Special Weather Statement, NWS La Crosse, WI

Area Forecast Discussion
National Weather Service La Crosse WI
238 PM CDT Sat Jul 22 2023

.DISCUSSION...(This evening through Saturday)
Issued at 223 PM CDT Sat Jul 22 2023

Key Messages:

 - Scattered showers/storms possible into this evening; perhaps a
   stronger storm or two with small hail and gusty winds

 - Temperatures increasing through midweek with some river valleys
   possibly achieving heat indices of 100-105 Wed and especially
   Thu, but the heat will be seasonable by late July standards

 - Occasional rain/storm chances next week

Rest of today into tonight...

An upper level shortwave can be seen well on current satellite
imagery this afternoon, with showers and thunderstorms continuing
to track southeast from Minnesota into northwestern Wisconsin. A
cu field is also visible across Wisconsin, with showers and
thunderstorms developing across some areas of the state. A look
at SPC mesoscale analysis data shows areas of around 1000 J/kg of
CAPE available for storms to work with, though very little deep
layer shear. The stronger storms have had a fair amount of
lightning, with some reports of hail with the cells
developing/moving from MN into northwest Wisconsin this afternoon.
These shower and storm chances will continue into the evening as
the shortwave continues to dive down across the area, with chances
diminishing and leaving fairly quiet conditions into the
overnight.

Main story next week -- turning seasonably hot...

Bottom line...heat will be building across the region next week,
peaking Wednesday and Thursday with highs likely in the low to mid
90s and some river valleys possibly achieving heat indices of 100 to
105. However, signals continue to trend towards just a period of
seasonable summer heat for our area, nothing record breaking or
excessive by late July standards.

Background...Guidance continues to trend towards a less amplified
mid-level ridge extending into the region by midweek, which looks to
suppress the potential for any excessive heat locally. Once the +2
to 4 sigma western ridge tries to advance east of the Rockies by
Tuesday, guidance shows a deep upper low coming ashore British
Columbia that will quickly dampen the ridge over the Plains. The
pattern gives way to broad, low-amplitude ridging over the nation's
midsection by Wednesday and transitions to nearly zonal flow for the
Upper Mississippi Valley by Thursday. So what had earlier looked
like potential for a stretch of excessive heat for our area
continues to look more like just a typical mid-summer warm spell.
Nothing really stands out for our area with regard to ensemble
height/temperature anomalies, though the GEFS continues to be on the
extreme warm end of guidance. With the GFS's dry soil temperature
bias, the 22.12Z GEFS has increased probabilities for Wed highs of
100+ to 70% within the Mississippi River Valley from La Crosse
southward. That's quite excessive when compared to the 12.06Z EPS
which shows a 0% chance of that occurring. That said, current
forecast indicates a possibility for heat indices to climb into the
100 to 105 range (particularly on Thursday) in the river valleys,
suggesting some areas could perhaps need a heat advisory eventually.

Occasional rain chances next week...

Periodic opportunities for showers and storms will continue into
next week, mostly driven by weak embedded shortwaves in the
northwesterly flow aloft. Model differences in timing and track of
these individual shortwaves lends to lower confidence in specific
timing of these rounds of convection and locations impacted.

An area of convection on our western doorstep driven by a weakening
low level jet is hinted at Monday morning by several solutions,
including the CAMs. Low confidence in how and exactly where this
might play out (perhaps staying just west of our area) as well as
what impacts it might have on additional convection developing
later in the day. Some guidance suggests a better opportunity for
another round of convection in the afternoon-evening timeframe as
the shortwave moves through, accompanied by a feed of moisture
transport directed into the area. There are questions though of
how much instability and shear might come into play due to an
elevated warm layer.

Low predictability with mostly small occasional rain chances from
Wednesday onward tied to passing weak shortwaves in the flow aloft.
Right now a cold front is progged to usher in some relief from the
heat Thursday night into Friday, but it also looks to introduce the
relatively better rain chances (40%) through this period.

&&

.AVIATION...(For the 18Z TAFS through 18Z Sunday afternoon)
Issued at 106 PM CDT Sat Jul 22 2023

Current satellite imagery shows a cu field has started to develop
across much of Wisconsin, with a few showers beginning to pop up
east of the TAF sites. The main concern this afternoon and evening
will revolve around any impacts at TAF sites with developing
showers and storms as a shortwave continues to drop across the
area. These showers/storms will be more scattered in nature, so
have only highlighted a short window this afternoon/early evening
with a tempo group at both KLSE and KRST. Will continue to monitor
for any updates, especially to timing, that may be needed in the
shorter term. With any showers or storms, gusty winds and short
periods of visibility restrictions may be possible. Otherwise, VFR
conditions look to prevail.

&&

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

DISCUSSION...EMS/Kurz
AVIATION...EMS

Hazardous Weather Outlook
National Weather Service La Crosse WI
251 PM CDT Sat Jul 22 2023

IAZ008>011-018-019-029-030-MNZ079-086>088-094>096-WIZ017-029-
032>034-041>044-053>055-061-231200-
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-
251 PM CDT Sat Jul 22 2023

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

.DAY ONE...This Afternoon into Tonight

Scattered storms are expected this afternoon and evening across
most of the area. Lightning will be the main threat, though any
stronger storm could also produce small hail and gusty winds.

.DAYS TWO THROUGH SEVEN...Sunday through Friday

Periodic storm chances dot the extended forecast. Monday evening
boasts an isolated chance for severe weather, especially in
portions of northeast Iowa. Large hail looks to be the primary
threat at this time.

Heat and humidity will steadily build through the middle of next
week with highs climbing into the low to mid 90s for most
locations by Wednesday. Thursday will be the hottest day with heat
indices expected to climb into the 95 to 105 range.

.SPOTTER INFORMATION STATEMENT...

No action is needed.

.EMERGENCY MANAGEMENT COORDINATION...

No webinar is scheduled.

Kurz

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