442
FXUS63 KARX 092031
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
331 PM CDT Thu May 9 2024

.KEY MESSAGES...

- Showers with funnel clouds possible this afternoon

- Frost advisory for parts of north central Wisconsin

- Showers and storms will move southeast across the region during
the afternoon and evening on Friday. A few strong storms are
possible with small hail and gusty winds being the primary threats.

- Periodic shower and storm chances persist through the weekend and
into next week. Temperatures will trend warmer than normal with
Sunday appearing to be the warmest day with highs in the upper 70s
to lower 80s.

&&

.DISCUSSION...
Issued at 325 PM CDT Thu May 9 2024

Overview/This Afternoon and Tonight:

Water vapor satellite imagery, lightning, 500mb heights and
radar show a couple of upper level waves embedded within the
main longwave trough from the New England through the Great
Lakes toward the local area. An area of showers pushed through
southeast MN and northeast IA. One of the showers increased with
lightning and quickly developed a funnel cloud/possible tornado.
With continued breaks this afternoon and additional heating this
afternoon, the latest RAP is showing some potential for NST
tornadoes with the next area of showers pivoting south across
parts of western WI and southeast MN as SBCAPE increases to
around 500 J/kg.

Clearing skies later tonight along with decreasing winds and
temperatures dropping into the lower to mid 30s should allow
areas of frost to develop, especially for Taylor County. In
coordination with neighbors, did include Trempealeau, Jackson,
Clark and Taylor Counties in a frost advisory.

Friday Afternoon and Overnight: Showers and Storms

As we head into the afternoon and onward cloud cover will begin to
build ahead of an incoming 850mb shortwave caught in the northwest
flow regime. As the wave progresses through our area during the
afternoon and early evening, the 09.15z RAP shows a fairly robust
low-level jet behind it to 40-50 kts. This in combination with
MLCAPE of up to 1000 J/kg will help to initiate a line of showers
and storms that will quickly push southeast along with the
aforementioned low-level jet. In this case, instability appears to be
the limiting factor with strong (around 50 kts) 0-6 km bulk shear in
place. However, with fairly steep lapse rates and an inverted-v
subcloud layer, cannot rule out some gusty winds to 50 mph or some
small hail with any stronger storms when considering the freezing
levels of around 6kft. Consequently, SPC has a marginal (level 1 of
5) risk for severe storms for much of our area on Friday. Showers
and storms will quickly progress southeast through the forecast area
and will move out of the local area later into the evening.

This Weekend and Next Week: Warmer Trend with Some Shower & Storm
Chances

Northwest upper-level flow will be in place for the weekend with the
aforementioned departing shortwave trough departing leaving room for
a weak ridge to build into the region allowing for a warming trend
to take hold with surface southwesterly flow. As a result, the inter-
quartile range of for high temperatures at La Crosse on Sunday in
the 09.13z NBM is 80 to 85 degrees. Additionally, with the 09.12z
GFS/NAM showing fairly well mixed low-levels it could be a day with
overachieving warming but will feature lower dewpoints. With fairly
modest instability (1000-2000 J/kg of MUCAPE) present at peaking
heating on Sunday will have to watch for any subtle forcing
mechanisms that could set off any convection. Currently, the 09.12z
NAM/GFS shows some subtle vorticity advection associated with a weak
wave that progress through northern MN and into northwestern WI. As
this synoptic forcing pushes into the aforementioned higher
instability, a stronger 800-900mb frontogenetic signal is noted in
both the 09.12z GFS/NAM which when combined with modest precipitable
waters of around 1 inch and marginal 850mb moisture transport could
set off pulse convection across portions of the region. With any
potential convection for Sunday being outside of the range of the
short-range CAMs, hard to put confidence into localized
thermodynamics and shear profiles too much but the severe threat
with any storms appears to be minimal at this point in time with 0-
6km bulk shear values appearing very low across portions of northern
WI.

As we head into next week, northwest flow turns more quasi-zonal
with a couple of weaker short-wave troughs hinted by deterministic
guidance. However, still lots of uncertainty with how these systems
will manifest. Consequently, generally expecting a calmer period
into the first half of next week at this point in time with fairly
pleasant temperatures and occasional shower/storm chances in
accordance with the NBM. However, the EC ensemble has fairly low
probabilities for measurable precipitation (0-40% chance) early next
week.

&&

.AVIATION /18Z TAFS THROUGH 18Z FRIDAY/...
Issued at 1248 PM CDT Thu May 9 2024

MVFR/VFR ceilings with scattered showers this afternoon.
Conditions will improve with gradual clearing this evening.
Light northeast winds become northwest and then light southwest.

&&

.ARX WATCHES/WARNINGS/ADVISORIES...
WI...Frost Advisory from 2 AM to 8 AM CDT Friday for WIZ017-029-033-
     034.
MN...None.
IA...None.

&&

$$

DISCUSSION...Naylor/Zapotocny
AVIATION...Zapotocny