752
FXUS63 KARX 190824
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
324 AM CDT Thu Jun 19 2025

.KEY MESSAGES...

- Extreme heat this weekend with heat indices from 95 to 105
  degrees across the forecast area, warmest south of Interstate
  94 locally in southeast Minnesota, northeast Iowa, and western
  Wisconsin.

- Persistent precipitation and storm chances primarily north of
  Interstate 90. Widespread storm chances tonight push
  southeast. Confidence in strong to severe storm chances
  locally is low.

- Consistent heavy rainfall through next raises flooding
  concerns across the forecast area. Many details to discern at
  the current forecast area before confidence increases.

&&

.DISCUSSION...
Issued at 241 AM CDT Thu Jun 19 2025

A massive anticyclone over the Desert Southwest on early morning
water vapor imagery will dictate the forecast over the next week.
Initial contention between its northeastern periphery building
northeast through the Upper Mississippi River Valley today and a
diving perturbation drives potential severe storm risk along
our northern peripheral counties in central Wisconsin.
Increased uncertainty in southwestern extent, again influenced
by strength and timing of capping inversion tied to the anticyclone.
Rapidly building heights with a tightened low level pressure
gradient overnight into Friday morning ushers in strong return
low level flow causing widespread precipitation and storm
chances through Friday morning. The anomalous strength of the
anticyclone overhead through the weekend fuels extreme heat
across the Upper Mississippi River Valley. As the ridge breaks
down through next week, precipitation chances frequent the
forecast with heavy rain and flooding concerns.

Extreme Heat This Weekend:

Anomalous temperatures accompany the anomalous ridge, warming to ne
near 30C at 850mb (GEPS/GEFS/EPS) over the Upper Midwest through the
weekend. LREF trends exhibit strong agreement for warmest
temperatures, 15C above normal with an ECMWF EFI from 0.8 to 1.0
with >0 SOT, settling in Saturday locally. See Climate section
below for further information.

Storm Potential Later This Afternoon & Evening:

Initial storm chances later today will be tied to a southeast
diving mid level wave seen on upper level water vapor imagery
over southern Canada running ridge periphery. Similar to
previous forecasts, exact placement remains difficult to nail
down with a capping inversion building northeast battling the
wave diving southeast. High resolution models show a narrow
area of low level convergence along our northern periphery in
central Wisconsin. Model instability appears clouded by strong
level mixing as we may see more cloud cover with the corridor
of moisture. Regardless, sufficient instability expected with
cooling mid level lapse rates. The strong jet provides ample
shear with all model hodograph plumes exhibiting a very long
hodograph with mean 0-3km HREF shear from 30 to 40kts and 0-6km
of 50+kts In other words splitting moving supercells will be
possible. Question of forcing strength and exact location remains
the main discerning factor, mesoscale forcing and location of
capping strength will be ultimate details to monitor through the
afternoon.

Precipitation Chances Overnight Through Friday Morning:

Subsequent widespread precipitation chances form on the nose of a
very strong low level jet and moisture axis of 1000 kg m-1 s-1
Integrated Vapor Transport. This strong axis rounds the ridge
periphery subsequent sagging southeast, bringing storm chances
with it. Storms expected to degrade on southwestern periphery as
they contact capping associated with the high pressure and
build along the northeastward periphery. Very long hodographs
and 55+ kt shear suggest a potential overnight MCS. Low level
inversion limits surface based convection but damaging winds
will be the main concern. Current confidence places highest
impacts along and to our north but will be very important to
monitor over coming high resolution forecast models for
southwestern periphery scooting farther into the forecast area.

Storm Chances Friday Overnight Into Saturday Morning:

A short reprieve in higher precipitation chances through Friday
afternoon from anticyclonic appendages and low level dry air
comes to an end Friday night primarily along our northern
periphery as a trough appendage from a lee Rocky Mountain low
advects a low level warm front north across the forecast area on
the nose of a 70kt 850mb jet according to the most recent RAP
(19.03Z) forecast. Difficulty will be an extremely strong low
level capping inversion taking shape locally. Confidence for
initial convection is extremely low. Overnight convection will
also be a concern as a ripple runs through the ridge periphery.
Again, confidence is very low as the ridge is expected to
strongly impact any storm potential.

Persistent Heavy Rain Chances Through Next Week:

This weekends anomalously strong upper level ridge breaks down
through the start of next week ushering in 1.5" PWATs according
to LREF models. The Bermuda high then quickly cuts off additional
synoptic forcing floating a surface boundary over the forecast
area along mid level zonal flow through much of the week.
Concerning heavy rain setup as the front will waiver north and
south slightly through the week and may cause multiple days of
heavy precipitation over the same spots.

The dependency on longer range ensembles with wider grid spacing
resolution due to the current longer forecast hour leaves much
to be desired for resolving confidence of highest impacts.
Multiple inches of rainfall are possible through the week likely
causing river and areal flooding should rainfall repeat over the
same areas. Current areal confidence isn`t representative as
narrow bands don`t align spatially and skews any true signal for
impacts.

&&

.AVIATION /06Z TAFS THROUGH 06Z FRIDAY/...
Issued at 1233 AM CDT Thu Jun 19 2025

VFR conditions expected at both TAF sites through the 19.06Z TAF
period. Should aviation impacts of TSRA occur, highest
confidence will lie in central Wisconsin. Could see a stray
shower as far south as KLSE but very low confidence. Winds turn
counter-clockwise through today, becoming west through the
afternoon and south-southwest towards the end of the 19.06Z TAF
period.

Widespread storm chances accompanying strong south-southwest
winds shortly after the 19.06Z TAF period. Line of storms sags
southeast through Friday morning. Expect TSRA mention in the
next couple of TAF periods.

&&

.CLIMATE...
Issued at 1255 PM CDT Wed Jun 18 2025

Record Warmth This Weekend (Record / Forecast):

Anomalous warmth builds in through the weekend with daytime
temperatures in the low to mid 90s in 25th to 75th LREF member
spread. While these temperatures align with <25th percentile
NBM, it appears there is a warm bias coming to fruition and
should be monitored for accuracy over the coming forecasts.

Similarly high confidence for dewpoints in the low to mid 70s
provide 80-100% confidence for 100F heat indices from the
Missouri River Valley to the Mississippi River Valley for
Saturday. Sunday lacks such agreement as the GEFS maxes out at
35 while EPS and GEPS continue the same probabilities. While
confidence is high, dependency on the strength of the building
ridge remains the deciding factor.

  Maximum              Warm Low
June 21
La Crosse:    102 (1988) / 9574 (1943) / 79
Rochester:      99 (1988) / 9372 (1943) / 77

June 22
La Crosse:    97 (1995) / 9575 (1983) / 79
Rochester:     94 (1911) / 93        71 (1983) / 77

&&

.ARX WATCHES/WARNINGS/ADVISORIES...
WI...Extreme Heat Watch from Saturday afternoon through Sunday
     evening for WIZ032-033-041-053>055-061.
MN...Extreme Heat Watch from Saturday afternoon through Sunday
     evening for MNZ079-086>088-094>096.
IA...Extreme Heat Watch from Saturday afternoon through Sunday
     evening for IAZ008>011-018-019-029-030.

&&

$$

DISCUSSION...JAR
AVIATION...JAR
CLIMATE...JAR