319
FXUS63 KARX 091159
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
659 AM CDT Thu May 9 2024

.KEY MESSAGES...

- Rainfall amounts just over an inch have been received in
  northeast Iowa and rain showers will continue today there into
  southwest WI. Additional rainfall amounts around 0.25" are
  expected today. Showers diminish early this evening.

- Some frost looks possible (~30% chance) Friday morning in
  central and northcentral WI.

- Showers and storms track southeast across the area Friday
  afternoon/evening. Rain chances have been increased areawide
  with a slightly more western track too. Severe storms not
  expected.

- Periodic lower rain chances (20-40%) from the Sunday into next
  week. Temperatures will be 5 to 10 degrees above the
  seasonable normals Sunday-Tuesday.

&&

.DISCUSSION...
Issued at 317 AM CDT Thu May 9 2024

Additional Rainfall Today Across Northeast IA and Southwest WI

At 3 am, radar indicated rain over northeast Iowa and southwest
Wisconsin associated with a deformation band and two shortwave
troughs within the broader long wave. Rainfall amounts have been
around an inch thus far in the Charles City and New Hampton
areas with lesser amounts /0.25 to 0.75/ for the remainder of
northeast IA, less than 0.25" in southwest WI. Occasional
lightning has been observed along the highway 3 corridor and
south in Iowa.

The two longwave trough elements over the central U.S /Great
Lakes and Neb/SD/ will phase today as upstream ridging over
Canada drives the pattern to evolve southeast. This will take
the lingering deformation rain band and slowly shift it
southeast with increased weak lift renewed this afternoon ahead
of the Great Lakes low heading south through WI. 09.06Z RAP
indicates a moderate signal of potential vorticity advection in
the 300-500MB layer with this sw->ne oriented trough heading
south through WI this afternoon aiding in popping off some
showers in weak instability. This exists southeast this evening
finally ending the rain chances and skies will clear.

Frost Possible Friday Morning in Parts of Wisconsin

With clearing skies tonight and winds going light with surface
ridging building in, frost is looking possible near and north
of the Highway 29 corridor in northcentral WI - 10% increasing
to 60% on Taylor county northern border. Some patchy frost may
also be found further south to the I-94 corridor in WI (15-40%
chances). Frost Advisories may be issued later today for this
possibility once the area is better defined.

Friday and Friday Night Showers and Storms

Signals continue to be consistent and impressive with model
guidance forecasting a vigorous northwest flow shortwave trough
to move through the area later on Friday and Friday evening.
Impressive 300-500mb potential vorticity advection, excellent
exit region jet dynamical divergence /curvature term is
outstanding!/, steepening mid-level lapse rates, and modest low-
level theta-e convergence in the 850-925 mb layer all in play
Friday later afternoon for organized lift. Trends suggest a
noticeable western outlier track through MN in the latest RAP
runs /08.21Z and 09.03Z/. Probabilities in the 09.00Z HREF
confirm a modest westward shift with 70%+ probabilities of
measurable rain northeast of a Rochester MN to Prairie Du Chien
WI line by Friday at 7 pm. Thus, have pulled the rain chances
southwestward and also increased them for this system much above
the NBM guidance. Current rain window is Friday 4-10 pm across
the area. Should be a fairly quick hitter with clearing by
Saturday morning.

Severe storm chances: The wind shear field increases
impressively across the area south of the mid-level low /southwest
of I-94/ but the big limiting factor is CAPE. With dewpoints in
the lower 40s, MLCAPE/SBCAPES are in the low 100s J/Kg. Even so,
the 09.00Z HREF CAMS show a 70% chance of 40dBZ or higher
Friday afternoon along the Miss river. So, weak instability and
strong dynamical lift are probably enough for pea-penny sized hail
and gusts to 35 mph with this system as it is coming through at
peak heating.

Sunday through Thursday: Warmer with Low Shower Chances (20-40%)

Weak ridging aloft with increasing southerly flow and moisture
returning Sunday will bring an airmass into play that could pop
some showers. Overall, no weather system looks particularly
strong until possibly mid-week. So, this weak forcing with mild
temperatures and low shower chances defines the period. A bit
swifter near-zonal flow looks in play for mid-week and beyond
but confidence is quite low for any details with abundant
spread in the forecast pattern suggested.

&&

.AVIATION /12Z TAFS THROUGH 12Z FRIDAY/...
Issued at 631 AM CDT Thu May 9 2024

Mostly VFR conditions seen across the Upper Mississippi River
Valley early this morning during 09.12Z TAF issuance.
Precipitation impacts primarily impact locally southern counties
from northeast Iowa into southwest Wisconsin. Confidence
decreases on northern edge of this precipitation, along am
east-west line between both TAF sites (KLSE/KRST). Any thunder
chances will be limited, higher confidence along southern
periphery. Regardless, low level moisture expected to introduce
lower MVFR ceilings from south to north, affecting both TAF
sites later this morning potentially. Lasting most of the day.

&&

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

&&

$$

DISCUSSION...Baumgardt
AVIATION...JAR