106
FXUS63 KARX 031647
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
1145 AM CDT Mon Jun 3 2024

.KEY MESSAGES...

- Multiple rounds of showers and thunderstorms are expected
  today. A few severe thunderstorms cannot be ruled out,
  primarily in far southwestern Wisconsin this afternoon if
  skies can clear.

- Widespread rainfall (80-99% chance) is expected Tuesday
  afternoon into Tuesday night with the entirety of the forecast
  area favored (70-80% chance) to receive at least half an inch
  of rainfall with the potential for locally higher amounts. A
  stray severe storm could occur as well, but the probability is
  very low (5% or less).

- Below normal temperatures are expected for the second half of
  the week with most afternoons featuring a chance for showers.
  Additionally, strong wind gusts could occur Wednesday and
  Thursday afternoon.

&&

.DISCUSSION...
Issued at 345 AM CDT Mon Jun 3 2024

Severe thunderstorm potential today:

A surface low in the Northern Plains has been progressing east
overnight, generating precipitation across much of Minnesota and
northern Wisconsin. The lows associated surface frontal boundaries
lie to its east and southeast - the eastern boundary predominantly
exists as a moisture boundary extending into central Wisconsin and
the southeastern boundary boundary extends southeast into central
Iowa. This boundary places the primary warm sector along and east of
the Missouri River Valley. Similar trends can be seen in the ongoing
associated precipitation shields - the northern shield progressing
northeast and weakening has been grazing our northern and western
periphery over the last ~18 hours.

Initial precipitation chances this morning will shift southeast
through the morning, remnants of the lifting mesoscale perturbation.
Subsequent, higher and more impactful precipitation will be tied to
the actual warm sector with a more amplified low level perturbation
arriving in the afternoon. These storm chances will lift northeast.
Otherwise, near term impacts overnight have been increased
southeasterly winds as the pressure gradient tightens.

In summary, storm threat increases this afternoon as the main warm
sector lifts northeast through the Upper Mississippi River Valley.
High resolution model guidance differs in instability, dependent on
sufficient reduction in sky cover allowing surface heating to
overcome the warm frontal off deck weak inversion. Comparing high
resolution model soundings, the RAP exhibits the warmest and most
unstable solution: building 1500+ J/kg of surface based CAPE from
3pm CDT onward into the nighttime hours. The ARW and HRRR build a 2
hour window this afternoon with 1000 J/kg of SBCAPE while the Fv3
has nil SBCAPE. Intra model dProg/dt has pushed this CAPE axis
farther northwest.

All appreciable shear is expected to remain in the lower levels (0-
3km = 0-6km = 20-30kts) with very weak mid level unidirectional
shear. While most likely storm mode will be scattered, pulsey strong
storms, the increased low level shear profile and helicity of 200
m^2/s^2 suggests a brief tornado cannot be ruled out this afternoon
if surface destabilization, dependent on a reduction in cloud cover
as discussed above, occurs.

Tuesday and Tuesday night rain:

Guidance continues to depict a longwave trough and an associated
cold front advancing from the northern Rockies east to WI Tuesday
into Tuesday night. South-southwesterly low level moist advection
looks to occur for much of the day ahead of this wave, with the
03.00z GFS suggesting 1.75" of PWAT will be present with NAEFS/ECENS
values above the 90th percentile. Thus, while strong forcing for
ascent will likely remain confined to the fast-moving cold front,
limiting the duration of rainfall, healthy rainfall totals should
still occur, with latest NBM probabilities suggest at least half an
inch is likely (70-80%) with some areas having a slight (15-20%)
chance of topping two inches. Trends over the past few model cycles
suggest the cold front will arrive a bit later than previously
forecast, so attempted to reduce PoPs Tuesday morning afternoon and
increase them towards 100 after 00z.

Additionally, this moist advection, coupled with cooling
aloft ahead of the trough, should allow for around 1000 J/kg of
MLCAPE to build. Thus, while deep layer shear appears weak - progged
0-6km bulk shear values are generally less than 20 knots - a stray
severe storm could occur. That said, progged mid-level lapse rates
are below 7 C/km, surface mixed layer is virtually non-existent
given the robust moist advection, and low to mid level winds are
generally 40 knots or less, all suggesting a severe storm is a
remote possibility (5% or less).

Wednesday through Next Monday:

Northwesterly flow aloft looks to dominate proceedings from
Wednesday through the end of the seven day forecast period. With
many of these days looking to feature cool temperatures aloft
and partly cloudy skies, resulting modest destabilization should
result in some diurnal showers most afternoons. Finally, progged
700/850/925mb winds suggest breezy conditions could occur
Wednesday and Thursday, with NBM probabilities suggesting a 20
and 40 percent chance for a 40 knot gust at Rochester each
afternoon, respectively. Thus, will need to monitor wind
potential during this period as well.

&&

.AVIATION /18Z TAFS THROUGH 18Z TUESDAY/...
Issued at 1145 AM CDT Mon Jun 3 2024

CIGS: low saturation has led to some MVFR cigs creeping across/near
the TAF sites for late morning/early this afternoon. RAP/HRRR
soundings suggest this will mix out/lift moving into the afternoon.
The propensity of short term guidance then suggest mostly BKN/SCT
mid-high level (VFR) clouds into 12z Tue, with increasing chances
for MVFR for Tue morning. Will steer the forecast this way.

WX/vsby: further shra/ts expected this afternoon, but CAMS and main
forcing mechanisms shift the bulk of the threat east/southeast of
the TAF sites. Will keep dry for now - update if radar/meso model
trends suggest different.

WINDS: generally southerly through the period and around 10 kts or
less into Tue morning. A few gusts this afternoon. Uptick in winds
moving into late morning/afternoon of Tue withe more gusts into the
lower 20 kts.

&&

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

&&

$$

DISCUSSION...Ferguson/JAR
AVIATION.....Rieck