139
FXUS63 KARX 151750
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
1250 PM 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 /18Z TAFS THROUGH 18Z FRIDAY/...
Issued at 1245 PM CDT Thu Aug 15 2024

IFR to low MVFR conditions persist east of the Mississippi River
for the rest of the day with MVFR to VFR ceilings west of the
river by mid-afternoon. Scattered showers and storms move
through the region this afternoon and into this evening, but
impacts will be isolated to scattered in nature (20-30%
probability of rain). Forecast confidence is low for tonight
with two scenarios possible. If clouds clear, LIFR fog could
quickly develop overnight along and east of the Mississippi
River. If clouds persist, fog will have a harder time forming.
MVFR ceilings are expected to develop again Friday by midday.
Winds will generally be from the SW to W at 5-10kts, increasing
to 10-15G20kts for Friday midday.

&&

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

&&

$$

DISCUSSION...Ferguson
AVIATION...Skow