425
FXUS63 KARX 171112
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
612 AM CDT Sat May 17 2025

.KEY MESSAGES...

- Scattered showers (mainly northern WI) today with a dry, mostly
sunny end to the weekend. Cool.

- Rain returns for the start of the new work week with the highest
chances on Tue, mostly along and south of I-90.

&&

.DISCUSSION...
Issued at 135 AM CDT Sat May 17 2025

- WEEKEND: scattered showers north, cloudy and much cooler today.
Drier, not as windy, mostly sunny for Sunday.

Upper level trough/closed low currently churning over northern WI
early this morning, serving as the main focusing mechanism for
scattered showers that are sweeping eastward across the local area.

The low will make steady progress east today, moving over the
eastern Great Lakes by 00z this evening. The bits of shortwave
energy, cyclonic flow aloft and decent 1000:850 mb lapse rates will
continue the showery conditions, mostly for northern WI, into the
afternoon. Some suggestions of very weak instability in RAP/HRRR
soundings, but not enough to warrant thunder inclusion into the
forecast.

While not nearly as windy as Friday, still a bit blustery as the sfc
pressure gradient doesn`t look to slacken until this evening when
shortwave upper level ridging builds in.

Sunday shaping up to be relatively cool, but with light winds and
some sunshine (and no rain), a fairly nice end to the weekend.


- START OF THE NEW WORK WEEK: rain chances return with Tue harboring
the higher chances. Greatest threat/amounts currently favored mostly
along/south of I-90.

A longwave upper level trough remains set to move east from the west
coast today, sliding over the mid mississippi river valley by Tue.
While a bit of shortwave energy will rotate around the trough on
Monday, the bulk of its influence will be west of the local area.
It`s a secondary shortwave dropping in from the PAC NW, then lifting
northeast and rotating around a weak 500 mb low Tue, that promises
widespread rains for portions of the region. At the sfc, the
system`s low is progged to stay well south of the forecast area,
holding across northern MO-central IL. Low level moisture will push
north/west to feed the system`s deformation region with ample
forcing via the shortwave, left exit region of a 300 mb jet, along
with sloping Fgen response, to produce widespread rain. NAEFS and EC
PW anomalies are close to +2 while EFIs run around 0.7 with a non-
zero SoT. NAM/GFS upwards of 150% or normal for PWs. Where that
deformation region sets up isn`t quite clear though, with some of
the EPS members bringing it up to the I-90 corridor, others holding
it over IA. The bulk of the GEFS favors a more southern solution -
but still brings rain up to I-90. What the models do agree upon is
for the bulk of the rain to fall Tue.

How much rain falls is also a question mark. Favorable west-east
running orientation of the deformation region would favor
time/residence enhancement of potential amounts. Large spread in
both the GEFS and EPS 25-75% for amounts - ranging from less than
1/10" on the low end to 1+" on the higher end. Much of those
differences are due to uncertainties in placement.

Will continue to run with model blend for rain chances. Expect
refinement to the northern extent of those chances, along with
potential amounts, as we move into Monday.

&&

.AVIATION /12Z TAFS THROUGH 12Z SUNDAY/...
Issued at 602 AM CDT Sat May 17 2025

Main area of rain has shifted mostly southeast of the forecast
area into central Wisconsin. Low level cold air advection will
keep increased winds and low level saturation through much of
today. Highest confidence in IFR ceilings for the northern half
of the forecast area in southeast Minnesota through western
into central Wisconsin. Low confidence in IFR ceilings for KLSE
TAF site due to sitting on the threshold of low level cold air
advection and lower elevation within the River Valley. Have
opted for OVC010 but could fluctuate 100-200` into and out of
IFR. Expect MVFR ceilings in southern half of the forecast area
from northeast Iowa into southwest Wisconsin. VFR reaches
smaller airports in these areas tonight as skies lift from
southwest to northeast.

Ceilings lift and winds decrease tonight. Moderate probability
for VFR at both TAF sites towards the end of the 17.12Z
forecast period.

&&

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

&&

$$

DISCUSSION...Boyne
AVIATION...JAR