289
FXUS63 KARX 291736
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
1236 PM CDT Wed May 29 2024

.KEY MESSAGES...

- Quiet and seasonal weather for today and Thursday with morning
  fog in valley locales.

- Threat for scattered showers and storms from Friday through
  early next week, but confidence in exact timing and coverage
  details is currently low.

- Slowly warming temperatures through the end of the week to
  near or slightly above average, reaching the low 80s for highs
  by Sunday.

&&

.DISCUSSION...
Issued at 320 AM CDT Wed May 29 2024

Today through Thursday: Pleasant and Dry

Upper level ridging slowly amplifies and builds eastward into
the north-central CONUS today with a 1024-mb surface high
pressure cell reflection sliding through the forecast area
during this time. The ridge morphs into an Omega Block on
Thursday before breaking down and shunting eastward on Friday.

The main forecast challenge for this two day period revolves
around early morning fog potential, mainly in the favored river
valley locales. Deep, light flow and temperatures falling well
below the crossover temperature threshold early this morning and
again tonight will be conducive for fog development. A similar
setup occurred yesterday with widespread valley fog with areas
of 1/4 sm visibilities. Indeed, early morning nighttime
microphysics satellite imagery and surface obs/webcams show fog
once again developing in the valleys and there is little to stop
it from thickening before sunrise.

Once the fog burns off between 7-8am, we should reach our
convective temperatures area-wide by midday today, so have
leaned towards the more aggressive CAM-weighted guidance to
bring scattered diurnal cumulus to the region for this
afternoon, which decay at sunset. Some of the cumulus fields
will be thicker in nature towards central Wisconsin and cannot
fully rule out a sprinkle given some RAP profiles have upwards
of 50-100 J/kg of SBCAPE, but impacts will be very minimal. With
ample solar insolation, highs should bounce back into the low
70s today, and then the mid-70s for Thursday on the immediate
backside of the departing surface ridge.

Friday through Tuesday: Scattered Storm Potential

Return flow off the Gulf on the backside of surface high pulls
a modest corridor of 50 to 60 degree dewpoints into the northern
High Plains for today, but as the Omega Block amplifies and
shifts east, this warm sector narrows and the convection along
the trailing cold front wanes with the main lobe of upper
forcing lifting well north of the region. Recent trends in the
medium range solutions (not surprisingly) have been for a slower
decay of this block late Thursday into Friday and thus a later
arrival time for the upstream precipitation.

The precipitation potential forecast rightfully only has 30-50
percent probabilities for the day on Friday as confidence is low
on what convection will survive into our area within the
marginal environment. An approaching northern stream shortwave
should result in some convection redeveloping for Friday
afternoon into Friday night, but how convection plays out
Friday morning will dictate the location of storms later in the
day. The pattern becomes more zonal for the weekend and early
next week with multiple perturbations working through the flow
that could spark convection. Thus, while much of the forecast
for Friday into Tuesday has 20-50 percent probabilities of
rainfall, there will definitely be breaks between these rounds
of rain that remain tough to pin down at the present time. There
isn`t any real signal for severe weather during the weekend and
Monday, but will need to keep an eye on how things evolve for
Tuesday evening with the next negatively-tilted shortwave trough
passage.

Predominantly west to southwest flow aloft builds in for the
weekend and early next week, bringing in a higher theta-e
airmass and thus warmer surface high temperatures. The longer
range guidance is tightly clustered with highs rising into the
low 80s for Sunday and Monday, with the inter-quartile range
broadening beyond this point owing to uncertainties creeping
into the longwave pattern evolution--with the trends at this
point favoring slightly cooler weather for mid to late next
week.

&&

.AVIATION /18Z TAFS THROUGH 18Z THURSDAY/...
Issued at 1231 PM CDT Wed May 29 2024

Primary concern for aviation interests is the likelihood that
valley fog recurs tonight into Thursday morning. With similar
light winds and excellent cooling favored tonight, have opted
for a repeat event with LIFR conditions at LSE. In the meantime,
abundant fair weather cumulus in the 3-5 kft layer is present
this afternoon.

&&

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

&&

$$

DISCUSSION...Skow
AVIATION...Ferguson