507
FXUS63 KARX 261147
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
647 AM CDT Thu Sep 26 2024

.KEY MESSAGES...

- Favorable conditions for morning river valley fog through
  Friday.

- Forecast remains dry (85 to 100% probability) as mid-
  tropospheric l ow/remnants of Hurricane Helene continue to be
  forecast southeast of the area.

- Warmer than normal temperatures through Monday, then a pattern
  change to more seasonable weather.

&&

.DISCUSSION...
Issued at 255 AM CDT Thu Sep 26 2024

Overview:

Water vapor satellite imagery, heights, and lightning showed a
closed low over the Mid-Mississippi Valley with a ridge from the
Plains into the Southwest U.S. Helene is now a category 1 hurricane
and was located over the Gulf of Mexico. At the surface, broad high
pressure was located over the region. Similar to Wednesday morning
at this time, under mostly clear skies with calm or light winds and
low dewpoint depressions, river valley fog has already begun to
form.  Mississippi River temperatures are still in the 70s.

Morning river valley fog continues through Friday with above
normal temperatures and mainly dry conditions into early next
week:

The closed 500mb low over Illinois is forecast to settle south today
as Hurricane Helene is forecast to increase to a category 3.
Meanwhile, a ridge of high pressure continues to build into the
Upper Mississippi Valley. The current deterministic models continue
to be in good agreement with the evolution of the closed area of low
pressure sagging southward and merging with remnants of Hurricane
Helene, gradually lifting north and east toward the Ohio River
Valley and eventually the Mid-Atlantic States this weekend. Still
some differences for the weekend as the operational European is
slightly farther northwest with the track and slightly slower with
it exiting the region.  The ECS/GEFS/GEPS probabilities for rain
through the weekend remain quite low with 0-15% through Monday.

With the ridge building; rising heights and warmer 850mb
temperatures, we have highs in the 70s to lower 80s forecast into
early next week.  At this time, the ridge breaks down Monday with
colder air arriving Tuesday, where highs dip back into the 60s.
The 25.12Z probability for measurable rain Monday night was 5-20%.

The 25.12Z cluster analysis for early next week has a variety of
solutions related to the strength of the trough/ridge across Canada
and the northern U.S.  This plays into a widening of solutions for
Max and Min temperatures from Tuesday and beyond.

&&

.AVIATION /12Z TAFS THROUGH 12Z FRIDAY/...
Issued at 642 AM CDT Thu Sep 26 2024

Been backing off on the valley fog for KLSE this morning. With
much of the fog this morning over the main channel of the
Mississippi River, opted to go BCFG with a tempo of 1 1/2 statue
miles. If the fog moves into KLSE, ceilings will briefly drop
into the 100 to 300 foot range. Otherwise, skies will be clear.
For late tonight, did add a BCFG starting at 27.11z for KLSE.

&&

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

&&

$$

DISCUSSION...Zapotocny
AVIATION...Boyne