644
FXUS63 KARX 232356
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
656 PM CDT Thu May 23 2024

.KEY MESSAGES...

- Initial Storm Chances Return Late Tonight Into Friday Morning
  Primarily Providing Widespread Rainfall

- Strong to Severe Storms Possible Through Friday Afternoon As
  Surface Frontal Boundary Pushes from West to East. Low
  Confidence Regarding Hazard Type, None Can Be Ruled Out At
  This Time

- Main Impact Will Be Heavy Rainfall, Scattered Nature Of
  Afternoon Rainfall Presents Difficulty Discerning Location Of
  Highest Impacts. Overall Amounts Near & Above 1" Likely. More
  Precipitation Chances Return Sunday Night Into Monday,
  Furthering Water Concerns.

&&

.DISCUSSION...
Issued at 257 PM CDT Thu May 23 2024

Ongoing Rain & Storms Today:

Surface low pressure early this afternoon has been meagerly
deepening across the Northern Plains. A couple of surface frontal
boundaries extend from the low with the main low level theta axis
extending from the Southern to Central and Northern Plains.
Accompanying surface observations have temperatures in the 70s with
dewpoint temperatures in 50s. An exiting surface high pressure has
provided an anticyclonic arc to the low level theta e axis,
pushing the surface warm frontal boundary near Lake Superior.
Otherwise, a pocket of drier air mass near the surface high
pressure center extends from Chicago into southeast Iowa with a
storm east-west moisture boundary farther south with dewpoint
temperatures in the 60s. A narrow appendage of the
aforementioned dry air extends northwest from Chicago, into
Trempealeau County. As a result, isentropic lift along the 290K
isentropic surface providing weak storm and precipitation
chances this afternoon. Therefore, have added slight chances in
a limited area as the moist air continues advecting northeast.
Although, this area in central Wisconsin known for very sandy
soils, potentially persisting precipitation and storm chances
later than currently expected. Will be a very near term forecast
detail.

Precipitation & Storm Chances Late Tonight - Friday Morning:

More widespread precipitation chances return tonight from the
aforementioned surface low pressure seen over the Northern Plains
this afternoon. Highest chances move from southwest to northeast
along the initial warm frontal boundary as the surface low occludes
and phases through the Northern Plains into northern Minnesota
and southern Manitoba. Separation of the PBL overnight will
limit local impacts, keeping things elevated and limiting
potential storm strength. Irregardless, have increased thunder
chances from National Blend based on instability in high
resolution model forecast skew Ts.

Precipitation & Storm Chances Through Friday:

These initial storm and rain chances shift northeast through
Friday morning before the subsequent cold front pushes from west
to east Friday afternoon. An appendage to the phasing low will
drive and strengthen the cold front, causing strong convergence
along the boundary. The occlusion of the northern low also
advects a narrow filament of instability and attendant shear.
The strengthening return flow keeps warmest air off the deck
initially, persisting an inversion before surface air can
potentially recover.

Due to the morning convection, ability for surface temperatures
to recover remains a questionable factor. The RAP remains to be
the most keen model, quickly recovering surface temperatures,
becoming surface based in a very limited window. Although, it
also persists mid level saturation through this time so hard to
confidently say the surface temperature recovery is accurate.
The highest helicity remains tied to the tight, narrow
occluding area ahead of the cold front, offset of the highest
instability and quickening cold front.

Severe Potential Through Friday:

As the cold front provides the necessary lifting, hodographs
and shear becomes quite messy as storms are undercut from a west
to east frontal boundary and a mean storm motion of southwest
to northeast. Therefore, scattered convection looks to be the
most likely with wind and hail with the highest potential
hazard types. If these ingredients line up closer, storm mode
could also be linear along the cold front with popping off
outgoing cold pools.

However, given the potential for severe, machine learning
probability forecast have 5-10% probability for tornadoes, 15-30%
for severe hail, and 5-30% for severe wind. So, will be a limited
window to keep eyes fully peeled for.

Highest Impact Concerns:

Irregardless, highest concern and impact, especially given the
recent heavy rainfall, will be potential for flooding. Given
the longer residence time of the 1" PWAT isohyet through much of
Friday, highest confidence for a widespread 1" of rainfall.
Thankfully, most rivers have begun to recover since previous
rainfall. Initial heavy rainfall Friday morning will provide a
widespread amount while the secondary wave will be more
scattered, limiting further widespread impact. Could see some
longer residence time though with those scattered storms into
Friday evening. Something to keep an eye on in coming forecasts.

Local probability matched mean (HREF) 24 hour precipitation totals
as of 12Z Saturday push 2-2.5" in our southern areas while 48
hour probability matched means only push 1". HREF storm total
precipitation maximum further spreads the 2" isohyet primarily
from northeast Iowa into central Wisconsin with highest 48 hour amounts
pushing 4" while the mean remains 1.5". Quite a spread, but a
concern.

Late Weekend Precipitation & Storm Chances:

Subsequent precipitation chances return late weekend into Monday. A
phasing bowling ball of vorticity lifts northeast before
crashing into the Upper Mississippi River Valley. Strong low
level moisture advection from a stunted Midwest atmospheric
river taps into the Gulf airmass, again providing heavy
rainfall. Highest confidence for instability grazes southern
counties from northeast Iowa into southwest Wisconsin as the
surface low is expected to wrap up and provide heaps of low
level shear. Too far to nail out details at this time.

&&

.AVIATION /00Z TAFS THROUGH 00Z SATURDAY/...
Issued at 655 PM CDT Thu May 23 2024

A cold front will move through the area on Friday afternoon.
A pre-frontal trough will bring a round of showers and
storms. Used the CAMs to fine-tune the timing for the highest
chances of showers and storms. For KRST, this period looks to be
between 24.12z and 24.15z. For KLSE, this period looks to be
between 24.13z and 24.16z.

Winds will be southerly at 10 to 15 knots during the morning and
then shift to the west during the afternoon and increase into
the 10 to 20 knot range.

&&

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

&&

$$

DISCUSSION...JAR
AVIATION...Boyne