167
FXUS63 KARX 070315
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
1015 PM CDT Mon May 6 2024

.KEY MESSAGES...

- Showers and storms are expected tonight through Tuesday evening. A
few strong storms are possible Tuesday afternoon and evening with
small hail being the primary threat.

- Pattern stays active into early part of next week with periodic
shower/storm chances. Severe threat low.

- Warmest conditions tomorrow/Wed with coldest day Thu. Mostly
within 10 degrees of seasonable normals (plus or minus).

&&

.DISCUSSION...
Issued at 210 PM CDT Mon May 6 2024

OVERVIEW: a series of upper level shortwave troughs look to spin
across/near the upper Mississippi river valley through the week,
bringing periodic shots for rain and a few thunderstorms. Severe
risk looks low and mostly confined to Tue. Current model blend
suggest a 50-70% for more than 1" through Friday, the bulk of which
comes tonight.


TONIGHT INTO TUESDAY: Showers and storms, a few strong storms
possible Tuesday afternoon

The 06.18z CAMs continue to depict two rounds of showers and storms
that progress through the area through Tuesday afternoon/evening.
The first of which is associated with a upper-level low situated
across the Dakotas with a corresponding negatively tilted trough
pivoting around the eastern edge of the aforementioned low. This
first storm mode would feature a long QLCS that originated from
supercellular convection across the Central Plains that will
progress towards our region where it will become instability starved
during the overnight hours. As a result, the 06.18z CAMs generally
paint a decaying band of more stratiform precipitation after
midnight with some embedded sub-severe thunderstorms as model
soundings only have a around 200 J/kg of MUCAPE to work with.

As this first round of showers and storms exits the region during
the mid-morning hours, diurnal heating will allow for instability to
increase across the area with MUCAPE values to around 1000-2000 J/kg
in the 06.15z RAP. Where cloud cover is able to exit, diurnal
heating will be maximized which will allow for localized boundaries
to setup which could initiate additional convection as shown in many
of the recent CAMs. As a result, exact location of any convective
initiation during the afternoon remains unresolved at this time.
However, with the general lack of bulk shear in the 06.15z RAP
during the later afternoon, storms will likely have a pulse nature
with small hail being the primary threat as 06.15z RAP soundings
show fairly lower freezing level during the afternoon and evening on
Tuesday. Cannot rule out some near-severe (half inch to nickel
sized) hail in the strongest storms. DCAPE values generally remain
on the lower side with roughly 500 J/kg present, suggesting that
some gusty winds to 30-40 mph could be possible with some storms as
their cores collapse. With the loss of daytime heating overnight
Tuesday, storms will begin to push northeast with the mean wind and
quickly weaken.


WED NIGHT/THU: medium range guidance in solid agreement with hanging
a sfc trough northwest of its parent low near the Ohio River valley,
reaching into northern IA/IL. In the upper level flow, pieces of
energy of set to spin east from the plains and southward out of
Canada. All the forcing mingles together to produce an area of
showers from southern MN across IA and over southern WI/IL. Locally,
the model blend favors keeping the higher chances (60-70%)
along/south of I-90. Instability is weak and the deeper shear gets
shunted southeast. So a few storms possible, but main severe risk
should be farther south.

FRIDAY NIGHT/SAT: another shortwave trough is set to drop in from
Canada with low level warm air advection/sfc front leading it in. No
tap to southerly moisture but has ample saturation to produce areas
of showers. Meager instability could pop a thunderstorm or too, but
this shortwave is more about another round of rainfall. Showers
could linger through much of the day Sat (moreso Mississippi River
east) with cyclonic flow and the western edge of the shortwave not
yet exiting.


TEMPS: mild-ish temps for the next couple days, colder Thu

Southerly flow promises a couple more relatively warm days for the
region with highs expected to top out near, or above 70 degrees -
about 5 to 10 degrees above normal.

Shots of colder, Canadian air along with rainfall associated with
shortwave troughs promise bouts of cooler air for the end of the
work week. Thu is trending the "coldest" (model blend painting only
a 20-30% chance to climb out of the 50s), but still only 5 to 10
degrees off the early May normals. EPS and GEFS then favor building
shortwave upper level ridging toward the end of the weekend, with
more zonal flow to kick off the work week. Expect a bump upward in
temps - at or above normal (near to above 70 for most).

&&

.AVIATION /06Z TAFS THROUGH 06Z WEDNESDAY/...
Issued at 1015 PM CDT Mon May 6 2024

An area of showers and weakening storms will develop from the
west after 07.06Z tonight, spreading into Wisconsin through
Tuesday morning. Timing since the last update has slowed by an
hour or so. Ceilings may vary from VFR to MVFR with
probabilities for MVFR between 30-60% as the rain moves through.
Some reductions in visibility are possible within heavier
rains/storms. Additional scattered showers/storms will remain
possible into Tuesday afternoon/evening, but confidence in how
showers/storms will evolve later in the day is lower, so have
not included any explicit mention. With less mixing into the
predawn hours tonight at KLSE, did mention LLWS at KLSE, with
winds trending S/SW on Tuesday.

&&

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

&&

$$

DISCUSSION...Naylor/Rieck
AVIATION...JM