Area Forecast Discussion
Issued by NWS La Crosse, WI

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784
FXUS63 KARX 220408
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
1100 PM CDT Sun May 21 2017

.SHORT TERM...(This evening through Monday)
Issued at 313 PM CDT Sun May 21 2017

Main impactful weather concerns in the short term are on
thunderstorm chances Monday afternoon and evening with potential
for a few stronger storms.

Light rain and areas of drizzle will gradually lift north tonight,
but a secondary shortwave rotating around an upper low will move
through this evening into the overnight. This wave will generate a
few light showers across areas north of Interstate 90.

The main forecast concern will be on Monday as a frontal boundary
and upper level trough dive into the region. Afternoon surface based
CAPE values look to climb into the 500-700 J/kg range. 0-6 km bulk
shear values are expected to be in the 15 to 30 kt range with 0-6 km
bulk shear values of 35 to 40 kts by the time the storms form.
However, the strongest shear appears to be exiting the area as the
CAPE builds from the late afternoon into the evening. So what is
initially potential supercell shear early in the day weakens to
the point where storms will likely struggle to organize. Freezing
levels are fairly low at around 7 kft. So, the main severe weather
threat will mostly likely be some large hail (up to quarter
size?) and possibly some gusty winds given a dry sub-cloud
layer/inverted v type of soundings. The main area of concern in
along and south of the Interstate 90 corridor.

.LONG TERM...(Monday night through Sunday)
Issued at 313 PM CDT Sun May 21 2017

Low pressure will slowly slide south across the region late Monday
night through Tuesday night with continued chances for showers. Weak
CAPE attempts to build over the area Tuesday afternoon so we may
see some isolated thunderstorms. We stay in weak cyclonic flow
aloft going into Wednesday with continued chances for rain
showers. We will finally see a break in the rain chances on
Thursday as ridging moves across the Upper Mississippi River
Valley. Then another trough pushes through the region late
Thursday night into Friday. This feature looks to bring showers
and perhaps some thunderstorms to the region. Temperatures take a
slight trend upward for the weekend with highs climbing into the
70s at many locations. Another trough could bring showers and
thunderstorms to the region but details are far from certain
since forecast models are showing considerable differences.

&&

.AVIATION...(For the 06Z TAFS through 06Z Monday night)
Issued at 1100 PM CDT Sun May 21 2017

Cigs: blanket of low clouds exiting east at late evening, and should
clear KLSE before 06z. Expect VFR with generally sct skies for the
rest of the overnight through Mon morning. Clouds will be on the
increase for the afternoon though, and could eventually drop back
into MVFR territory Monday night as ripples in the upper level flow
spin across the region.

WX/Vsby: scattered light showers will persist well north tonight
with no impact to the TAF sites. SHRA/TS could move back across the
region by late Mon afternoon as upper level shortwave troughs track
east across the region. The threat increases Monday evening. How far
north the shra/ts threat extends is not certain through, with some
variability in the models. Going to cover the chances with VCSH for
now, and will refine forecast for the rain/storm potential as
confidence increases.

Wind: westerly though the period, hovering near 10 kts - some
lightening for Mon evening.

&&

.HYDROLOGY...
Issued at 313 PM CDT Sun May 21 2017

Heavy rains over the past week have resulted in flooding on many
area rivers. River Flood Warnings continue for the Mississippi
River at Wabasha, the Trempealeau River at Dodge, the Black River
at Galesville, and the Yellow River at Necedah. Monitor the
latest river forecasts closely this week as additional flooding is
possible along portions of the Mississippi River.

&&

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

$$

SHORT TERM...Wetenkamp
LONG TERM...Wetenkamp
AVIATION...Rieck
HYDROLOGY...Wetenkamp



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