706
FXUS63 KARX 230700
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
200 AM CDT Mon Sep 23 2024

.KEY MESSAGES...

- Near normal temps next couple days, then milder regime returns for
the rest of the week (5 to 10 degrees above normal).

- Trending dry for the new work week. More uncertainty in rain
potential for the weekend.

&&

.DISCUSSION...
Issued at 200 AM CDT Mon Sep 23 2024

OVERVIEW: weak area of surface high pressure builds across the area
today while a spin in the upper layers of the atmosphere tracks east
across the central plains. Northern fringes of this feature could
glance across parts of northeast IA/southwest WI.

A stronger, better developed shortwave trough is progged to drop
south/southeast across the northern plains tonight, diving across
the MO river valley by 00z Wed. As it does it will lift the weaker
ripple northeast across the eastern great lakes - perhaps skimming
across far eastern parts of the forecast area Tue.

The GFS and EC have been at odds with how to handle the stronger
shortwave, but last few deterministic runs of both models lean into
what was originally a GFS-centric solution - dropping the trough
over the lower mississippi river valley and developing a cutoff low.
Comparing to WPC cluster analysis, all 4 of the clusters have this
solution with some variety with how strong the cutoff low could be
and strength of upper level ridging. Compared to its last several
runs, all WPC clusters show a stronger/sharper ridge axis running
northeast-southeast across the great lakes at 00z Sat while also
strengthening the cutoff low. There is one, somewhat weaker solution
of the 4, but this is in the minority with only a handful of the
GEFS/EPS ensemble members favoring it.

To muddle the picture is a potential tropical system to track
northward out of the gulf Thu, looping counter-clockwise around the
cutoff low. This could spin remnants of the tropical system across
the region before both upper level features merge and get a shove
east/northeast from another shortwave trough dropping from western
Canada to across the northern plains for the start of next week. A
lot more uncertainty with how the weekend with play out with the
addition of the tropical system - and probably won`t get too much
clarity for a few days.

10- PCPN CHANCES: there could be low end chances (10-20%) across the
far south and then far east for Mon/Tue respectively thanks to the
aforementioned bit of upper level energy. Latest NBM keeps any
influence farther south and east for the time being and will run
with that for pcpn chances (thus dry).

Looks to stay dry for the rest of the work week with growing
uncertainties on the rain potential for the weekend. How the
interaction of the possible tropical system and the cutoff low is
murky at best. About 1/2 of the EPS members spread at least light
QPF over parts of the forecast area with only a handful of support
in the GEFS. For now, the grand ensemble of models paints light qpf
across the southern portion of the forecast area with the NBM
sketching low end pops (20%). This looks reasonable given the setup
and development questions.


TEMPS: a couple more days of near normal temps for the area before
an upper level ridging bends northeast from the plains. A return to
a more mild regime has been progged by both the GFS and EPS and the
ensemble suites continue to place 75% of their members at or above
the normals into the weekend. Very little spread between the 25-75%,
which has been a consistent trend and increases confidence in this
outcome. More spread creeps into ensembles later in the weekend into
the early part of next week with the aforementioned uncertainties of
the influences of a possible tropical system.

&&

.AVIATION /06Z TAFS THROUGH 06Z TUESDAY/...
Issued at 1006 PM CDT Sun Sep 22 2024

Primary concern over the next 24 hours is potential for river
fog/stratus affecting LSE. Surface dewpoint depressions have
decreased dramatically over the past few hours but still remain a
degree or two too big to point to dense fog at LSE.
Additionally, no fog as formed as of yet on satellite anywhere in
the river valleys of the region. Near surface light wind layers
remain 500 feet or less, so if anything occurs, would expect stratus
to be the concern. In any case, have refrained from including any
restrictions in the TAFs with this issuance with good flying weather
expected Monday.

&&

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

&&

$$

DISCUSSION...Rieck
AVIATION.....Ferguson