067
FXUS63 KARX 251814
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
114 PM CDT Wed Sep 25 2024

.KEY MESSAGES...

- Favorable radiation/steam fog (cool nights/warm water) conditions
  for valley/river valley fog each morning.

- Threat for rain decreases as mid-tropospheric low/remnants of tropical
  storm Helene currently forecast to remain south of the area.

- Warmer than normal temperatures through early next week, then
  a swing to more seasonable weather.

&&

.DISCUSSION...
Issued at 204 AM CDT Wed Sep 25 2024

Overview:

Water vapor satellite imagery, heights, and lightning showed a
closed low over the Mid-Mississippi Valley with a ridge over the
Rockies and another trough approaching California.  Tropical storm
Helene was located over the Caribbean. At the surface, weak high
pressure was located over the region with surface low pressure over
Michigan and also over the western High Plains. Under mostly clear
skies, the cool locations already had low dewpoint depressions.
We`ve had valley fog/river valley fog the last two mornings and
fog is developing about three hours ahead of the previous
mornings. Mississippi River temperatures are in the 70s...thus
with temperatures in the 40s to around 50...in addition to
favorable radiational conditions/calm winds at KLSE, the
water/air temperatures are conducive to fog formation. Nighttime
microphysics satellite imagery is also showing the smaller
Wisconsin river valleys with fog.

Valley fog continues each morning through Friday with above normal
temperatures and mainly dry conditions into early next week:

Conditions continue to remain favorable for valley fog through
Friday morning.  Calm to light winds in the low levels, cool nights
with few clouds, a low level inversion, and warm Mississippi River
water temperatures in the 70s.  Some hints at patchy clouds Thursday
morning and less of an inversion Friday morning, thus extent of fog
could be affected by these factors.

The closed 500mb low is forecast to remain over the Illinois with
the trough extending toward the Great Lakes. Over time, the closed
low retrogrades westward as tropical storm Helene heads toward the
Florida Coast. Meanwhile, a ridge of high pressure builds into the
Upper Mississippi Valley. The current deterministic models appear to
be in good agreement with this evolution and that the two areas of
low pressure merge into one over the Mid-Mississippi/Tennessee
Valley Friday.  Some differences by Sunday where the storm system
will edge closer to the forecast area or remain to the south and how
quickly the ridge breaks down with colder air arriving next
week.

With the ridge building; rising heights and warmer 850mb
temperatures, we have highs in the 70s to lower 80s forecast for the
rest of the week into early next week.  The 25.00Z cluster analysis
had shifted the rain south of the forecast area. The 24.12Z
probability for measurable rain was 0-10%, so would think when the
00Z data comes in it would be even lower. We`ll need to
continue to monitor the storm evolution with remnants of
tropical storm Helene as the track could change over the next
few days.

There are still some differences in when the ridge breaks down, but
it does look like a return to seasonable or cooler than seasonable
highs in the 60s could occur early next week.

&&

.AVIATION /18Z TAFS THROUGH 18Z THURSDAY/...
Issued at 107 PM CDT Wed Sep 25 2024

VFR conditions will prevail at both RST and LSE through the
afternoon under high pressure. A field of FEW-SCT cumulus has
developed across much of the region, generally around 4000ft.
This deck may vary BKN at times, especially south of I-90, but
is expected to dissipate this evening.

Valley fog is expected to impact LSE again late tonight into
Thursday morning as high pressure, clear skies, and weak winds
continue to influence the area. Currently thinking that fog
will begin to restrict visibilities to LIFR conditions around
26.10z although there may be drops in visibility in the hour or
two before. Have gone with a 1/2SM visibility for now, but
visibility of 1/4SM does appear possible again tonight. Fog
should dissipate by late Thursday morning with VFR conditions
again Thursday afternoon.

&&

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

&&

$$

DISCUSSION...Zapotocny
AVIATION...Falkinham