NWS-La Crosse Forecast Discussion

						FXUS63 KARX 031004
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
404 AM CST Tue Dec 3 2024

.KEY MESSAGES...

- Windy and cold weather is expected Wednesday afternoon into
  Thursday morning. Gusts of 40+ MPH are possible which combined
  with falling temperatures will lead to wind chills below zero
  Thursday morning (-5 to -15F).

- Snow showers are possible Wednesday (20-30%), but
  accumulations will be light as higher amounts are expected to
  remain further north.

&&

.DISCUSSION...
Issued at 345 AM CST Tue Dec 3 2024

Windy and Cold Wednesday Afternoon into Thursday Morning

A 500hPa shortwave trough will drop south out of Canada tonight into
Wednesday morning with an attendant jet streak in excess of 110kts
deepening the trough throughout the day Wednesday. This deepening
trough will be reflected to the surface as a surface low pressure
system which will drag a cold front through the region Wednesday.

The increased pressure gradient associated with these features will
allow for wind speeds upwards of 50kts (57 mph) develop at
850hPa, stronger west of the Mississippi River. Strong cold air
advection behind the aforementioned cold front will be very
effective at steepening low- level lapse rates with model
soundings suggesting that the boundary layer will be well mixed
to at least 850hPa, if not higher, allowing the strong winds
aloft to mix to the surface. Some model soundings suggest gusts
upwards of 40kts (45mph) are possible, especially west of the
Mississippi River. The deterministic NBM continues to be lower
than model soundings would suggest, so have increased winds
towards the 75th percentile.

Temperatures also fall quickly behind the cold front Wednesday as
strong cold air advection under northwest flow quickly moves across
the region. The 03.00z LREF continues to indicate 70-100%
probabilities of temperatures falling into the single digits across
the region Wednesday night into Thursday leading to apparent
temperatures below zero. Not expecting advisory criteria to be
reached as the 03.00z LREF 10th percentile apparent temperature
falls short, but apparent temperatures will be very cold
beginning Wednesday afternoon and continuing into Thursday morning.

Snow Showers Possible Wednesday

As the cold front moves through the region Wednesday afternoon,
ample forcing via 925-800hPa frontogenesis will move through the
region as well which will coincide with a saturated DGZ as well as
some MUCAPE given steep low-level lapse rates. This set-up results
in an environment conducive to snow shower development, which is
suggested in the majority of ensemble guidance with the 03.00z LREF
suggesting a 20-50% probability of precipitation, although these
probabilities are heavily weighted by the consistently more bullish
ENS which suggests probabilities as high as 80%. The GEFS/GEPS
remain much more reserved with 20-30% probabilities. Regardless of
probability, amounts will be light with accumulations less than 1
inch expected as higher amounts remain to the north. There is
some concern for freezing drizzle Wednesday morning before the
DGZ fully saturates, but confidence in this scenario is low (<
20%) and any super-cooled liquid should quickly turn to ice as
the stronger cold air advection gets going and the column
quickly cools.

&&

.AVIATION /06Z TAFS THROUGH 06Z WEDNESDAY/...
Issued at 1050 PM CST Mon Dec 2 2024

During the overnight, the 925 mb ridge axis will shift to the
east. As this occurs, the MVFR ceilings will exit the region.
This looks to be a bit slower than earlier in the night, so
shifted this time by 2 hours later.

North and northwest winds will be around 10 knots this evening
and then be light and variable overnight. Wind will then become
south and increase into the 10 to 15 knot range on Tuesday
morning.

&&

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

&&

$$

DISCUSSION...Falkinham
AVIATION...Boyne