761
FXUS63 KARX 011738
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
1238 PM CDT Wed May 1 2024

.KEY MESSAGES...

- Periodic showers and scattered storms from tonight into
  Thursday night. Rain amounts and severe weather chances have
  decreased some.

- Decreased or removed the rain chances for Friday for much of
  the forecast area.

- Scattered showers from Friday night into Saturday evening. 60
  to 80% of the grand ensemble have rain totals less than a
  tenth of an inch.

&&

.DISCUSSION...
Issued at 208 AM CDT Wed May 1 2024

Wednesday Night through Thursday Morning - Another round of showers
and scattered storms

A 500 mb shortwave will eject out of the Central Plains tonight and
then move northeast through the area on Thursday morning. As this
occurs, the 850 mb moisture transport will increase on the nose of a
45-knot jet which will be moving northeast through the area.
While this enhances the 0-3 km shear into the 30 to 40 knot
range, the 0-4 km most unstable CAPES remain less than 250 J/kg.
The highest values are south of Interstate 90. The 30.12z LREF
10th-90th percentiles for precipitable water increases on
Thursday morning from 1 inch to 1.4 inches. With less
instability, rainfall amounts have come down some in the 01.00z
HREF. Through Thursday morning, the HREF 10th to 90th
percentiles ranges from less than a tenth of an inch to three-
quarters of an inch north of Interstate 90 and from a tenth to
1-inch across the remainder of the area.

Thursday Afternoon through Thursday Night - Another round of showers
and scattered storms

On Thursday afternoon, negatively-tilted shortwave trough will
eject out of the Central Plains and then move northeast through
the region on Thursday night. The CAMs are in general agreement
that another round of showers and storms will move northeast
through the area on Thursday afternoon and night. The 01.00z
HREF 10th to 90th percentiles range for Thursday afternoon
ranges from 0 to 410 J/kg. The greatest instability will be
located southeast of an Oelwein, IA to Black River Falls, WI
line. This is also the area where the 0-3 km shear is the
weakest (20 to 30 knots), so severe weather chances have
decreased. For this period, additional rainfall amounts range
from a 0.25 to 0.50 inch west of the Mississippi River and 0.50
to 0.75 inches for the remainder of the area.

Friday - Looking Dry

In the wake of the front, subsidence quickly build across the area.
The NBM continues to hold onto a 20 to 25% chance of rain for
the entire area. Meanwhile, less than 10% of the GEFS,
Canadian, and ECMWF ECE have any precipitation at all. Since
this trend has been showing up the past several days, it was
agreed to remove these small rain chances for much of the
forecast area.

Friday Night through Saturday Evening - Scattered showers - Rain
amounts look light

A broad longwave trough will move east through the area. Our area is
on the southern extent of this trough, so the forcing is rather
weak. This results in the 60 to 80% 01.01z NBM grand ensemble having
less than a tenth of an inch of rain.

For Early Next Week - Uncertainty on rain chances

The models are in general agreement that a closed 500 mb low will
eject out of the southwest US into either the Northern Plains or the
Upper Mississippi River Valley. Due to uncertainty on how this
pattern will evolve stayed close to the NBM.

&&

.AVIATION /18Z TAFS THROUGH 18Z THURSDAY/...
Issued at 1238 PM CDT Wed May 1 2024

Besides some MVFR ceilings to the north, that are becoming more
cellular, VFR conditions prevail with a fairly brisk west to
northwest low level flow. The winds will diminish quickly going into
the evening as we are start to feel impacts from approaching trough
that promises to bring abundant convection back to the area starting
late tonight.

As ceilings gradually lower overnight, expect VFR conditions to last
for a bulk of the time but as lift and moisture advection continues,
we will see gradual drop to MVFR to IFR ceilings, especially as bulk
of convection impacts area early Thursday. Details on the impacts
from thunderstorms will have to be resolved in near-time.

&&

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

&&

$$

DISCUSSION...Boyne
AVIATION...Shea