270
FXUS63 KARX 160119
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
819 PM CDT Thu Aug 15 2024

.KEY MESSAGES...

- Showers and storms persist into the evening hours today with
  gusty winds being possible in any stronger storms that
  develop. Additional showers and storms are likely (50-80%
  chance) for Friday.

- Fog potential tonight into tomorrow morning east of the
  Mississippi River.

- Slightly below normal temperatures through next week with
  sporadic shower and storm chances.

&&

.DISCUSSION...
Issued at 309 PM CDT Thu Aug 15 2024

Rest of Today - Tonight: Showers and Storms This Evening, Fog
Potential Overnight East of the Mississippi River

The general synoptic setup this afternoon can be seen on GOES-16
water vapor imagery and 15.16z RAP 500mb heights with a surface low
and corresponding upper-low situated on the ND/MN state line. As
this low slowly works its way southeast, pieces of shortwave energy
can be noted in the cyclonic flow, aiding convective initiation
across the Great Plains and Midwest. Attention this afternoon turns
to how much instability we have managed to build in order to aid
convection as we approach peak heating. As of 1830z, noting some
developing cumulus across portions of southeast MN leading to shower
activity. Still some questions with how these showers will deepen
over the coming hours though. While the 15.17z RAP continues to
increase MLCAPE values to around 1000-2000 J/kg locally into the
evening, currently satellite trends have kept a fairly persistent
low-level cloud deck over much of the local area into the
afternoon. As a result, ongoing convection could struggle to
deepen with limited instability where cloud cover persisted. If
storms are able to tap into more instability where clearing is
present, DCAPE values of 600-900 J/kg in the 15.17z RAP would
support an isolated threat for some stronger wind gusts as
storms move east.

With the aforementioned cloud cover lingering over portions of the
area through peak heating this afternoon, particularly along and
east of the Mississippi River, diurnal mixing processes likely will
be minimized in these locations allowing low-level moisture to
remain in place going into the overnight. Consequently, noting
deterministic soundings developing a near surface inversion with
this low-level moisture, particularly across southwestern
Wisconsin. The main question will be if any cloud cover lingers
into the overnight hours which could hinder any fog development.
A key point of interest in the 15.12z HREF visibility
probabilities overnight is there is minimal deviation between
the 4 mile probabilities (40-70%) and the 1/4 mile probabilities
(20-50%) in southwestern Wisconsin. As a result, areas that
manage to develop fog would favor low visibilities that could
lead to fog becoming locally dense before fog dissipates Friday
morning.

Friday: Seasonable With Additional Showers/Storms Likely

Looking towards Friday, the previously stated low pressure system
pushes southeast into north-central Wisconsin on Friday afternoon.
Consequently, this will further instigate shower and storm
development throughout much of the day on Friday. The overall
setup generally shows a piece of vorticity associated with a
subtle shortwave that progresses through the area during the
late morning and early afternoon. As instability increases by
afternoon, convection will develop along an associated surface
boundary that will progressively push eastbound. With minimal
0-6km bulk shear in the 15.15z RAP to maintain any organized
convection, storm mode will be very pulse-like in nature. The
one caveat to the afternoon showers and storms tomorrow will be
the possibility for any non-supercell funnels that can develop.
With the vertically stacked low and a surface boundary pushing
through the area, the non-supercell tornado parameter in the
15.15z RAP is highlighting portions of central and north-central
Wisconsin with values greater than 1 during the afternoon.
After this round of convection moves through during the early
afternoon hours, limited instability in the 15.15z RAP would
suggest limited able for deep convection to occur. However,
given some of the CAMs holding onto showers later into the
evening with marginal instability and weak pieces of energy
pivoting around the low, held onto some precipitation chances
through the evening (20-40% chance).

Saturday - Wednesday: Continued Seasonable Temperatures With Spotty
Shower and Storm Potential

As we head into the weekend and next week, the aforementioned
vertically stacked low will slowly continue to migrate eastward. As
the associated trough axis becomes more neutrally tilted, a
deepening ridge to our west will help posture our region in
northwesterly flow pattern. Consequently, this will aid in
keeping our temperatures near to slightly below average into
next week. This is reflected in the 15.12z grand ensemble
(GEFS/EC ensemble/Canadian ensemble) with median high
temperatures trending in the middle 70s to lower 80s with inter-
quartile spread of generally around 5 degrees through the first
half of next week. Shower and storm chances will be possible
with any shortwaves embedded in the flow but guidance currently
having low confidence in how these may resolve at this time.

&&

.AVIATION /00Z TAFS THROUGH 00Z SATURDAY/...
Issued at 640 PM CDT Thu Aug 15 2024

Scattered showers and isolated thunderstorms will push out of
the area around mid evening with a surface front in the area and
a decrease in heating. Additional shower development is
forecast later Friday morning and Friday afternoon. Forecast
soundings show elevated winds off the surface tonight, however
will need to monitor for patchy fog development with recent rain
and should cloud coverage decrease more than expected. Deeper
low level moisture rotates around the surface low for Friday
with deteriorating conditions and MVFR ceilings.

&&

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

&&

$$

DISCUSSION...Naylor
AVIATION...Zapotocny