Severe Thunderstorm Warning 108, NWS La Crosse, WI

Area Forecast Discussion
National Weather Service La Crosse WI
1220 PM CDT Sat Jul 23 2022

.DISCUSSION...(Today through Friday)
Issued at 339 AM CDT Sat Jul 23 2022

Key Messages:

 - Today: Heat & significant severe weather event becoming more likely

 - Tuesday night/Wednesday: next chances for rain

 - Next week: cooler, 70s to low 80s afternoon temps

* Today: Severe Storms

This potentially significant severe weather event has been the main
talking point in previous forecasts of the past couple of days. Now
that the event has come within range of the CAMs, confidence has
increased for severe storms to impact a majority of the local
region. However, exact timing and placement details will depend on
ongoing convection, the capping inversion, and how well the cold
pool develops to dominate storm progression.

Satellite water vapor imagery shows an upper-level trough beginning
to exit the northern Rockies. This wave is slated to progress
eastward today with an associated surface low pressure system. Ahead
of this system, plenty of warm and moist air surges into the region
as the warm front lifts northward and low-level jet increases
advecting in these conditions along with increasing moisture
transport ahead of the surface cold front.

There's no question that there will be plenty of instability. The
23.00z HREF guidance for ensemble minimum SBCAPE has much of the
forecast area exceeding 2000 J/kg by noon. By mid afternoon, some
locations push near 4000 J/kg. Mean CIN values by this time would be
around between 100 and 25 J/kg as the HREF suggests. There is some
question on the exact timing and when the capping inversion breaks.
For example, the ARW 23.00z run has an initial round of convection
reaching southeast Minnesota before noon, whereas the FV3 23.00z run
is not until closer to 5 PM with the other high resolution guidance
falling between those times. Forecast soundings having the capping
inversion centered around 850 mb still in excess of 125 J/kg by late
morning lowers confidence that convection would be initiating by
that time. There could be multiple rounds of storms. The potential
initial round of storms breaking the early afternoon cap ahead of
the main convective line, the convective line itself being cold pool
driven, and a final round with the passing of the surface cold
front. The main focus for timing of severe storms are from 1 PM to 8
PM with HREF probabilities being over 50% for convection to enter
the forecast area around that time. Later into the overnight hours
is when flooding becomes the main potential hazard.

However, what could throw a wrench into this forecast pertains to
the ongoing convection in north central South Dakota and south
central North Dakota. Radar imagery has shown these storms have
developing over that area in the last few hours which is not
currently being handled well by the meso models. If this continues
on its current trajectory, then the local area could see convection
earlier than forecast as early as 15z.

As for the primary threats, damaging wind, large hail, and repeated
rounds of heavy rainfall are all anticipated. Although all threats
are possible depending on storm development and structure. Plenty of
shear in the lowest km would also introduce the threat for
tornadoes. So cannot rule them out, especially if discrete
supercellular storms become surface based at convection onset or
within any favorably oriented bowing segments within a QLCS as
storms cluster together by early evening. Mid-level lapse rates
exceeding 9 C/km would support large hail development mainly with
initial discreet storms this afternoon. The wind threat increases
later in the day into the evening as cells form into a line becoming
cold pool driven and dominant. Overall, this has the potential to be
a significant event with possible 2" hail and wind gusts up to 75
mph.

Last but not least, the heavy rain threat. PWs pushing over 2" and
warm cloud depths increasing to near 4 km would support very
efficient rainfall production. This in combination with multiple
rounds of rain over the same location increases the risk for urban
and flash flooding. After the main line of severe storms pass, then
lingering flooding impacts could last into the overnight hours. HREF
guidance for QPF LPMM has a near east to west oriented narrow swath
of 2-4" of accumulation just north of I-90 on the western edge
sagging a touch southward on the eastern end just south of the I-
90/I-94 corridor. Again this is still dependent on location and the
axis of highest accumulated amounts will likely shift depending on
storm tracks and evolution.

In addition to the severe storms, heat indices around 100 across the
southern tier of counties of the forecast area has called for a heat
advisory for today. If precipitation moves over the region earlier
than expected, this would result in lower afternoon temperatures
therefore decreasing the risk for hazardous heat conditions.

* Next Week

Overall cooler conditions are expected with afternoon temperatures
topping out in the 70s to 80s. The next chance for precipitation
comes Tuesday night into Wednesday. While there are still plenty of
differences in ensemble and deterministic model guidance with
specific timing and exact placement, the general consensus has an
upper-level trough and associated surface cold front to sweep
through the Upper Mississippi River Region. It is still to early to
discern the potential for severe storms at this time, and will
continue with the blended model guidance.

&&

.AVIATION...(For the 18Z TAFS through 18Z Sunday afternoon)
Issued at 1221 PM CDT Sat Jul 23 2022

Ongoing thunderstorms to the west will move into the area at the
beginning of the TAF period. When thunderstorms reach the
airports, expect deteriorated visibilities and lower ceilings.
Periods of MVFR conditions are expected with the storms. Winds
will also be gusty, particularly at KRST where gusts could be as
high as 50 kts. Showers and storms will linger into the late
evening at both sites before clearing out overnight. Ceilings
should remain lower overnight before skies become clearer tomorrow
morning. South/southeast winds will become more west/northwest and
less gusty overnight.

&&

.ARX WATCHES/WARNINGS/ADVISORIES...
WI...Heat Advisory until 8 PM CDT this evening for WIZ061.

MN...NONE.
IA...Heat Advisory until 8 PM CDT this evening for IAZ018-019-029-030.

&&

DISCUSSION...Peters
AVIATION...Dahl

Hazardous Weather Outlook
National Weather Service La Crosse WI
407 AM CDT Sat Jul 23 2022

IAZ008>011-018-019-029-030-MNZ079-086>088-094>096-WIZ017-029-
032>034-041>044-053>055-061-241315-
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-
407 AM CDT Sat Jul 23 2022

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

.DAY ONE...Today and Tonight

A potentially significant severe weather event has become
increasingly likely for this afternoon and evening. Damaging
winds in excess of 70 mph, large hail near 2", and heavy rainfall
are the main threats. Cannot rule out the possibility for
isolated tornadoes. Repeated rounds of heavy rain would increase
the threat for localized flash and urban flooding. There are some
questions on where the highest impacts may occur and this could
still shift north or south.

Afternoon and early evening heat indices around 100 are expected
this afternoon but mainly for areas south of highway 18 across
portions of northeast Iowa and far southwest Wisconsin.

.DAYS TWO THROUGH SEVEN...Sunday through Friday

Storms are possible Tuesday night into Wednesday. At this time,
organized severe weather is not expected.

.SPOTTER INFORMATION STATEMENT...

Spotters will likely be needed this afternoon and evening.

.EMERGENCY MANAGEMENT COORDINATION...

No webinar is scheduled.

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE LA CROSSE WI
124 PM CDT SAT JUL 23 2022

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0123 PM     TSTM WND GST     DODGE CENTER            44.03N 92.85W
07/23/2022  M48 MPH          DODGE              MN   AWOS

&&

SHEA

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