354
FXUS63 KARX 112018
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
318 PM CDT Thu Jul 11 2024

.KEY MESSAGES...

- Some fog potential overnight, otherwise a quieter day expected
  for Friday.

- Temperatures increasing into the weekend with highs in the
  upper 80s to low 90s and heat indices nearing 100 by Sunday.

- Monitoring how the risk for severe weather unfolds for
  Saturday evening through Tuesday. The forecast remains complex
  and of lower confidence.

&&

.DISCUSSION...
Issued at 318 PM CDT Thu Jul 11 2024

Rest of Today - Tomorrow:

Currently, GOES-16 water vapor imagery and 11.14z RAP 500mb heights
shows a synoptic trough slowly pushing eastbound with a shortwave
ridge positioned to our northwest. As we head through the afternoon,
increasing instability (1000-1500 J/kg of MLCAPE) and some weak
cyclonic flow may help develop showers, maybe an isolated storm.
However with very weak shear profiles in the 11.14z RAP, would not
expect any convective development to maintain in any organized
fashion. However, with the 11.12z HREF showing ensemble max values
for 3-hr QPF of up to around 1", could not completely rule out
extremely localized areas receiving some higher totals.

As we head into tonight, subsidence builds in with the
aforementioned ridge allowing for more clearing in sky cover and
calming winds. As a result, expecting some fog formation overnight,
primarily in river valley locations and in central Wisconsin. Still
some questions for coverage and density for fog with the 11.12z NAM
trying to hold onto some saturation at 850mb in addition to some
increased low-level winds at 925mb to around 10 kts. Regardless,
with the aforementioned subsidence in place, should be a drier
forecast for Friday outside of a few possible sprinkles across north-
central WI with temperatures reaching into the middle 80s for most.

Saturday - Sunday:

A challenging forecast remains for the weekend as we monitor the
potential for storms. The ridge that has persisted over the West is
forecast to build towards the region on Saturday with a series of
upper level shortwaves riding around it. With increasing moisture
and warm air advection beneath an EML, strong instability looks to
build across the region. Some indication in model guidance for
elevated convection to develop early as one of the aforementioned
waves moves through. However, as we continue to move into the day
there remains some concern revolving around capping. Latest ensemble
guidance, as an example/proxy, keeps us right on the eastern edge of
the 700 mb temperature probabilities for 12C+ with model forecast
soundings showing a bit less of a capped environment in the east.
With a weak moisture transport convergence signal into southern
Wisconsin along with the warm front, guidance suggests this could be
enough to get showers and thunderstorms to develop. However, there
seems to still be plenty of variability on how that evolves. Will
need to continue to monitor trends over the next day or so,
especially as we remain in a slight risk for severe weather from
SPC.

The story remains the same as with previous forecasts for Sunday,
which will be dependent on Saturday`s convection as well as the
strength of the cap building across the area. Model guidance again
diverges quite a bit on how exactly this unfolds and where storms
track. Overall though, current thinking would be that storms will
track along the gradient of the instability pool and weaker capping
(potentially favoring more north/northeast). With the strong
instability and better shear to work with will be closely monitoring
over the next couple days for the potential for severe weather.

Of higher confidence is for increasing temperatures into the
weekend. Sunday still looks to be the warmest day coming up, with
current forecast high temperatures in the upper 80s to low 90s. This
is supported by quite low spread (~2-3 degrees) between the 25th and
75th percentiles of the blended model guidance. There does not look
to be much relief for the overnight as temperatures only drop into
the 60s to low 70s. With the addition of increased dewpoint
temperatures, heat indices may rise into the 90s to near 100 for
some especially Sunday. Now, will note that one wrench in this would
be any impacts from potential convective systems Saturday night into
Sunday morning. Something to monitor as these details become more
clear, but for now will continue to message the heat.

Into Early Next Week:

Some signal for an upper level trough to move across the region with
additional showers and storms possible. However, it is uncertain how
any convection on Sunday will impact storms on Monday. Otherwise,
the heat does not look to last long as temperatures drop back to the
70s by mid-week.

&&

.AVIATION /18Z TAFS THROUGH 18Z FRIDAY/...
Issued at 1249 PM CDT Thu Jul 11 2024

VFR conditions are mostly expected through the TAF period with
potential valley fog being the main exception at KLSE. Currently,
with the lack of cloud cover and calmer winds being expected valley
fog is a consideration. However, with the 11.15z RAP showing
stronger wind (around 10 kts) just off the deck, which when
considering the longer day length, may aid keeping fog at bay. Due
to the uncertainty decided to hold with the BCFG mention at KLSE for
now. Otherwise, isolated showers and storms (10-20% chance) will be
possible across portions of western WI later this afternoon.
However, opted to keep out VCSH mention due to lower confidence
in how any convection will manifest. Winds will remain light
through the TAF period at around 5-10 kts or lower during the
overnight hours.

&&

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

&&

$$

DISCUSSION...Naylor/EMS
AVIATION...Naylor