676
FXUS63 KARX 140915
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
415 AM CDT Wed Aug 14 2024

.KEY MESSAGES...

- Wednesday night into Thursday morning will feature showers and
  thunderstorms bringing potential for heavy rain, with a 40 to
  70 percent chance for an inch of rain across the southwestern
  two thirds of the forecast area.

- Thursday afternoon and evening will likely feature an
  additional round of showers and thunderstorms. While rain
  rates look to take a small step downward, a severe
  thunderstorm could occur in the southern portions of the CWA.

- The remainder of the seven day forecast features seasonable
  temperatures with the potential for an afternoon shower or
  thunderstorm.

&&

.DISCUSSION...
Issued at 413 AM CDT Wed Aug 14 2024

Wednesday night into Thursday morning: Potential heavy rain

This afternoon, upper shortwave, over western Nebraska as of 08z,
will eject eastward over the forecast area. Cooling aloft associated
with this feature may lead to a few showers and perhaps a rumble of
thunder. Progged instability is very low - 14.00z GFS suggests
MUCAPE will be less than 100 J/kg - so think severe thunderstorms
are not in the cards this afternoon and early evening. Additionally,
PWAT values at this time remain under 1.75" so am not too concerned
with heavy rain potential with this shortwave.

That assessment changes as we move into the nighttime hours. While
severe chances continue to appear minimal given available
instability, an assessment reflected in the scant (<5%) potential
for severe storms in post-processed 14.06z HRRR output, the threat
for localized heavy rain increases as robust low level jet causes
PWAT values to increase to around or greater than 2", around the
99th percentile of GEFS/ENS climatology. With deep warm cloud depths
also present, rain rates should be highly efficient as showers and
thunderstorms develop across the area as the low level jet
increases as another shortwave approaches from the west. Given the
convective nature of the rain, while most areas should receive at
least some precip, narrow pockets of higher amounts look to be the
norm. In other words, rain distributions have a long tail on the
high side. Indeed, NBM probabilities suggest a 40 to 70 percent
chance for at least an inch of rainfall and a 10-25 percent chance
for at least three inches. Saving grace will be the progressive
nature of the pattern and progged storm motions near 40 mph,
limiting flood risk to only those unlucky enough to experience the
heaviest thunderstorms of the bunch.

Thursday afternoon and evening: Additional rain, potential severe

Thursday afternoon, next upper trough approaches from the west,
likely kicking off another round of convection. A marked reduction
in low level moisture transport will mean PWAT values back down to
around or below 1.75", limiting the risk for additional heavy rain.
However, the chance for a severe storm may re-enter the picture as
MLCAPE values build to around 1500 J/kg with 0-6km bulk shear of
around 35 knots. Given some turning seen in progged hodographs and
steep mid-level lapse rates, severe hail would be possible if that
amount of instability is indeed realized. Additionally, sfc-500m SRH
of 50-100 m2/s2 suggests a low but greater than zero risk for a
tornado, primarily in SW WI. Will need to monitor Thursday closely,
particularly if any daytime clearing of skies occurs in SW WI.

Friday through Tuesday: Seasonable, low potential for afternoon
showers/storms

Northwesterly flow aloft looks to dominate the remainder of the
seven day forecast, a pattern that usually yields a stray afternoon
shower/storm or two if an upper shortwave arrives coincident with
peak surface heating. With an upper low present nearby to the east
Friday and Saturday, expect temperatures to run a bit under normal
and for the probability for that stray afternoon convection
potential to be realized to approach 30 percent given the cold air
aloft. Aside from potential lightning risk to those outside during
the mid to late afternoon, am not concerned with either heavy rain
or severe thunderstorms Friday through Tuesday.

&&

.AVIATION /06Z TAFS THROUGH 06Z THURSDAY/...
Issued at 1055 PM CDT Tue Aug 13 2024

Predominant VFR conditions expected. Some areas of fog will be
possible overnight in river valleys. Southeast winds will
continue overnight and increase to 10 to 20 knots during the
day Wednesday. Rain chances increase in the mid to late
afternoon Wednesday and remain through the rest of the TAF
period. Along with these showers, there is a 25 to 40% chance
for thunderstorms to occur.

&&

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

&&

$$

DISCUSSION...Ferguson
AVIATION...Cecava