555
FXUS63 KARX 050834
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
334 AM CDT Sun May 5 2024

.KEY MESSAGES...

- Pleasant weather for today and Monday, warming back up into
  the upper 60s to mid-70s.

- Periods of showers dot the forecast from Monday through
  Friday. Best chance for rain at this point is late Monday
  night/Tuesday morning and again late Wednesday.

- Cooler air settles in after midweek, but overall look for
  seasonal temperatures into next weekend.

&&

.DISCUSSION...
Issued at 330 AM CDT Sun May 5 2024

Today through Monday: Pleasant!

Fog once again developed across the region in the wake of
yesterday`s cooler and wetter weather, densest within the river
valleys per nighttime microphysics satellite imagery. After
this fog burns off, look for a few days of dry and seasonably
warm weather. Split flow aloft coupled with weak surface high
pressure results in light winds today with the only notable
sensible weather coming in the form of some thin daytime cumulus
clouds, mainly along and north of I-90 where forecast soundings
show convective temperatures being reached under a cooler mid-
tropospheric thermal trough. Clouds may be a bit thicker north
of I-94 per MOS guidance solutions, but overall expect more sun
than clouds.

Winds and temperatures increase for Monday as the upper ridge
pushes to the east and southeasterly flow ensues ahead of an
approaching upper trough that will end up driving our weather
pattern for the remainder of the week. Overall trends in the
guidance have been for a slower pattern transition Monday
afternoon, resulting in the wind/sky/PoP forecast lowering with
successive forecast updates. This latest forecast continues the
trend and thus aside from some high cirrus we look to see
another nice day in store.

Monday Night through Saturday: Periods of Rain

Convection over the Central Plains Monday afternoon grows
upscale in the evening as it moves into Iowa, becoming
increasingly elevated as the boundary layer cools and decouples
overnight. MUCAPE values fall to around 300 J/kg or less by the
time the system arrives after midnight, thus these showers and
isolated thunderstorms look to pose little in the way of any
risk as they decay during their transit. The negatively-tilted
upper trough responsible for this convection takes on a nearly
zonal orientation as the upstream longwave closes off into an
elongated low stretching from the Rockies to the Great Lakes.
This pattern evolution, coupled with the early morning timing of
the convection, nullifies our the risk of severe weather on
Tuesday afternoon with the instability axis shunting off to the
south/east. Indeed, the latest SPC Day 3 outlook mirrors this
thought in its southeastward shift.

Te remainder of the week`s forecast hinges on the evolution of
the closed upper low that meanders over the region. The 6-hourly
precipitation forecast for Tuesday through Friday features
nearly a continuous smattering of 20-40% PoPs owing to
subtle/difficult to resolve undulations within this cyclonic
flow pattern driving the risk for rainfall. We certainly are not
expecting a washout by any means and with the deeper moisture
transport shunted well to the south, any showers will be of
lower impact. One break in the precipitation looks to come late
Tuesday night into Wednesday morning as low amplitude ridging
builds in the wake of the northward-lifting zonal shortwave.
Convection developing along the central Missouri River basin
will lift into the region Wednesday night, with 80+% of the
EPS/GEFS members bringing measurable rainfall with this complex.

Forecast confidence in rainfall timing degrades beyond
Wednesday evening as the upstream omega block pinches off and
energy associated with a Hudson Bay vort lobe interacts with the
bifurcating low and keep some semblance of cyclonic flow in
place through Thursday. However, there is good agreement that
such a pattern will funnel cooler air southward and drop
temperatures back into the upper 50s to lower 60s for highs to
round out the week. The pattern does trend drier for the
weekend, but there exists plenty of variability in the longwave
pattern and such stagnant setups have a tendency to linger
longer than initially advertised.

&&

.AVIATION /06Z TAFS THROUGH 06Z MONDAY/...
Issued at 1045 PM CDT Sat May 4 2024

Main aviation forecast question for this TAF period revolves
around fog potential towards daybreak at LSE. Rainfall earlier
today combined with clear skies and light winds will prove
favorable, but the highest likelihood for fog development looks
to be along and north of I-94 in Wisconsin. Models have
struggled with recent post-rain fog events and are not very
supportive for fog at LSE late tonight, but observed dewpoint
depressions are dropping at this hour, so trends suggest the
possibility exists. Will introduce MVFR visibility (20%
probability) in the 06Z TAF, but confidence is only medium in
overall fog potential and how low visibility might drop. Right
now there is a small (10-15%) chance for IFR visibility at LSE
by daybreak and possibly some accompanying low stratus if this
full-on fog development scenario would pan out.

Otherwise VFR conditions for the TAF sites with light winds
under 10 knots, fluctuating between westerly and southerly as
high pressure drifts overhead.

&&

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

&&

$$

DISCUSSION...Skow
AVIATION...Kurz