FXUS63 KARX 310300

Area Forecast Discussion
National Weather Service La Crosse WI
1000 PM CDT Thu Mar 30 2023

.DISCUSSION...(This evening through Thursday)
Issued at 319 PM CDT Thu Mar 30 2023

Key Messages:

 - Potent spring storm system to bring a wide range of impactful
   weather to the region tonight into Saturday morning.

 - Severe storms expected over northeast Iowa and southwest Wisconsin
   Friday afternoon and evening. The threats from this fast-
   moving complex of storms could include damaging winds of 60-70
   mph, tornadoes (some possibly strong), and large hail of 1-2".

 - Heavy rain potential tonight into Friday night from south-
   central Minnesota to central Wisconsin with amounts of 1 to 2
   inches likely. River flooding risks along smaller rivers such
   as the Black and Trempealeau are on the rise.

 - Heavy snow threat from south-central Minnesota through west-central
   and north-central Wisconsin Friday night into Saturday
   morning. Amounts of 5-10 inches are expected under this snow
   band, though confidence in amounts at a given location remain
   in flux. Strong winds may result in blowing snow and reduced
   visibilities during the times the snow is falling.


Surface and satellite analysis shows the warm front and moisture
surge with ~0.7" precipitable water into the Topeka/Kansas City area
steadily moving north. This surge arrives across the area through
the mid-evening spreading north through the overnight with excellent
moisture transport convergence on the nose of the low-level jet,
which is also isentropically lifting. Text book case of elevated
convective forcing with MUCAPE in the CAMs suggesting >0 J/Kg
southwest of I-94 overnight and by sunrise 750 J/Kg into northeast
Iowa. Overall this means showers and thunderstorms should become
widespread overnight. There may be enough moisture convergence
remaining later tonight as this somewhat higher CAPE moves in, and
RAP indicated bulk wind shear from 2-7 km at about 35 kts, to
provide a low severe sized hail threat there.

Further north the surface temperatures will remain quite cool at the
surface. Trends today in the CAM guidance indicate a well formed
deep warm layer aloft building into the area under strong warm air
advection this afternoon and early evening. By mid-evening, the warm
layer will shift into northcentral WI where surface wet-bulb
temperatures will be in the 30-32F range, promoting a scenario of
freezing rain and icing. With the "most likely" precipitation
amounts in the 0.50 to 1.00" range overnight, it will be challenging
to know the impact and how much ice will form. SPC HREF FRAM mean
output from the morning 30.12Z run indicates 0.25-0.50" of ice
accumulation in eastern Taylor county by Friday morning, decreasing
southwestward. Forecast soundings confirm the threat but also
indicate surface T/Td right near freezing. So, a challenging
forecast right on the edge. NBM and HRRR guidance are very similar
with 0.25-0.35" of ice in northeast Taylor county. Continued warming
and higher dewpoints in the morning Friday should diminish the
threat with mainly all rain expected. This will need to be monitored
for an upgrade to an ice storm warning, but for now have issued a
Winter Weather Advisory to cover the threat.

Precipitation is expected to move off or diminish as the main low
shifts into the Plains in the morning.


The forecast remains on track for numerous weather impacts Friday
afternoon through Saturday morning. The probability of severe
weather south of Interstate 90 on Friday afternoon continues to
increase. Then, gusty synoptic winds and heavy accumulating snow
combine for winter storm impacts north of Interstate 90 Friday night
into Saturday morning. Lastly, river rises are expected this weekend
in response to the heavy rainfall and snow melt from this system.
The Black and Yellow Rivers in Wisconsin have a high likelihood of
reaching flood stage this weekend.

The culprit, a potent low pressure system, is currently located over
the Rocky Mountains near the Colorado/Wyoming border. After ejecting
into the Plains this evening, the cyclone intensifies as it slides
east-northeast towards the Upper Mississippi River Valley. The
following ensues...


There is good agreement in the hi-res models for the warm sector to
set up along and south of Interstate 90 with dewpoints nearing 60
degrees by late morning. As dewpoints climb, the HREF mean surface-
based CAPE builds to 1000-1500 J/kg in northeast Iowa, southwest
Wisconsin, and southeast Minnesota counties that border Iowa. This
is where the highest risk of severe storms in the local area resides.

Little to no CIN is present in model soundings which increases the
potential for storms ahead of the cold front. Storms could initiate
as early as 1 to 2 PM. If supercells form, the hail and tornado
threat increases. Hodographs are mostly linear in model soundings
which indicates no preference between right-moving or left-moving
supercells, but instead highlights the potential splitting
supercells. There is a lot of uncertainty surrounding how many
storms develop ahead of the moisture boundary and what type of
interactions they may have with each other. Storms could split,
merge, or destructively interfere.

A less likely scenario of storms developing and tracking along the
warm front would lead to an increased tornado threat near the
Interstate 90 corridor. It is more likely that storms would become
elevated if they encounter the warm front, making large hail the
primary threat.

A moisture gradient will precede the cold frontal passage, and hi-
res models all show linear storm development along this boundary.
This is where the highest threat for widespread damaging wind lives.
Bowing segments could easily form, especially if/when the squall
line catches up and merges with isolated storms in the warm sector.
The high speed of the squall line itself poses a damaging wind
threat as it will not take much to mix down stronger wind speeds.
The severe weather threat pushes east of the local area by 7 PM with
non-severe showers and storms lingering across portions of the area
through the evening, especially north of Interstate 90.


Rain transitions to snow from northwest to southeast overnight
Friday as temperatures fall below freezing. There continues to be a
heavy snow threat, especially north of Highway 10 in Clark County,
Wisconsin. The EPS, GEFS, and hi-res models all favor a slightly
different location for the heavy snow band in the deformation zone.
The EPS is a southern outlier, the GEFS is 2-3 counties north of the
EPS, and hi-res guidance is somewhere in between. Because of the
model differences, it is likely that the southern gradient of the
accumulating snow will exhibit a sharper cutoff in higher
accumulations than currently forecast. There is a 50 to 70% chance
of snow rates greater than two inches per hour in the heart of the
heavy snow band. Rapid accumulations will lead to hazardous travel.

Light snow is likely across much of the forecast area into Saturday
morning as the deformation zone rotates around the back side of the
surface low. Light accumulating snow could continue as late as mid-
morning, especially in central Wisconsin.


Confidence remains high for gusty winds (35-45 mph) Friday night
into early Saturday morning as the strong pressure gradient sets up
on the backside of the surface low and ridge to the west. There is a
high (80%+) chance for 40 mph gusts but the probabilities fall
under 50% for gusts greater than 45 mph. This poses a blowing snow
risk, but the ability for the snow to blow after the snow ceases
falling is less certain given the potential wet nature of the snow.
However, during the falling snow, these is high confidence for
reduced visibilities, even where lower accumulations are forecast.

Winds subside quickly Saturday mid-morning with high pressure
building into the region.


A zonal upper-level flow pattern sets up on Sunday and persists into
the new week. Periodic precipitation chances exist with a train of
shortwaves embedded in the mid-level flow. Rain will be the primary
precipitation type except in north-central Wisconsin where cooler
temperatures could lead to some snow or a rain/snow mix, especially
precipitation falls overnight.

Another low pressure system looks likely around mid-week. It is too
early to dive into the details, but all of the ensemble clusters
bring sensible weather to the Upper Mississippi River Valley. The
global deterministic models currently show the surface low tracking
just to the northwest of the forecast area, but there are a lot of
differences in the timing, strength, track, and progressiveness.


.AVIATION...(For the 06Z TAFS through 06Z Friday night)
Issued at 1000 PM CDT Thu Mar 30 2023

CIGS: cigs will lower into IFR/MVFR tonight, continuing to drop to
mostly LIFR/IFR overnight through Fri morning. Some improvement Fri
afternoon, but trends favor keeping low cigs until responsible storm
system exits east Sat.

WX/vsby: scattered/numerous showers expected to develop overnight at
the TAF sites. Could be a rumble or two of thunder. This activity
should wane toward mid/late morning Friday with more vigorous showers
and storms developing by mid afternoon, a few could be strong-
possibly severe (higher threat south of the TAF sites). Will add
VCTS for thunder threat for now. The band of showers/storms looks to
lift north of the TAF sites by late Friday evening, with periods of
rain/snow or just snow overnight. Some snow accums expected.

WINDS: east/southeast around 10 kts (or so) into Fri evening,
becoming  north as sfc low exits east. Some LLWS concerns for a few
hours later tonight...mostly from differences in direction from sfc
to 2 kft (roughly 40 kts at top of that layer).


Issued at 319 PM CDT Thu Mar 30 2023

Consistent signal, with just a smidge of southward drift, for 1-2
inches of rain along and north of the I-90 corridor should lead to
minor to moderate flooding on many rivers in that area. This will be
exacerbated by snowmelt runoff in the northernmost basins. River
forecasts have steadily increased in their peak crests on the Black,
Yellow, Trempealeau, and Zumbro as confidence increases. There are
certainly some signals for hitting major flood stage, mainly on the
Black river, and those signals are growing. Most of the crest
information suggests late weekend to early next week for the


WI...Winter Storm Watch from Friday evening through Saturday
     afternoon for WIZ032-033.

     Winter Weather Advisory from 10 PM this evening to 10 AM CDT
     Friday for WIZ017.

     Winter Storm Watch from Friday evening through Saturday
     afternoon for WIZ017-029-034.

MN...Winter Storm Watch from Friday evening through Saturday
     afternoon for MNZ079-086>088.




NWS ARX Office Area Forecast Discussion