624
FXUS63 KARX 202329
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
629 PM CDT Sat Jul 20 2024

.KEY MESSAGES...

- Fog may occur late tonight into early Sunday morning,
  particularly in southeastern Minnesota and northeastern Iowa.

- Seasonable temperatures and largely diurnal showers and storms
  are favored through Wednesday afternoon.

&&

.DISCUSSION...
Issued at 248 PM CDT Sat Jul 20 2024

Tonight: Potential Fog

19z WV satellite shows an upper low over north central IA with
multiple perturbations rotating around this feature which, along
with the pocket of cooler temperatures aloft, have led to scattered
showers in the western portions of the CWA. Elsewhere, convection has
struggled to develop due to capping and a lack of notable forcing
for ascent. This afternoon, expect this general pattern to continue
with spotty showers and perhaps a rumble of thunder possible in the
western portions of the forecast area and in Clark/Taylor Counties
where a weak, slow moving surface boundary may help a few parcels
become freely buoyant.

Moving ahead to tonight, deterministic guidance is insistent that
fog will develop, particularly in SE MN/NE IA. Given that
temperatures there may drop into the lower 60s due to light winds
and dewpoints this afternoon are in the low to mid 60s in many
locations, fog does appear plausible. That said, the potential for
high clouds introduces uncertainty on this point. Additionally,
while deterministic GLAMP suggests many area could reach 1/4 mile of
visibility, related NBM probabilities for less than 1 mile of
visibility are only 20-40 percent. Therefore, am not thinking dense
fog will occur and have generally kept fog mentions to the patchy
category.

Sunday through Wednesday: Seasonable Temperatures, Diurnal Convection

As northwesterly flow aloft and pooled 925/850mb moisture remain in
place, each afternoon will likely feature a few showers and
thunderstorms. Best chances for more coverage appear to reside
Sunday and Tuesday afternoon and evening as guidance is in decent
agreement that an upper wave will help cool temperatures aloft a bit
as peak surface heating is reached. Additionally, Tuesday may
feature a non-zero but still quite low chance for a strong storm as
deep shear trends upward from near zero to 20 knots.

As drier air slowly filters in following a frontal passage
Wednesday, the chance for diurnal showers looks to decrease to
around 10 percent or less for Thursday and, as heights aloft build,
closer to zero for Friday afternoon.

&&

.AVIATION /00Z TAFS THROUGH 00Z SUNDAY/...
Issued at 628 PM CDT Sat Jul 20 2024

This evening, surface dewpoints were 5-7F higher over the KRST
airfield versus KLSE with skies partly cloudy over both and
some further clearing expected after sunset. Overnight, winds
will be light and variable with a rather pronounced deep wind
layer and mid-level shortwave ridge building in from the north.
Dewpoint depressions were only 13F at KRST at 23Z, quite easy
to overcome with cooling to arrive at fog by morning. A bit
different story further east at KLSE, where dewpoints were a
bit lower...and temperatures warmed about 5F more in more
sunshine. Dewpoint depressions were 23F at KLSE...a bit more
work to overcome to saturate and generate fog.

Will continue to monitor but there is a fair amount of model
guidance suggesting fog will be around in the morning and based
on current observations, confidence is highest at KRST for an
IFR fog /1SM/ in the pre- and post sunrise hours and thus have
included it in the TAF. While conditions are a bit less
favorable at KLSE due to lower surface dewpoints, it could be a
river valley fog night. Have decided to include 2SM fog in the
KLSE TAF...and will continue to monitor cooling this evening to
build confidence in a lower visibility fog.

What could go wrong? With the front and mid-level shortwave
trough action coming in from the north right behind the
shortwave ridge axis /showers along it right now across northern
WI/, mid-level clouds could spread into the KLSE area near
sunrise to halt the fog process. Also, just not moist enough to
cause lower visibility fog at KLSE.

&&

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

&&

$$

DISCUSSION...Ferguson
AVIATION...Baumgardt