180
FXUS63 KARX 192230
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
530 PM CDT Sun May 19 2024

.KEY MESSAGES...

- Another round of showers and thunderstorms late tonight
  through Monday afternoon. A few stronger storms possible.

- Confidence continues to increase for severe thunderstorm
  potential Tuesday across the region. Severe risk level 3 out
  of 5 for most.

- Another system moves in Friday into Saturday. Best chances
  (60%) on Friday evening across the area.

&&

.DISCUSSION...
Issued at 303 PM CDT Sun May 19 2024

Overview: Active period of weather this week. Increasing
chance of severe thunderstorms Tuesday.

Broad troughing across the western CONUS continues to dig south into
the Four Corners this evening through Monday. Meanwhile, a low off
the southern CA coast is progged to phase with the digging trough as
it moves onshore into the Baja region. As it does, SW flow takes
hold across the CONUS with a downstream elongated ridge across the
Appalachians. This SW flow will allow PWATs to climb above 1.5
inches on both Monday and Tuesday, which remains near the
climatological max of DVNs sounding. The aforementioned trough and
associated shortwave trough will eject from the Rockies Tuesday
morning as a leeside Colorado Low develops and lifts NE through the
Central Plains. This trough takes on a negative tilt as the surface
low is expected to deepen quickly into a seasonably strong low (into
the 990s hPa). Tuesday remains the day to watch for severe weather
as we have a deepening surface low passing through the region,
bringing ample forcing for ascent with strong, deep layer shear all
while peak daytime heating is occurring. All the ingredients remain
in place for a convective severe weather event, but the details
remain uncertain with an expected MCV on Monday that may perturb the
environment. After Tuesday, the upper-level pattern becomes
temporarily more zonal and progressive with another upstream trough
dropping south along the West Coast. This trough moves inland across
the northern Rockies Thursday and through the Northern Plains Friday
through Saturday bringing another surface low and chance for
widespread precipitation. Variability among the medium range
guidance will bring low chance PoPs Friday through Saturday, but the
best timing looks to be on Friday evening through Saturday morning.

Tonight through Monday: Thunderstorms Tonight into Monday. Severe
Risk Level 1 out 5.

Warm air advection and isentropic ascent continue to increase this
afternoon with clouds increasing from the west all day. A shortwave
trough tracks through the area after midnight tonight, with
increasing PoPs after 06Z across the area. These precipitation
chances appear more likely than specific details on any severe
weather threat. Evolution on that threat depends on how the upstream
MCS evolves through Central Plains this afternoon. It`s possible
this MCS perturbs the warm front enough that it doesn`t lift far
enough into our region, which would decrease any severe threat
considerably. Large scale shear outside of the upstream convection
is on the lower end, which decreases threat as well.

By Monday afternoon, better large scale forcing appears to be just
east of our local area across SE WI where a 40-50kt LLJ lifts into
the region with 40-45kts bulk shear. So overall, higher chances for
showers and thunderstorms tonight into Monday, but uncertainty
remains on severe evolution into the afternoon with convective cloud
debris and better shear/instability trending east.

Tuesday: Strong Low Pressure with Increasing Confidence for Severe
Thunderstorms. Severe risk level 3 out of 5.

There are two separate chances for storms on Tuesday, with the first
one occurring as a warm front lifts into southern MN/WI. The next,
potentially more significant, round of thunderstorms will occur as a
strong low pressure system and associated fronts move into the area
Tuesday afternoon/evening.

A potent negatively tilted trough and seasonably strong surface low
pressure system are expected to lift NE from the Central Plains into
the Upper Midwest Tuesday, bringing severe thunderstorm chances
along with it. There is ample moisture (PWATs over 1.5 inches),
large scale ascent, and instability for severe thunderstorms to
occur, but will need to wait for more specific details. After the
potential morning convection, models favor the surface low passing
to the west of our local area. This puts our area firmly in the warm
sector of the low pressure system with dewpoints in the 60s
expected. As the low lifts north, steep mid-level lapse rates and
surface destabilization will bring increased instability as
sufficient shear moves in. At this time it seems like wind would be
the primary hazard as the threat evolves with time with upscale
growth, but initial development of isolated storms could pose a
tornado or hail threat. With the upscale growth in the afternoon and
tightly wound low pressure system, embedded QLCS tornados would also
be possible. Continue to monitor this forecast as this is the
highest severe weather threat of the season so far.

Wednesday and Beyond: Remaining Active

There could be some lingering showers and thunderstorms on
Wednesday, but overall a drier trend is favored Wed into Thursday.
The next wave advancing from the Rockies across the Northern Plains
Friday, bringing a widespread chance (50-60%) of showers and
thunderstorms Friday afternoon into Saturday morning.

&&

.AVIATION /00Z TAFS THROUGH 00Z TUESDAY/...
Issued at 530 PM CDT Sun May 19 2024

CIGS: mid level cigs into the overnight with a lowering into MVFR
expected near 12z. Some differences in short term guidance in how
long these will linger, but consensus generally favors through the
morning with improvement to VFR cigs for the afternoon. Another
round of low (MVFR or lower) cigs are expected Tue, although
timing/how long those impact the TAF sites is unclear at this time.

WX/vsby: showers become widespread toward 06z tonight, lasting for
through the overnight/morning hours of Monday. Enough instability in
the overnight for a few storms, but confidence in coverage isn`t
high yet - so will hold VCTS. Will refine/update forecast as needed
based off radar/short term trends.

While the model blend favors keeping low rain chances through Monday
afternoon/evening, the CAMS and other deterministic model runs
suggest there will be a break from the pcpn. This looks reasonable
and will trend the forecast that way. After Monday morning, next
shot for shra/ts could hold off until Tue morning.

WINDS: southerly tonight, shifting westward post a sfc front toward
18z Mon.


&&

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

&&

$$

DISCUSSION...JAW
AVIATION.....Rieck