849
FXUS63 KARX 242323
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
623 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...
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 similiar
in idea of temperatures cooling back to more near or below normal
for late September via global ensembles.

&&

.AVIATION /00Z TAFS THROUGH 00Z WEDNESDAY/...
Issued at 623 PM CDT Tue Sep 24 2024

Main taf concern is fog development in low lying areas and river
valleys tonight. Light winds/clear skies and strong inversion
develops over the region tonight. This will allow for fog to form
after 08z in low lying areas and river valleys. The main concern is
how widespread fog forms in the Mississippi River Vally to impact
the LSE taf site. At this time...kept previous taf mention of IFR
visibility potentially from 11z to 14z Wednesday...as confidence is
medium on fog impacting the LSE taf site. After 14z Wednesday...VFR
conditions should prevail at both taf sites.

&&

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

&&

$$

DISCUSSION...Shea
AVIATION...DTJ