816
FXUS63 KARX 191737
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
1237 PM CDT Thu Sep 19 2024

.KEY MESSAGES...

- Scattered showers and storms this morning, especially west of the
Mississippi. Higher chances (70 to 90%) for storms this afternoon
and evening. A slight risk (level 2 of 5) for severe weather is in
place for much of the area. Main hazards are large hail and damaging
winds.

- Above normal temperatures continue through Saturday before cooling
down to near or slightly below normal temperatures.

- Chances of rain for Saturday and Sunday as next system moves into
the region.

&&

.DISCUSSION...
Issued at 315 AM CDT Thu Sep 19 2024

Today: Scattered AM Showers/Storms, Strong to Severe PM Storms

A low pressure system moving into southern Saskatchewan and Manitoba
will be the main synoptic focus for today. An axis of stronger
moisture transport shifts into the Upper Midwest early this morning
as a shortwave riding along the southern edge of the main low
pressure system moves eastward. As it does so, this shortwave will
strengthen a little allowing for so showers and thunderstorms to
form in the early to mid-morning period. Areas along and west of the
Mississippi will have the best chance at seeing these showers and
thunderstorms this morning. Moisture transport is expected to
decrease into the afternoon resulting in the showers and
thunderstorms diminishing.

After the aforementioned storms diminish, there will be a lull in
the action for a few hours. By mid-afternoon, a cold front oriented
north to south across Minnesota and Iowa and into the Central
Plains, will slide eastward. Ahead of this front, a corridor of 1000
to 2000 J/kg of SBCAPE will exist. Strong deep-layer (0-6km)
shear of 35 to 45kts will be perpendicular to the front
orientation. This will allow the potential for supercells to
form initially in the late afternoon before becoming more linear
in the evening. With the initial onset of storms potentially
being supercells, the main threat will be hail. As storms become
a little more linear and cold pools congeal, wind will become
the primary threat for the evening. The tornado potential is
more conditional and may occur early on with discrete convection
or as storms become more linear, QLCS tornadoes could be
possible. With this severe weather potential, SPC has continued
the slight (level 2 of 5) risk for severe weather across much of
the forecast area.

Another threat to watch out for, due to moisture transport picking
back up again during the afternoon and the slower storm motions, is
the potential for locally heavy rainfall. With how dry we have been,
flooding does not look to be much of a concern, however if any
impacts are felt, they would be mainly in the urban and small stream
locations. The HREF probability matched mean QPF shows a broad area
of 0.5 to 1.5" along and north of I-90, with some smaller pockets of
0.5 to 1.5" along the line of storms in northeast Iowa and
southwestern Wisconsin. During the overnight, the main low pressure
system continues to lift northeastward and the main severe
threat diminishes, however showers and storms will continue on
and shift eastward and be out of the forecast area between 06
and 08Z.

Friday through Middle of Next Week: More Showers and Storms

Weak ridging returns for Friday with high temperatures remaining
above average in the mid 70s to mid 80s. Heading into the weekend,
we get into zonal flow, allowing for a couple of shortwaves to move
into the area. The first comes through Saturday bringing about the
first chance of showers and storms to occur. The next chance is
Saturday evening into Sunday. Looking at ensemble and deterministic
guidance, this shortwave appears a little stronger than the first.
This has resulted in higher PoPs (50 to 70%) for Saturday night
into Sunday morning.

By Monday and Tuesday an area of low pressure moves through the
Central Plains and into the Upper Midwest. There remains uncertainty
in the exact path the storm system takes. Comparing the GEFS and
EPS, the GEFS is wetter than the EPS for early next week. This is
seen when looking at the 25th and 75th percentiles for 24 hour
precipitation. For example, on Sunday afternoon, GEFS 25th to 75th
percentiles are roughly between 0.5 and 1.0" whereas the EPS
percentiles are between 0 and 0.5". Looking into this more, EPS
members have around 20 to 30% of them being dry through the weekend,
whereas almost all GEFS members have measurable precipitation
through the weekend.

After Tuesday, uncertainty in precipitation chances exist as we
remain in zonal flow and then northwest flow by the middle of next
week. This may allow for weak shortwaves to pass through the area.
Due to this uncertainty, most PoPs are between 20 and 30% from
Tuesday through Thursday.

&&

.AVIATION /18Z TAFS THROUGH 18Z FRIDAY/...
Issued at 1236 PM CDT Thu Sep 19 2024

Thunderstorms will be the main aviation concern this evening with
IFR category reductions possible at times. As a result, have
included a prevailing MVFR conditions with a tempo for IFR visbys
where confidence is highest on timing. Winds may shift to westerly
with strong wind gusts being possible as storms move through. With
convective initiation still 4-5 hours away, likely will need to make
forecast adjustments prior to the 00z issuance depending on
convective trends. Storms move eastbound through the region during
the evening and eventually exit towards midnight. As a front passes
through the region, cooling overnight temperatures and recent
rainfall in combination with clearing skies will create a fairly
favorable fog/low-stratus setup into the morning hours on Friday.
However, there remains some questions with how some stronger winds
to around 20 kts in model soundings (recent HRRR/RAP/NAM) just off
the surface will impact any fog/low-stratus formation. Consequently,
held with the 3SM but will likely introduce IFR/LIFR reductions in
subsequent issuances as forecast confidence increases.

&&

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

&&

$$

DISCUSSION...Cecava
AVIATION...Naylor