207
FXUS63 KARX 232321
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
621 PM CDT Mon Sep 23 2024

.KEY MESSAGES...

- Potential (30 to 50%) for river valley fog again tonight.

- Remaining dry through the week, low chances (20 to 40%) for
  precipitation return Saturday.

&&

.DISCUSSION...
Issued at 210 PM CDT Mon Sep 23 2024

Tonight into Tuesday: Passing Clouds, River Valley Fog, A Few
Sprinkles?

Remnants of a weak low pressure system will be lifting north through
the mid-Mississippi River Valley tonight as an upper-level wave
drops SE across the Dakotas. A weak surface ridge of high pressure
overhead will remain as the dropping system skirts by and the
remnant surface low moves into the Ohio River Valley. These two
systems will both bring passing clouds overhead tonight, with the
biggest impact being low temperatures and fog potential. Should
clouds clear to the east quicker, temperatures will likely fall
another degree or two with radiational river valley fog developing.
Both of these systems will bring a small chance (10% or so) of a
brief rain shower, but the low-levels of the atmosphere are dry
which will limit hydrometeors reaching the surface. Did keep the
prevailing fog forecast for tonight in the river valleys.

Midweek Forecast: Dry, With Above Normal Temps (mid 70s to near 80)

The system dropping from the Dakotas reaches mid-Missouri by
Wednesday afternoon as global ensembles overwhelmingly favor this
system cutting off as a western CONUS ridge amplifies. This ridge
continues to build and elongates itself from the desert SW towards
the Upper Great Lakes region as the cutoff low remains along the mid-
Mississippi River Valley and another trough moves into the Pacific
NW. This pattern will favor continued dry conditions with above-
normal temperatures (5 to 10 degrees above normal).

This Weekend: Remaining Warm, Uncertainty for Rain Chances this
Weekend

Global ensembles show considerable uncertainty as to the evolution
of any potential tropical system this weekend and the cutoff low
during the middle of the week. Ensembles suggest the ridge axis
shifts east over the Upper Midwest as the cutoff low and potential
tropical system remain to the south along the Gulf Coast. A 100-
member ensemble consisting of 30 GEFS, 50 EPS, and 20 Canadian
members suggest a 20 to 30% chance of showers from a remnant
tropical system by Saturday across the area. Canadian and ECMWF
members suggest this remnant tropical system will bring a low
pressure system into the Missouri and Tennessee border by Friday
evening, but the GEFS mean is more diffuse with the low pressure
fizzling after it moves on shore. Either way, global ensembles all
suggest that any precipitation that does shift north, will be
fighting a ridge axis that will be along and north of the US/Canada
border that will likely keep PoPs on the lower end until models get
a more certain handle on upper-level flow evolution. However it
shakes out, the ridge overtop of the cut off low pressure system
will serve as a quasi-Rex Block with the overall pattern evolution
slowing down this week.

&&

.AVIATION /00Z TAFS THROUGH 00Z TUESDAY/...
Issued at 619 PM CDT Mon Sep 23 2024

Signs continue to point toward a recurrence of valley fog
tonight into Tuesday morning. Considering this overachieved last
night in a bit less favorable conditions, have elected to
include LIFR fog with this issuance for LSE. Will continue to
refine timing over the next cycle. Otherwise, a stray light
shower may occur near LSE over the next 3 hours but no impact to
flight rules is expected. Expect light winds generally out of
the southeast tonight and out of the northwest tomorrow.

&&

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

&&

$$

DISCUSSION...JAW
AVIATION...Ferguson