763
FXUS63 KARX 090903
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
403 AM CDT Wed Jul 9 2025

.KEY MESSAGES...

- Sunny today with high temperatures in the low to mid 80s.

- Precipitation and storm chances initially overnight into
  Thursday morning slowly progress east into Thursday night,
  increasing overnight into Friday morning.

- Highest confidence for local storms Friday into early Saturday
  morning. Heavy rain appears to be the main threat with 1" to
  1.5" or more possible in spots. Exact location limits overall
  confidence (30-50%) for impacts including strong to severe
  storm potential.

&&

.DISCUSSION...
Issued at 349 AM CDT Wed Jul 9 2025

Today & Tonight; Sunny & Late Night Rain Potential:

A seasonable, mostly sunny day is on tap as an upper level synoptic
ridge over the Rocky Mountain West, shown on early morning GOES
water vapor imagery loop, shifts east. A line of storms will
initiate in the Northern Plains later today into tonight and
push east towards the Upper Mississippi River Valley. The (mostly)
driving open wave of upper level cyclonic flow off the California
Coast on early GOES water vapor imagery and accompanying ageostrophic
ascent from an attendant upstream anticyclonically curved jet
streak will rapidly weaken as it contends and skirts around anticyclonic
flow over the Desert Southwest early this morning. As a result,expect
a rapid weakening trend as leftover precipitation and storms reach
the Upper Mississippi River Valley late tonight into early Thursday
morning. However, supplementary synoptic areas of ascent do attempt
to drive a low level moisture boundary, oriented north-northwest
to south- southeast, through the forecast area, challenging the
drier air in place. The resultant narrow corridor of isentropic upglide
along the frontal boundary is causing high resolution model reflectivities
to suggest a disjointed line of scattered storms along our western
peripheral counties in southeast Minnesota and northeast Iowa into
early Thursday morning. The aforementioned weakening in overall synoptic
forcing and resultant dependency on minute disturbances limits confidence
and any impacts, but will be immediate term forecast detail to remain
cognizant of.

Preciptiation & Storm Chances Through Thursday:

Precipitation chances slowly increase through late Thursday morning
as low level moisture transport surges through the Upper Mississippi
River Valley. Given the ample moisture, have increased PoPs above a
mostly dry NBM (09.01Z) through Thursday afternoon in collaboration
with neighbors. However these may still require increasing as high
resolution models peer further into the longer forecast hour as a
wave of mid level ascent has some models suggesting mesoscale cyclogenesis
between two areas of increased low level theta-e lobes. High resolution
model soundings show ample instability 500-1000 J/kg, little to
no deep shear, very slow storm motions, and surface relative
vorticity maxima causing splotches of 1+ NST values in the RAP
over the forecast area.

Heavier Rainfall & Moderate Storm Confidence (50%) Friday:

The highest confidence for storms reaches the local area on Friday
with the main longwave trough over the Gulf of Alaska on early
morning water vapor imagery traversing the Upper Midwest. LREF/Cluster
confidence suggests highest potential Friday afternoon and evening.
However, specifics are murky due to the length of potential lull
in rainfall Friday morning and a large spread between 500 mb cluster
heights. Most noticeable in location of heaviest rainfall as 50%
probabilities for 1" of rainfall in 24 hours varies from northern
to southern Minnesota between clusters with 37% of members placing
a bullseye over southeast Minnesota by Saturday morning.

Rainfall Amounts & Severe Storm Potential:

While there is quite good agreement between clusters for 2" PWATs
traversing the forecast area, uncertainty in overall amounts remains.
The cluster with the highest members also exhibits the heaviest accumulations
(1.25") due to the deepest and slowest solution of the mid level trough;
preventing rapid lifting of the low while leveraging more moisture.
Intra-cluster 25th to 75th spreads 0.5" to 0.9" with inter-cluster
25th from 0.5"-0.75" to 75th from 1" to 1.75"; exhibiting a wet tail
or more uncertainty in the highest amounts instead of the
overall amounts.

Strong to severe storms also present a concern Friday as LREF & LREF
time-lagged cluster sounding plumes show 1000-1500+ J/kg of CAPE with
30kt of 0- 3km shear. Joint probabilities (CIN/Shear/CAPE) of 30-50%
suggest a very narrow area of concern likely along an associated frontal
boundary given stunted ensemble hodographs.

Uncertainty For Next Week:

Expect a short break in storms Sunday before a mostly active pattern
through next week. LREF (EPS/GEFS) mean and probabilities for
500mb heights suggest a succession of perturbations along quasi-
zonal flow. Agreed upon probabilities for 0.01" over 24 hours is
highest (50-80%) Tuesday and Wednesday with disagreement for
Monday and Friday as the EPS suggests 10-40% both days.

&&

.AVIATION /06Z TAFS THROUGH 06Z THURSDAY/...
Issued at 1221 AM CDT Wed Jul 9 2025

Introduced immediate term potential aviation impacts. Surface
and satellite observations have exhibited surface temperatures
decrease near saturation with clear skies and light winds over
the forecast area. Besides KOVS in the climatologically dense
fog Wisconsin River Valley, only noticing scattered BR mention
at AWOS in southeast Minnesota and central Wisconsin as of
09.06Z. Given ongoing observations and timing have reduced
visibility grids across the forecast area from southeast
Minnesota, northeast Iowa, and western into central Wisconsin
through the early morning hours. Unusually, confidence is lowest
along the Mississippi River Valley where warm spots have been
keeping RHs lower. Fog concerns cease with diurnal heating
onset.

VFR expected through the rest of the 09.06Z TAF period.

&&

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

&&

$$

DISCUSSION...JAR
AVIATION...JAR