NWS-La Crosse Forecast Discussion

						FXUS63 KARX 270855
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
355 AM CDT Sat Jul 27 2024

.KEY MESSAGES...

- Summer heat/humidity return over the next 7 days, with heat indices
  in the low to mid 90s favored with a triple digit reading or
  two possible Wednesday.

- Shower/storm chances return Sunday and Monday and again
  Wednesday and Thursday. Locally heavy rain appears possible.
  Additionally, a stray severe storm cannot be ruled out.

&&

.DISCUSSION...
Issued at 354 AM CDT Sat Jul 27 2024

Next Seven Days: Heat and Humidity

Over the next week, operational guidance is in very rough agreement
that ridging aloft will build first over TX with a shift toward
stronger ridging over the intermountain west next weekend. For our
forecast area, this should result in heights aloft building through
Wednesday before a subtle decrease moving ahead to next weekend.
Heat each afternoon will depend a lot on the details and timing of
various shortwaves and associated precip (see below) but GFS/EC
suggest heights aloft will be maximized Wednesday afternoon. In any
case, most afternoons will feature highs in the mid 80s and, with
occasional periods of southerly surface winds expected, 60s and 70s
dewpoints look to be the norm. Thus, heat indices in the 90s are
favored each afternoon with Wednesday having a small (25%) chance
for a triple digit reading in NE IA per latest LREF guidance.

Sunday/Monday and Wednesday/Thursday: Potential for locally heavy
rain and a stray severe storm

Sunday, an upper low, currently over the ArkLaTex region, looks to
churn northeastward and rejoin the west-southwesterly flow aloft
just to our southeast over NW IL. With best forcing for ascent
favored to skirt just to our southeast - and progged soundings
showing a good amount of capping remaining - am increasingly
doubtful precip will occur in the forecast area.

The same cannot be said for Monday as guidance, while differing in
exact timing, is in pretty good agreement that a robust shortwave
will eject eastward over the forecast area. This should help erode
remaining capping, leading to a round of showers and storms. Progged
PWATs increase above 1.75", near the 90th percentile of GEFS/ENS
model climatology. While widespread risk looks to be limited by the
overall progressive nature of the system, could still get a few
pockets of locally heavy rainfall given this amount of moisture and
storm motion vectors of around 20 knots. Finally, should this
shortwave arrive coincident with peak afternoon heating, would have
around 1500 J/kg of MLCAPE to work with as deep layer shear
approaches 40 knots, so would have a risk for a severe thunderstorm
were this to occur, a small possiblity also suggested by LREF joint
probabilities (10-15%) and GEFS-based machine learning outlooks
(~10%).

While guidance continues to show a variety of solutions moving into
the middle of next week, next strong wave looks to approach sometime
during the Wednesday/Thursday period. With modeled PWATs again
increasing above 1.75" with storm motions of around 20 knots,
localized heavy rain would be possible during this period as well.
LREF joint probabilities suggest around a 10-20% chance for
sufficient shear and CAPE for a low end severe storm, so will need
to keep an eye on this possibility as well.

&&

.AVIATION /06Z TAFS THROUGH 06Z SUNDAY/...
Issued at 1040 PM CDT Fri Jul 26 2024

Predominantly VFR conditions are forecast for the TAF period
with a few periods of high MVFR ceilings before 12Z this morning--
mainly west of the Mississippi River. Winds of 5-10 kts from the
SSE tonight increase to around 10-15 kts from the south during
the afternoon before decreasing and backing to the southeast
overnight.

&&

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

&&

$$

DISCUSSION...Ferguson
AVIATION...Skow