784
FXUS63 KARX 150859
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
359 AM CDT Thu Aug 15 2024

.KEY MESSAGES...

- Widespread showers with a few thunderstorms will sweep east
  this morning. Some areas may receive more than 1" of rain
  (15-60% chance) with a small chance (10%) for an isolated
  pocket with 2"+, mainly south of Highway 18.

- A second round of thunderstorms may develop this afternoon and
  evening. If thunderstorms occur, a few may be strong to severe
  storms, particularly if a bit of clearing occurs after the
  morning rain departs.

- Friday through next Wednesday looks to feature temperatures
  just a bit below normal with potential for afternoon showers
  and storms.

&&

.DISCUSSION...
Issued at 355 AM CDT Thu Aug 15 2024

This morning: Widespread rain

08z composite radar shows a large area of showers and thunderstorms
ongoing across MN/IA with this advancing eastward toward our CWA.
Strongest activity and heaviest rain rates remain to our south and
east in IA, in line with the trend seen in the past few runs of
guidance to take the main body of the upper wave responsible for
this precip farther south. Similarly, 2"+ PWATs should remain just
to our south through the morning hours. Thus, while widespread
rainfall is still expected, final rain amounts will likely be less
than previously thought. Indeed, NBM/HREF probabilities now suggest
a 15-60% chance to exceed 1" and a 10-15% chance to exceed 2", with
higher probabilities residing in southern portions of the forecast
area, mainly south of US Highway 18. That said, deep warm cloud
depths suggest efficient rain production should still occur, so will
need to remain vigilant, particularly if an embedded thunderstorm
takes aim at a more urbanized area.

This afternoon and evening: Additional rain, potential severe

Next shortwave looks to eject downstream this afternoon, opening the
potential for a round of thunderstorms. While progged MLCAPE values
will likely climb into the 1500-2000 J/kg range in the presence of 0-
6km bulk shear increasing to around 30 knots, two major limiting
factors for the development of strong to severe thunderstorms look
to be present as well - convective inhibition and suboptimal shear
profiles in the mid-levels. In the case of convective inhibition, if
more clearing in cloud cover occurs late this morning and early this
afternoon, the aforementioned MLCAPE values would be realized. As
for shear, progged hodographs suggest deeper rotating updrafts may
struggle to sustain themselves due to relatively weak winds above
3km. Thus, while sfc-500m SRH values of 50 to 100 m2/s2 - sufficient
for a tornado risk - look to develop in SW WI, would need to have
everything come together for that risk to be realized. Primary
concern with any thunderstorms this afternoon instead looks to be
damaging winds given modeled DCAPE of around 1000 J/kg.

Friday through Wednesday: Seasonable, potential for afternoon
showers/storms

Friday, as the upper low becomes centered over north central WI,
shortwave rotating around this feature should, in concert with peak
afternoon surface heating, lead to numerous showers and a few
thunderstorms in a textbook high PoP, modest QPF setup. With the
upper low potentially directly over Taylor/Clark Counties, will need
to watch out for any remnant surface boundaries, as a shower
becoming rooted on one of these boundaries could provide enough
stretching potential for a funnel cloud or non-supercell tornado to
develop.

Saturday through next Wednesday, with an upper ridge firmly in place
over NM, northwesterly flow aloft will likely be predominant. Thus,
expect temperatures to run just a bit below normal for mid to late
August with potential for a stray afternoon shower or thunderstorm,
particularly if a fortuitously timed upper shortwave helps cool the
mid levels.

&&

.AVIATION /06Z TAFS THROUGH 06Z FRIDAY/...
Issued at 1105 PM CDT Wed Aug 14 2024

Scattered showers will continue into the early morning hours.
Shower coverage is expected to increase as a band of
precipitation moves through the area roughly between 07Z and 15Z.
As coverage increases, there is an increased chance for
thunderstorms to occur, between 20 and 40%. After this band
exits the area, scattered showers will remain in the area into
the afternoon before scattered thunderstorms become possible
through the afternoon. CIGS will lower into MVFR/IFR flight
categories during the overnight before going back to VFR for
Thursday afternoon. Predominant southeast winds through Thursday
morning before shifting more westerly for the afternoon and
evening.

&&

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

&&

$$

DISCUSSION...Ferguson
AVIATION...Cecava