446
FXUS63 KARX 012323
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
620 PM CDT Tue Apr 1 2025

.KEY MESSAGES...

- Snow moves in late this afternoon and evening near and north
  of I-90 with periods of heavy snow possible at times. The
  snow will transition to rain throughout the evening south of
  I-94 and overnight north of I-94 where a mix of sleet/freezing
  rain will occur early Wednesday morning before switching to
  rain.

- Snowfall accumulations of 2 to 4 inches are expected north of
  I-94 by daybreak Wednesday. Accumulations of up to 1 inch will
  be possible near and north of I-90 where heavier snowfall
  rates occur this evening.

- Potential for strong to severe storms Wednesday morning and
  early afternoon, mainly in southwest WI and northeast IA. The
  main hazards with any severe storms will be large hail and
  damaging wind gusts.

- Seasonable to slightly cooler than normal temperatures for Thursday
  and onward with some precipitation chances (20-40%) for the
  weekend.

&&

.DISCUSSION...
Issued at 141 PM CDT Tue Apr 1 2025

Rest of Today - Tonight: Snow, Wintry Mix Changing to Rain

As we progress later into this afternoon and this evening, a wing of
mid-level theta-e advection and associated 500-700mb frontogenesis
pushes through the area. As this occurs, sounding profiles in the
HREF sounding ensemble suggesting fairly uniform residence below the
freezing layer to the surface with cloud ice present near and north
of I-90. As a result, expecting precipitation with this band to
begin as snow, maybe mixed with rain near the I-90 corridor but then
switch over to rain as the low-levels warm up ahead of an incoming
warm front. Important to note that this transition time and the rate
at which snow falls will be crucial for any accumulations at all
south of I-94. The recent HREF has some probabilities for inch per
hour snowfall rates (10-40%) during the evening between I-94 and I-
90 for a brief period before the transition to rain occurs. As a
result, a few of the CAMs have some snowfall accumulations with
modest probabilities (40-70%) for an inch or greater of accumulation
in this area assuming a 10:1 snow-to-liquid ratio which likely is
not wet enough. So the general idea south of I-94 and to the I-90
corridor is if rates can manage to be heavy enough, some minor
accumulations of up to an inch may occur before transitioning to
rain.

Further north across Clark and Taylor counties, this mid-level
frontogenetic band shifts northward into the evening hours with more
favorable thermodynamics for a purely snow solution to persist for
several hours. As a result, the 01.12z HREF has high confidence for
accumulations of 2 inches or greater (60-90% chance) in Taylor and
Clark counties. As we head into the overnight and early Wednesday
morning, a warm front will move towards the area allowing for low-
level warming aloft. As this occurs, surface temperatures at
freezing will enable a period of sleet and freezing rain as
suggested in several of the short-range models. This is reflected in
the 01.12z HREF as well which has medium probabilities (30-60%) for
measurable freezing rain across both Clark and Taylor counties.
Eventually later into the morning, the low-level warm advection
regime wins out and a complete transition to rain occurs in north-
central WI.

Tomorrow Morning - Afternoon: Widespread Rain, Strong to Severe
Storms Possible For Southwest WI/Northeast IA

As previously mentioned, the early morning hours on Wednesday
feature a strong warm advection signal pushing northward through the
area. As this occurs, widespread rainfall will quickly move through
the area during the morning hours with some storms possible across
portions of southeast MN and northeast IA. Given the warm advection
taking place, a fairly stout capping inversion can be noted in this
area at 700-800mb which will keep any convection elevated. So while
low-level hodograph curvature looks fairly spooky during the morning
hours, this would not be realized as it is maintained in a stable
layer. Regardless, given the over cyclogenesis and synoptic setup,
cannot completely rule out some gravity wave enhancement which
may lead to localized damaging wind gusts out ahead of the warm
front Wednesday morning. Some isolated hail of up to quarter
sized may be possible as well with mid-level lapse rates of 7-8
C/km and freezing levels of 7-9kft. However, very limited MUCAPE
would mitigate this potential with the 90th percentile HREF
only having 50-200 J/kg at Platteville, WI during the morning.
So general thinking with the morning convection is very low
confidence for any severe potential with the 01.12z HRRR neural
network probabilities agreeing with this with minimal severe
weather probabilities (5-15% with a 120km neighborhood radius).

The question as we head towards the afternoon will be exactly if and
how much recovery we see when the warm advective convection attempts
to exit the area. Currently, the 01.15z RAP displays a narrow window
where some 850mb moisture transport could advect in some increased
instability of 1000-1500 J/kg MUCAPE that based on afternoon
RAP/HRRR soundings could be surface based as the cap gets
removed. Low-level shear profiles change profoundly during this
period as well with the surface low progressing northeast of
the area and shifting the wind direction at the surface to
southwesterly/westerly. Consequently, a shift from sharp low-
level curvature to straight line hodographs are noted. As a
result, thinking the potential for a weak tornado in Grant
County is in a very narrow window, possibly less than an hour
even and still conditional on if we managed to get the
instability in. With the stronger synoptic winds aloft and very
heavily sheared profiles, both hail and damaging winds would be
in play where SPC has a slight risk (threat level 2 of 5) in
portions of southwestern WI and northeast IA if healthier
convection does materialize. However, by middle to late
afternoon the cold front will push through the area and
effectively wipe out any chances for severe potential, again
highlighting that this window is very brief. Overall, HRRR
neural network guidance is slightly more confident in this
period with 20 to 40 percent probabilities for all severe
weather hazards, mostly driven by damaging wind gusts.

Thursday - Sunday: Quieter Conditions and Remaining Cooler

As we look past this spring system, upper-level southwesterly flow
takes place over the region with a broad synoptic ridge to our east
and a trough situated to the west. Overall, minimal waves in the
flow are noted into the weekend. As a result, not expecting much for
precipitation on Thursday and Friday, however without a more
substantial ridge setting up shop in the area may be difficult to
clear our skies out completely. Regardless, as we head into the
weekend, a northern stream trough will begin to phase with the
previously mentioned western trough which will cool down our
temperatures as it ushers in a slightly cooler airmass for Sunday
and Monday with highs in the 40s for much of the area. Cannot rule
out some precipitation chances with the grand ensemble
(GEFS/Canadian/EC ens) having medium probabilities (30-50% south of
I-90) for measurable precipitation if the two synoptic troughs phase
in such a way that allows the deformation zone of a developing
surface low to sneak into southern portions of the area.

&&

.AVIATION /00Z TAFS THROUGH 00Z THURSDAY/...
Issued at 620 PM CDT Tue Apr 1 2025

CIGS: MVFR/IFR with passing band of sn/rasn early this evening, then a
period of VFR before dropping back into mostly IFR overnight through
Wed morning. KLSE expected to see a bump into VFR, potentially
scattering out for a few hours. MVFR (or lower) favored to return
Wed evening at KLSE. KRST looking to hold with cig restrictions
through Wed night.

WX/vsby: quick moving, narrow band of sn/rasn quickly pushing
northeast this evening...likely past KRST by 00z TAF and KLSE by
01z. Could see a break with only low end chances until the overnight
hours when more rain returns. Some thunder chances, but look mostly
south of the TAF sites. Expect a break from steady rains for much of
Wed morning with scattered shra/ts possible mostly for the
afternoon.

WINDS; east southeast into Wed afternoon, swinging west/southwest as
a sfc low and cold front move northeast. Going to stay windy with
gusts in the mid 20s to low 30 kts.

&&

.ARX WATCHES/WARNINGS/ADVISORIES...
WI...Winter Weather Advisory this evening to 10 AM CDT
     Wednesday for WIZ017-029.
MN...None.
IA...None.

&&

$$

DISCUSSION...Naylor
AVIATION.....Rieck