945
FXUS63 KARX 270846
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
346 AM CDT Fri Sep 27 2024

.KEY MESSAGES...

- Above normal temperatures and mainly dry conditions into early
  next week. After this morning, the valley fog should be more
  limited this weekend due to the stronger winds aloft.

- 10-20% probability for a shower north of I94 with the cold
  front Tuesday night...then turning cooler for Tuesday.

&&

.DISCUSSION...
Issued at 330 AM CDT Fri Sep 27 2024

Overview:

Water vapor satellite imagery, heights, and lightning showed a
closed low over the Tennessee River Valley/northern Mississippi with
Hurricane Helene, a category 4, already dropping to a category
2 and centered over southern Georgia. A ridge of high pressure
is over parts of the Southwest U.S. into the Upper Mississippi
Valley with a trough over Manitoba with zonal flow farther west.
At the surface, high pressure was located over the region,
however the trough over Manitoba was dragging a cold front
through the Dakotas. With the clear skies,calm winds, and low
dewpoint depressions, river valley fog had re-developed. Of note
also is the cirrus shield making it into southern Wisconsin and
parts of eastern Iowa.

Above normal temperatures and mainly dry conditions into early next
week.  After this morning, the valley fog should be more limited
this weekend due to the stronger northeast winds aloft:

Hurricane Helene is forecast to merge with the 500mb low to the
west. This area of 500mb low pressure stalls over the
Ohio/Tennessee Valleys through Sunday, gradually lifting
northeast into the Mid- Atlantic region Tuesday. Meanwhile, a
cold front over the Dakotas will moves through Minnesota and
wash out. Mid-tropospheric ridging builds back in for Saturday.
Cirrus will increase, but with time, patchy low and possibly mid
level moisture will try to work into parts of southern WI. The
strengthening northeasterly flow is forecast to mitigate fog
development for Saturday and Sunday morning. That being said,
the low levels do decouple and could see some isolated valley
fog develop, thus will still need to monitor this through the
weekend. The ECS/GEFS/GEPS probabilities for rain through the
weekend remain quite low, only 0-10% through Monday.

The 850mb thermal ridge is more over MPX`s area with similar
temperatures today locally compared to yesterday.  For Sunday, the
850mb temperatures rise, but then some cooling occurs at 850mb for
Monday.   Highs in the 70s to lower 80s remain on track into early
next week.

10-20% probability for showers north of I94 then turning cooler
for Tuesday:

The deterministic EC/GFS are consistent in bringing a cold front
through Tuesday night.  The ECS/GEFS/GEPS probabilities for rain
through 01/12Z are 10 to 20% north of I94. A more zonal flow
pattern develops for Wednesday through Friday with chances for
light rain returning sometime Wednesday night through Thursday
night. Only one day of the cooler highs in the 60s Tuesday,
then back to seasonable temperatures to end the week.

&&

.AVIATION /06Z TAFS THROUGH 06Z SATURDAY/...
Issued at 1108 PM CDT Thu Sep 26 2024

Valley fog remains the main aviation concern with LIFR/IFR fog
and low-stratus at KLSE for the approaching morning hours.
Conditions appear to remain favorable for valley fog development
across much of the local area with a light wind layer to around
5-6kft in model soundings and falling dewpoint depressions.
Minimal cloud cover is expected through early morning, although
may need to watch if any cloud cover can sneak (10-20% chance)
from the southeast which may hinder fog development. Otherwise,
trends appear similar to previous nights with dewpoint
depressions at KLSE being 6 degrees F as of 27.03z. As a result,
have introduced a tempo for 1/4 SM at KLSE, favoring later into
the morning when considering potentially drier soils hindering
fog formation some and also seasonal climatology allowing fog to
linger later into the morning. After fog/low-stratus lifts, VFR
conditions are expected through the remainder of the TAF period
with some high clouds and generally light winds under 8 kts.

&&

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

&&

$$

DISCUSSION...Zapotocny
AVIATION...Naylor