La Crosse, Wisconsin 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

379
FXUS63 KARX 211142
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
542 AM CST Thu Nov 21 2024

.KEY MESSAGES...

- Snow & Subsequent Rain Move Through Central Wisconsin Counties
  Through Much Of Today.

- Strong Wind Gusts, Highest In Far Southwest Wisconsin & Far
  Northeast Iowa In Excess Of 45 mph At Times. Near 40mph
  expected elsewhere.

&&

.DISCUSSION...
Issued at 333 AM CST Thu Nov 21 2024

Early Morning Precipitation In Central Wisconsin:

The extratropical cyclone that provided widespread precipitation
yesterday (Wednesday) has been slowly progressing southeast over
Lake Michigan while tightening on early morning GOES satellite
imagery loops. Can expect a more southerly progression of the closed
low today attributed to the northwest to southeast polar jet
streak along it`s base in GOES derived winds and 21.00Z RAOBS.
Frontogenetical bands accompanying the low are expected to
drive additional precipitation chances primary through central
Wisconsin today. Have seen an attempt of initial precipitation
chances through central Wisconsin early this morning. However,
the low hasn`t fully committed to it`s southerly trajectory
which has been limiting overnight precipitation extent. As the
low does eventually shift south, highest confidence locally will
be in Juneau/Adams (nearest the low).

Precipitation Type:

Initial precipitation type expected to be snow. While some high
resolution soundings suggest a limited window of freezing
precipitation, early this morning, overall saturation struggles
before ice introduction returns later this morning. Regardless,
through late morning a quick switch to liquid precipitation
type is expected as low level warm air wraps around the low.
Unfortunately, freezing precipitation chances are non zero
though.


Limited Impacts:

Overall snowfall amounts will be dependent on timing of
saturation onset relative to amount of surface warming. MRMS
probability for subfreezing road temperatures in our central
Wisconsin counties remains low (30-40%). Therefore don`t expect
high impacts from snowfall at this time. Similarly, rainfall
amounts are also expected to be low as best moisture remains
close to the low as it shifts south-southeast. A north-south
orientation of this moisture axis will result in quite variable
accumulations. Therefore, immediate term updates may need to be
accounted for through the morning hours as the low takes shape.

Wind Setup Today:

As the aforementioned low slowly shifts over the Great Lakes today
an upstream ridge collides with the upstream, tightening the
pressure/isoheight gradient as a result. Off deck winds at 850mb
push 60kts while sustained surface winds are forecasted to be
near 20 kts sustained. Higher concern will be ability of gusts
to reach the surface. However, the wrap around low level warm
air advection is presenting some difficulties regarding mixing
heights and associated discrepancies between high resolution
models as a result of mixing heights.

Wind Impacts Today & Advisory Issuance:

The maximum anomaly of 50kt gusts seen southeast of Lone Rock,
WI in the ARW, while the 90th percentile NBM remains near
40kts. Most of the high resolution guidance shows a weakening
and easterly shift in dProg/dT for these highest gusts as well.
In summary, there`s quite a spread in model guidance and
ability to tap into the higher off deck gusts remains in
question. Therefore, in collaboration with neighboring offices
to the east and south, have issued a Wind Advisory in Vernon,
Richland, Crawford, Grant, and Clayton counties where wind
gusts are most likely to exceed 45 mph today. Ridge tops will be
most susceptible.

Through The Weekend, Subsequent Precipitation Chances:

The synoptic pattern progresses tonight into Friday as the
aforementioned upper level heights try to build. Weak return
flow provides meager warming to the Upper Mississippi River
Valley on Friday. Best return flow shunts to the southwest as
the ridge flattens as it crosses the Northern Plains due to upstream
synoptic perturbations.

Long term global ensemble (EPS/GEFS) confidence is strong
(500mb heights 100% confidence) for this initial perturbation
to swing through the Upper Mississippi River Valley during the
start of the new week. Precipitable water values above 0.5" are
expected (EPS/GEFS 100% confidence) while precipitation type
remains in question. A much more southerly track of the surface
in the GEFS keeps surface temperatures lower, slightly below
freezing. A more northerly track in the current EPS (21.00Z)
leverages the better moisture and return flow for rain.
Regardless there`s only a few degree difference between long
term global ensemble temperatures and minimal impacts are
currently expected as probabilities for >1" of snow are only
40%.

7 Day Forecast:

Current Thanksgiving Day forecast (7 day) is quite ambiguous as one
would expect. Confidence and agreement in 500mb heights between
the EPS & GEFS is nonexistent. Regardless, EPS probabilities
for 1" of snow are 50% while GEFS is 10%. As for temperatures,
the EPS places the 0C isotherm near the Upper Mississippi River
Valley while the GEFS sags across the Mid Mississippi River
Valley.

&&

.AVIATION /12Z TAFS THROUGH 12Z FRIDAY/...
Issued at 542 AM CST Thu Nov 21 2024

Low-stratus season is certainly under way with MVFR cigs enveloping
much of  the local area this morning. Without a clear mechanism to
mitigate the low-level saturation in place, expecting MVFR cigs to
persist throughout the forecast period. Additionally, as the
marginal diurnal mixing wanes by evening, expecting some slight
reductions in cigs with IFR restrictions possible (30-60%
chance) west of the Mississippi River. Rain/snow will be
prevalent east of the Mississippi River with highest confidence
across portions of Adams/Juneau counties (50-80% chance).
Otherwise, winds will be strong today from the northwest of
20-25 kts late this morning and this afternoon with gusts of 30
to 40 kts. Winds will gradually diminish to around 10-15 kts
after sunset while maintaining from the northwest.

&&

.ARX WATCHES/WARNINGS/ADVISORIES...
WI...Wind Advisory from 9 AM this morning to 6 PM CST this evening
     for WIZ053>055-061.
MN...None.
IA...Wind Advisory from 9 AM this morning to 6 PM CST this evening
     for IAZ030.

&&

$$

DISCUSSION...JAR
AVIATION...Naylor