870
FXUS63 KARX 182003
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
303 PM CDT Wed Sep 18 2024

.KEY MESSAGES...

- Above average temperatures continue into the weekend with some cooling
  through early next week.

- Scattered non-severe storms possible tonight into Thursday
  morning, with higher chances (60-80%) for storms late Thursday
  afternoon/evening, potentially severe, especially near and
  west of the Mississippi River.

- Low to medium (30-50%) rain chances continue later in the
  weekend into early next week.

&&

.DISCUSSION...
Issued at 303 PM CDT Wed Sep 18 2024

Tonight-Thursday night:

GOES water vapor channels early this afternoon showed a trough
across the northern Rockies with a plume of moisture extending north
and west towards it from the plains and much drier low-level air
over the Great Lakes. The axis of stronger moisture transport will
shift east into the area later tonight and Thursday ahead of the
trough, likely resulting in scattered showers/storms, most numerous
near and west of the MS River, gradually diminishing through early
afternoon as the moisture transport weakens a bit before refocusing
late in the day ahead of the trough/front. Storms through the
morning should be non-severe.

The main focus of attention will be late in the afternoon and
evening as the front approaches and ascent increases with the
main shortwave passing by to the north. A narrow corridor of
2000+ J/kg of MLCAPE is indicated by the RAP along/ahead of the
front with cross frontal deep layer shear profiles sufficient
for supercells/updraft rotation before some upscaling/clustering
occurs through the evening. Any initial supercells could pose a
risk for large hail in excess of 1" with a plume of steeper
mid-level lapse rates ahead of the trough, especially west of
the Mississippi River. A tornado could not be completely ruled
out with any supercell with modest curvature noted in low-level
hodographs. These threats likely would be greatest earlier in
the storm life cycle before upscale growth occurs with a
wind/hail risk. Eventually, storms should weaken eastward
through the evening as diurnal cooling occurs/instability wanes.


Friday-Wednesday:

Weak high pressure builds back into the area on Friday with temps
staying above average in the mid 70s to mid 80s given only slight
low-level cooling. There continues to be spread in the model suite
with the evolution of upper level troughs progressing from the
Desert Southwest and from northern plains/southern Canada through the
weekend into early next week, especially with the amplified ridging
across western Canada. With the uncertainty in the flow progression,
rain chances remain in the 30 to 50% range Saturday night through
early in the week. A general cooling trend is expected early next
week, although global ensemble members show notably higher spread
during this time. NBM 25th-75th percentile max temps are mainly in
the mid 60s through the 70s during this time.

&&

.AVIATION /18Z TAFS THROUGH 18Z THURSDAY/...
Issued at 1238 PM CDT Wed Sep 18 2024

VFR conditions will prevail through the first half of the TAF
period. An approaching cold front from the west will increase
southerly winds Thursday morning and bring a 30% chance for
showers and thunderstorms in the morning. I have included a
prob30 at RST and LSE for these chances. Brief MVFR visibilities
will be possible with any thunderstorms that develop, but
ceilings are expected to remain VFR. Beyond this TAF period, the
cold front sweeps through the area Thursday evening, bringing
gusty winds and a line of showers and thunderstorms Thursday
evening.

&&

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

&&

$$

DISCUSSION...JM
AVIATION...JAW