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420
FXUS63 KARX 050346
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
1046 PM CDT Sat May 4 2024

.KEY MESSAGES...

- Fog possible tonight along and east of the Mississippi River.

- An active stretch of weather from Monday night through at
  least Thursday as multiple rounds of showers move through.
  Severe weather risk for Tuesday is trending south.

&&

.DISCUSSION...
Issued at 200 PM CDT Sat May 4 2024

TONIGHT: fog, frost?

With a day of wetting rains saturating the boundary layer and clouds
not slated to clear until near or after sundown, decent setup for
fog. Add in diminishing winds and the potential increases. Short
term models aren`t overly enthusiastic on the widespread possiblity
for thick/dense fog though. HREF and RAP hold any "thicker" type fog
to the I-94 corridor northward. Can`t rule out a thin fog layer in
the river layers too, ala what occurred a couple days ago. Will hold
with some mention of fog mostly along, east of the Mississippi river
for now.

In addition, passage of a cold front will bring in colder air with
lows expected to bottom out in the upper 30s to lower 40s. Some
potential for frost in low lying areas as a result, mostly in
central and north-central WI. Not widespread enough, nor high enough
confidence, to warrant a frost adv at this time.

Active Weather Pattern Next Week:

Shortwave ridging will be over the area through Monday. Warm
southerly flow and increasing PWATs are expected out ahead of a low
moving through the Upper Plains. Temperatures in the low 70s
expected. An 850mb jet with winds between 30 and 45 kts will be over
our area during the late evening on Monday and overnight period into
Tuesday. Convection across the Plains is expected to begin Monday
afternoon and push eastward throughout the day. Moisture
transport increases late in the day on Monday across eastern
Minnesota and Iowa. As the storms push eastward into Wisconsin,
even though shear and life potential increases, instability will
decrease. Thus the severe threat with this first round of
storms is low. Another factor that plays into the severe threat
is timing. These storms will push through the overnight period,
so daytime heating and the gradually weakening warm sector will
help to minimize the severe threat.

After this first round moves out by early Tuesdsay afternoon, model
guidance continues to show another round of storms forming over Iowa
as a shortwave pushes around the main low. While this wave does look
stronger with higher CAPE and lift potential, moisture advection is
weaker and so is the low-level shear. Even though the ingredients
are there to at least have some thunderstorm development, the
biggest inhibitors for development is that environment will
probably be worked over the morning thunderstorms and that the
warm sector will be further southeast of our area. At the
moment, southeastern Iowa and northern Illinois will have the
better chances at afternoon thunderstorm development.

From Wednesday through the rest of the week, this shortwave becomes
a cutoff low and the original low weakens and gets absorbed by the
new low. This cutoff low will be over the Upper Midwest providing
the area with northwest flow and periodic chances for precipitation
to occur.  With this flow pattern, temperatures will cool down and
remain in the low to mid 60s from Thursday through the weekend.

&&

.AVIATION /06Z TAFS THROUGH 06Z MONDAY/...
-- Changed Discussion --
Issued at 1045 PM CDT Sat May 4 2024

Main aviation forecast question for this TAF period revolves
around fog potential towards daybreak at LSE. Rainfall earlier
today combined with clear skies and light winds will prove
favorable, but the highest likelihood for fog development looks
to be along and north of I-94 in Wisconsin. Models have
struggled with recent post-rain fog events and are not very
supportive for fog at LSE late tonight, but observed dewpoint
depressions are dropping at this hour, so trends suggest the
possibility exists. Will introduce MVFR visibility (20%
probability) in the 06Z TAF, but confidence is only medium in
overall fog potential and how low visibility might drop. Right
now there is a small (10-15%) chance for IFR visibility at LSE
by daybreak and possibly some accompanying low stratus if this
full-on fog development scenario would pan out.

Otherwise VFR conditions for the TAF sites with light winds
under 10 knots, fluctuating between westerly and southerly as
high pressure drifts overhead.
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&&

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

&&

$$

DISCUSSION...Cecava/Rieck
AVIATION...Kurz