307
FXUS63 KARX 071740
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
1240 PM CDT Tue May 7 2024

.KEY MESSAGES...

- Line of Showers and Thunderstorms Early, Scattered Storms this
  Afternoon and Evening

- Active pattern continues with additional thunderstorms
  Wednesday evening into Thursday morning.

- Cooler Saturday, Potential Shower or Two Friday Night through
  Saturday

&&

.DISCUSSION...
Issued at 244 AM CDT Tue May 7 2024

Today: Line of Showers and Thunderstorms Early, Scattered Storms
this Afternoon and Evening

Line of showers and thunderstorms are making their way into the area
from the west this morning and will continue to shift east through
the rest of the morning. Latest RAP analysis suggests up to 700 J/kg
of MUCAPE with around 40kts of bulk shear; however model soundings
suggest parcels are lifting from around 5kft with nearly uniform
winds through the instability - bringing the effective shear down
considerably. There`s ample upper-level forcing to support
convection through the rest of the overnight/morning hours across
the area, but the threat for any severe storms at this time appears
low when assessing the radar trends.

There remains uncertainty on the chances for thunderstorms later
this afternoon across the area as remnant cloud cover could hinder
instability. After the line of thunderstorms wraps up by mid-
morning, cloud cover will begin to decrease from the SW to NE. While
low-level destabilization is less certain, there is good agreement
on steepening mid-level lapse rates this afternoon across the area,
which will increase fuel for thunderstorms should surface parcels
get going. Mean mixed-layer CAPE values for this afternoon are
forecast around 1000-1400 J/kg, but the shear decreases as
instability increases. CAMs this afternoon show a mixed bag of
solutions with some models not resolving any convection and others
continue convective rather quickly after the first round exits. So
while there`s decent chances (20 to 50%) for afternoon
thunderstorms, strong to severe storm chances are rather low with
hail being the primary threat.

Temps today are highly dependent on exiting cloud cover. Model
spread is rather large, with the 25th to 75th percentile temp spread
over 10 degrees in spots. NE Iowa is most favored to see clearing
skies this afternoon, with temps climbing into the low 70s. SE MN
will likely see temps near 70 as well. As for SW into central WI,
modeled sky coverage has a larger spread which makes temps less
certain. If cloud cover remains throughout the day, could see temps
closer to 60; however, if clouds diminish temperatures will climb
into the 70s.

Wednesday into Thursday: Another Round of Thunderstorms

A remnant surface low and longwave trough from the Dakotas dives
south Wednesday, bringing upper-level divergence and fgen forcing
across the region. While there has been a slight shift to the south
from previous runs, an inverted trough and Fgen forcing along 850 to
600mb coupled with a mid-level low shifting over IA and a skinny
CAPE profile will bring the potential for more showers and
thunderstorms and potentially a localized heavy rain threat. Modeled
rainfall amounts among the GEFS/EPS/CMCE rain from near 0 inches of
precipitation to as high as 2.50 inches across NE Iowa and SE MN,
but as a whole, there`s roughly a 10-20% chance of rainfall
exceeding 1 inch across the aforementioned areas Wednesday evening
into Thursday morning. This threat will diminish Thursday morning as
cooler air moves in behind the exiting trough. Behind the cooler air
Thursday, temps will back down a bit to the upper 50s to low 60s.

This weekend: Not as Warm, Some Showers

Long wave trough drops south from Canada across the Upper Midwest
into the weekend bringing temps into the low to mid 60s with
scattered showers as the upper level low pivots overhead. The best
chances (30 to 40%) for precipitation appear to be on Friday night
into Saturday morning at this time with the initial shortwave vort
max moving overhead. As broad cyclonic flow remains through Saturday
night, expecting additional showers across the area at times - best
chances east of the river.

&&

.AVIATION /18Z TAFS THROUGH 18Z WEDNESDAY/...
Issued at 1241 PM CDT Tue May 7 2024

VFR conditions are generally expected through the TAF period with
showers and a few storms possible (20-50% chance) later this
afternoon and evening. As a result, included some VCSH mention in
the KLSE TAF from 21z to 02z to account for anything that may
intiate. However, confidence is low in exact coverage of
showers/storms and where they will initiate due to uncertainty
on where localized boundaries will setup. Otherwise, cigs will
vary throughout the TAF period between middle and high-level
clouds, perhaps a few low-VFR cigs to begin the TAF period.
Southwesterly winds will diminish late this evening and
overnight to around 5 kts.

&&

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

&&

$$

DISCUSSION...JAW
AVIATION...Naylor