260
FXUS63 KARX 250712
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
212 AM 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 /06Z TAFS THROUGH 06Z THURSDAY/...
Issued at 1035 PM CDT Tue Sep 24 2024

Main aviation weather forecast concern this period will be
potential for valley fog impacting LSE late tonight. Skies
should remain mostly clear with light winds at the surface,
allowing for a strong surface inversion. On the plus side for
fog development, recent forecast soundings show a trend toward a
deeper light wind layer just off the surface, and 03Z dewpoint
depressions of 5 degrees is very favorable. On the negative
side, although winds will remain near calm at LSE, the
prevailing direction is more westerly and not the favored
south/southeast that typically brings thicker fog from the
channel into the airfield. Some model guidance isn`t as "sold"
on fog at LSE as they were last night, but their dewpoint
depressions at this hour are also larger than current
observations. So, medium to high confidence that fog is likely
and have steered TAFs that direction, but lower confidence on
just how low visibility may tank. Have gone low end IFR
visibility and LIFR ceilings and will amend as necessary. VFR
conditions for the remainder of the TAF period at both RST/LSE.

&&

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

&&

$$

DISCUSSION...Zapotocny
AVIATION...Kurz