827
FXUS63 KARX 081040
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
540 AM CDT Tue Jul 8 2025

.KEY MESSAGES...

- Seasonable this week with highs in the upper 70s to middle
  80s.

- Chance (20-40%) for showers & storms this afternoon and
  evening.

- More widespread showers and storms likely (55-75% chance) for
  Friday. Storms on Friday may be strong and capable of locally
  heavy rainfall.

&&

.DISCUSSION...
Issued at 346 AM CDT Tue Jul 8 2025

Today: Isolated to Scattered Showers and Storms Possible

GOES-19 water vapor imagery and 08.06z RAP 500mb heights show a
fairly pronounced mid-level trough situated over the region early
this morning as weakly forced showers slowly dissipated overnight.
As this trough pushes eastward, northwesterly mid-level flow will
aid in some cooling aloft which will allow MLCAPE values to rise to
around 1000-2000 J/kg concurrent with peak heating as shown in the
08.06z RAP. The key question that remains is if there will be enough
convective forcing. The recent long-range RAP tries to suggest a
theta-e gradient over the region which coupled with any remnant
differential heating boundaries may be enough to support convection.
The CAMs have mixed schools of thought on exact coverage for
convection this afternoon and evening, however have maintained
some low-end precipitation chances (20-40%) for the area to
address this potential. Regardless, would expect convective
coverage to decrease towards sunset as the nocturnal inversion
builds in.

Wednesday - Thursday: Dry, Seasonable

As we head into Wednesday, upper-level ridging moves into the region
which should allow for mostly sunny conditions on Wednesday. Minimal
ensemble spread observed in the grand ensemble (GEFS/EC/Canadian
ensemble) for high temperatures on both Wednesday and Thursday in
the upper 70s to middle 80s as little modification to the ambient
airmass is observed. Certainly looks like the best period of the
week to get outdoors!

Friday - This Weekend: Storms For Friday, Strong Storms Possible

Conditions turn more active for Friday as a paired shortwave impulse
and broader mid-level trough eject out of southwestern Canada and
into the Northern Plains later on Thursday. As this occurs,
increasing low-level moisture advection and a warm sector will
attempt to push into the region. However, many questions still
remain on any severe potential with this system. Overall question
will be the location of the warm sector, dependent on exact position
of the synoptic upper-level trough as well as if any convective
contamination pushes eastward. If the warm sector is kept south into
IA/IL and near I-80 (as shown in the 08.00z GFS), instability will
be weak which would preclude any severe threat for the local area as
opposed to a further north solution. The second piece of this puzzle
is the aforementioned shortwave impulse pivoting around parent upper-
level trough. Deterministic guidance in the GFS/EC/Canadian would
suggest a narrow corridor of more favorable shear dynamics in the
exit region of this impulse. The trouble with this is there still
remains uncertainty on the exact trajectory of this impulse and
potential collocation with the aforementioned warm sector. Where and
if the more favorable shear overlaps the greater, uncapped
instability in the warm sector, an organized severe threat seems
plausible.

Regardless of severe potential, certainly looks like a more favored
airmass for heavier rainfall as increasing 850mb moisture transport
and mean precipitable waters in the grand ensemble (GEFS/EC/Canadian
ensemble) approach 1.75" to 2". Overall probabilities in the grand
ensemble for 0.5" of rain or greater are fairly modest (30-60%)
throughout the day on Friday. The main question will be the
northerly extent of the warm sects and higher instability causes
more vigorous rainfall rates locally. Either way, the recent Extreme
Forecast Index does highlight portions of the region with a 50 to 60
percent chance to exceed model climatology for QPF with a shift of
tails of 0 indicating a few EC members showing a the percentile QPF
event.

By the weekend, cannot rule out (15-25% chance) some lingering
showers and storms in the northwesterly flow behind the broader
upper-level trough. However, building subsidence behind the trough
may keep these precipitation chances to a minimum. In any case, will
likely cool down from the week long steady-state temperatures
slightly for Saturday with the northwesterly flow in place. As a
result, median high temperatures in the NBM range in the middle 70s
to lower 80s for Saturday.

&&

.AVIATION /12Z TAFS THROUGH 12Z WEDNESDAY/...
Issued at 540 AM CDT Tue Jul 8 2025

VFR conditions are expected through much of the TAF period with
winds remaining between 5-10 kts from the northwest. Isolated to
scattered showers and thunderstorms will be possible (20-40% chance)
later this afternoon and evening, especially east of the Mississippi
River. As a result, have included a prob30 for -shra at KLSE for a
few hours around 00z.  Otherwise, winds trend under 5 kts overnight
with some patchy fog development possible towards 12z, however
confidence remains too low (under 20% chance) to include in the TAF
at this time.

&&

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

&&

$$

DISCUSSION...Naylor
AVIATION...Naylor