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769
FXUS63 KARX 250336
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
1036 PM CDT Tue Sep 24 2024

.KEY MESSAGES...

- More valley fog expected next couple nights.

- Rain chances return to the region late in the week, especially
  this weekend, as tropical system remnants approach from the
  south.

- A weather pattern early next week could lead to more typical
  autumn temperatures.

&&

.DISCUSSION...
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Issued at 140 PM CDT Tue Sep 24 2024

Valley Fog Next Couple Nights

Tis the season...for valley fog that is. With several more
nights of mostly clear skies, light low level flow, and longer
nights, likely will see more bouts of valley fog. Obviously
will need to be watching the subtle day to day details for
specific impacts, especially as it relates to aviation
interests.

Rain Chances Return This Weekend

Highly amplified upper air pattern and related evolution will be key
to future rain chances. This is especially true as upper trough
becomes cutoff to our south and tropical system Helene enters
the picture. Approaching tropical systems always seem to disrupt
flow patterns and often can stall normal progression.

As low becomes established more centered on lower to mid Mississippi
River valley, upper ridge axis will shift and tilt further east,
likely keeping our weather quiet and pleasant. Expecting dry regime
for much of the week with above normal temperatures.

Interestingly medium range guidance suggest remnants of
tropical system wrapping up into cutoff low which could
gradually weaken over time, but still looks to push ridge axis
back west by late week. Precipitation shield expected to rotate
through parts of the mid Mississippi and Ohio River valleys but
probabilities decrease as you move northwest based on strength
of ridge and drift of reformed upper low from previous system
and tropical feature.

This is the biggest differences in guidance - how far north this low
will get. Cluster analysis shows differences between low locations
roughly 250 miles apart with ECMWF the furthest south and GEFS more
north. In fact, from late Friday on, the Canadian ensemble members
and some GEFS are the main solutions producing rain closer to
our service area with the mean and bulk of other outcomes
remaining completely dry. This is a lot of detail to say small
rain probabilities enter the forecast in our southeast service
area over the weekend and gradually work northwestward by Sunday.
But confidence in how far north this might get, and when,
remains on the lower side. Higher confidence for drier outcomes,
especially in west and northern areas. This bears out with
24.12z guidance suggesting even a drier look with lower rain
chances and drier ensemble changes loaded in. Probabilities for
measurable rain Friday/Saturday went from roughly 30% to 10%.

Pattern Shift Early Next Week

Nearly a week out but medium range suggesting broad upper ridge
sets up across the western CONUS, putting upper Midwest back
into a cooler northwest flow regime. Certainly differences in
deterministic runs with regards to ridge strength but similar
in idea of temperatures cooling back to more near or below
normal for late September via global ensembles.
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&&

.AVIATION /06Z TAFS THROUGH 06Z THURSDAY/...
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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.
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&&

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

&&

$$

DISCUSSION...Shea
AVIATION...Kurz