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000
FXUS63 KARX 090837
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
337 AM CDT Tue Apr 9 2024

.KEY MESSAGES...

- Possible Showers Along Northwest Periphery From Southeast
  Minnesota Into Central Wisconsin Today.

- Precipitation Chances Return Wednesday Night, Initially Along
  Aforementioned Northwest Periphery, Increasing From West To
  East Thursday Night Into Friday Morning. Near Nil Thunder
  Chances.

- Warming Into The Weekend, Precipitation Chances Return To
  Start Next Week

&&

.DISCUSSION...
Issued at 310 AM CDT Tue Apr 9 2024

Shower Chances West to North of La Crosse Later Today:

GOES water vapor imagery at 07Z was showing large scale troughing
across the Upper Midwest with numerous shortwave trough spokes of
energy rotating through the region. Radar and surface observations
showing light rain showers north of La Crosse associated with a
stronger trough near Red wing MN lifting northeast. Another wave is
over eastern ND making the turn southeast toward the area, with
another over eastern MT forecasted to deepen as it moves into
the longwave toward the area for evening arrival. Later this
afternoon, differential temperature advection in the
surface->850mb (warm) layer and 700 mb (cold) will steepen lapse
rates per consensus short term models/CAMS. Depending on
moisture in that ~800mb layer, MUCAPE of 50-200 J/Kg looks like
it will materialize north and west of La Crosse, maximized
toward central MN. A few showers will be possible in the 3-8 pm
window with this instability and troughing moving through. CAMS
have differing outcomes from scattered showers to isolated
vigorous cumulus/low reflectivity due to what appears to be
instability above the surface mixed layer. Forecast shower
chances were increased some to 20-30%...but that may be too low
should these shortwave troughs bring a bit more lift with them.
In any case, just enough to wet the ground in most cases and
updates to the forecast for later today may be needed.

Driving of Precipitation Chances Wednesday:

A siesta from showers over the Upper Mississippi River Valley
Wednesday morning-afternoon ceases Wednesday night as synoptic
forcing returns. A shortwave trough, seen on GOES 18 Water Vapor
imagery early this morning over the Pacific Northwest, is expected
to dig into the central CONUS Tuesday night, fresh on the heels of
the exiting low. Supplemental synoptic forcing originating from
a closed low, seen on GOES 16 water imagery along the Desert
Southwest International border earlier this morning, will also
be the primary driver for highest low level moisture.

Wednesday Night Precipitation Confidence & Impacts:

Initial precipitation chances Wednesday along our northwest are
tied to this northern disturbance and any leftover moisture
within northwest flow. Highest confidence remains to our west as
timing with diurnal heating and frontal boundary better aligns.
An overall slowing trend in upper level forcing limits
confidence locally as the PV 1.5 fold wraps and phases through
the Northern Plains. Therefore, highest confidence (30-50%) for
precipitation chances lies along our northwestern periphery as
moisture leftovers appear more likely.

Increasing Precipitation Chances Through Thursday

Overnight into Thursday, the northern and southern streams get their
act together, conglomerating over the Upper Mississippi River
Valley. Precipitation chances squeeze the local forecast area from
the north and south into Thursday morning. Timing and location of
conglomeration impacting surface low track and subsequent location
of highest moisture. Additionally, an initial low level height
saddle shrinks and stretches a south-southwest to north-northeast
low level theta e filament across the forecast area. Highest
confidence places the low track to our southeast, across southern
Illinois with the warm sector zipping shut along the aforementioned
moisture filament.

Differences in long term global ensembles (EPS/GEFS 09.00Z) surround
timing of synoptic forcing joinery, as the earlier solution
allows the surface low to lift farther northwest towards the
Mississippi River Valley. In the GEFS, a later conglomeration of
synoptic upper level energy keeps the surface low into the Ohio
River Valley. Therefore, differences in PoPs, QPF, and thunder
potential remain. EPS dProg/dT has shown a northwesterly shift,
poking highest probabilities (80-100%) for the 24-hour 0.1"
isohyet along local counties in central Wisconsin. Meanwhile
keeps probabilities for the same from near 50%.

Instability & Thunder Chances:

GEFS members exhibit some SBCAPE whereas very few members can
say the same for the EPS. The more northerly, later digging of
the perturbation allows the GEFS to better tap into peak
diurnal heating whereas the EPS remains slightly slower.
Given sounding profiles, have removed Thunder mentions given
the very low instability magnitude, 200 J/kg mean MUCAPE SREF
plume. Irregardless, will be another forecast detail to keep an
eye on.

Temperatures Wednesday Into The Weekend:

Higher impact of any moisture and low level warm air advection
Wednesday ahead of the trough will be high temperatures. Exact
location in strands of moisture will limit the higher April sun
angle from influencing daytime highs in spots. Advection of any
upstream cloud deck with higher precipitation chances may
result in underachievement along our western periphery while an
incoming cloud deck along our southern periphery from the
stronger, better low level moisture could similarly bust
temperatures. Given lower confidence, have only slightly bumped
sky cover away from National Blend, keeping daytime high
temperatures in mid 60s. As confidence increases into Wednesday
night, overnight lows then may be too low. Eyes remain peeled.

While daytime high temperatures remain near seasonable through the
end of the work week, an amplified upper level ridge is
expected to build into the weekend, bringing above normal
temperatures across the Upper Mississippi River Valley.
Long term ensemble (GEFS/EPS 09.00Z) confidence for the 70
degree isotherm has pushed farther northeast, 20-40% on Saturday
and 50-70% on Sunday, bifurcating the southwestern half of the
local forecast area. The higher upper level heights combat a
weak, open ridge runner late weekend followed by another closed,
phasing low through the Central Plains early next week. A CONUS
bifurcating spread in surface low location exemplifies the
limited confidence in local impacts. Associated 24 hour QPF
probabilities are 20-40% late weekend from the ridge runner and
40-60% with the closed low into the new work week.

&&

.AVIATION /06Z TAFS THROUGH 06Z WEDNESDAY/...
Issued at 1039 PM CDT Mon Apr 8 2024

Light showers remain possible overnight, particularly north of
KLSE/KRST. A period of MVFR ceilings is probable (60-90% chance)
from west to east overnight into Tuesday with some
clearing/rising ceilings through the day. Winds will trend
westerly tonight through Tuesday with occasional gusts up to 20
kts possible before subsiding by sunset on Tuesday.

&&

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

&&

$$

DISCUSSION...Baumgardt/JAR
AVIATION...JM