areawide Monday morning.

.SPOTTER INFORMATION STATEMENT...

No action needed.

.EMERGENCY MANAGEMENT COORDINATION...

No webinar is scheduled.

nofwd

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