918
FXUS63 KARX 011720
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
1220 PM CDT Sat Jun 1 2024

.KEY MESSAGES...

- Rain chances were increased in southwest and central Wisconsin
  this morning. Showers most numerous this morning areawide,
  becoming more scattered this afternoon. Slight storm/lightning
  chances.

- Some morning fog may be around Sunday with light winds and
  clearing skies, especially favorable where it rains longer today.

- Showers and non-severe, waning storms move through Sunday
  night into Monday, delayed from previous forecasts, with
  Sunday now looking mainly dry and seasonal.

- Tuesday afternoon and evening is the next potential for
  organized severe weather.

&&

.DISCUSSION...
Issued at 338 AM CDT Sat Jun 1 2024

Showers and Isolated Storms Today

Radar and GOES water vapor showing a number of shortwave troughs
affecting todays weather converging on the area. One is
approaching I-35 in northern IA while another more vigorous
system is over northern MO with widespread rain ahead of it. Per
latest RAP guidance and low-level radar winds, the area is under
moisture advection in southwest flow, however many radar echoes
aloft over northeast IA and southeast MN have just started to
reach the surface (after many hours).

As the southern system approaches and meets a more westerly
component over the forecast area, an area of deformation should
be favored across far northeast IA, southwest into central WI
this morning for a more steady light rain. Have increased rain
chances there and really it is just a matter of how far north
this rain area moves. Further north, the influence of the weak
IA shortwave trough and diminishing frontogenesis in a westerly
push will provide more scattered and less predicable showers,
most numerous this morning it appears. By afternoon, some weak
instability may grow for isolated storms/lightning, especially
if skies can clear a bit more. This may be north and west of La
Crosse first, shifting east into WI during the afternoon into
evening. This all diminishes during the early evening. Some
clearing and calm winds favor fog formation for Sunday morning,
but confidence on exactly where and how dense is still lower.

Rain Chances Sunday Night into Monday

Model consensus is pretty strong on an identifiable, moderate
intensity shortwave trough streaming east across the Northern
Plains Sunday in zonal flow with an abundant moisture return as
heights fall across the Dakotas Sunday. Probabilities suggest
the low-level warm advection and moisture transport convergence
remain just west/on I-35 with the ridge holding over WI. Thus,
have dried the forecast for Sunday with only small /30% or less/ rain
chances closer to I-35. Severe storms are probable over the
Dakotas into wrn MN Sunday, shifting east and into sern MN and
nern IA in a waning state after midnight. While it looks like
the storms are waning as CAPE rapidly diminishes after sunset,
there is still good signal for rainfall with 70-80% chances in
the 31.12Z Grand Ensemble for 0.05"/6hr, and each model family
has the mean shortwave trough moving through Sunday night into
Monday with moisture transport support. While the 01.02Z NBM
rain chances are 80%+, have lowered these slightly to near 60%
in WI as there is a diminishing nocturnal rain signal as the
system shifts east Monday /e.g., coverage decreases/.

Tuesday`s Severe Storm Potential

Medium range forecasts suggest some consistency in a stronger
trough moving east in the zonal flow across the northwest CONUS
as amplification in the pattern begins Monday. Downstream
ridging over the eastern CONUS will influence the incoming
trough depth and track and some spread still exists on the
amount of synoptic lift this wave will provide the local area.
Current timing for the cold front is later on Tuesday, providing
the ideal situation for a warm day and instability being near
diurnal peak. MLCAPE of ~2000 J/Kg should be in place but wind
shear looks a bit more shallow and limited with meager venting
at storm top with 40-50kts winds. Stay tuned as the 31.00Z GEFS
CSU severe probabilities have the area on the northern sector of
the 15%+ region while the 01.08Z SPC does not have a current
risk Tuesday.

Cooler For Later Next Week

The Tuesday trough begins a large scale transition over North
America where deep broad troughing and cooler than normal
conditions would be expected across the Great Lakes. Periodic
showery periods would also be expected with this pattern.

&&

.AVIATION /18Z TAFS THROUGH 18Z SUNDAY/...
Issued at 1220 PM CDT Sat Jun 1 2024

CIGS/VSBY: some hints for a short (2 hour) window of MVFR cigs this
afternoon, but most meso models favor keeping 3k+ ft for cigs and
will stick with that. Should see clearing by 00z with mostly SKC/SCT
conditions through the overnight.

Questions arise with fog and stratus potential overnight. Mixed
signals in some of the short term guidance, but with rain and
clearing skies/light winds, setup is favorable for fog formation. Is
it widespread or just river valley is another question - which isn`t
clear. For now, probabilities favor along the Mississippi River
eastward, but will bring fog in for both TAF sites. See the
potential for sub 1SM, moreso at KLSE where the RAP/HRRR soundings
mirror what a fall valley fog event would look like. If thick fog
manifests, it would eventually lift as a low stratus layer,
continuing LIFR/IFR conditions into mid morning Sun. How this
evolves will be closely monitored. Anticipate updates to the
forecast based on trends as we move through the evening hours.

PCPN: temp discontinuity will result in a weak sfc boundary
developing in between the sunny/cloudy regions on the western flank
of the current cloud shield. This will work in a ribbon of
instability to spark isolated shra/ts as we move through the
afternoon. Coverage is very spotty, but this has been a consistent
signal in the meso models over the last 24 hours. Won`t add to the
forecast but will monitor and adjust forecast if a TAF site
becomes more likely to be impacted.

WINDS: light wind field, mostly south to southeast through the
night. Should see an uptick in speeds later Sunday as pressure
gradient starts to tighten from the west.


&&

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

&&

$$

DISCUSSION...Baumgardt
AVIATION.....Rieck