107
FXUS63 KARX 111934
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
234 PM CDT Sun Aug 11 2024

.KEY MESSAGES...

- Low potential for valley fog tonight driven by the coverage
  and thickness of high clouds.

- Seasonal temperatures on the docket for this week with highs
  generally in the mid-70s to low 80s.

- Slight shower risk for a few locales on Monday, otherwise dry
  through midweek. Increasing risk for several days of showers
  for the latter part of the week, but confidence in specifics
  is low.

&&

.DISCUSSION...
Issued at 230 PM CDT Sun Aug 11 2024

Tonight: Fog Potential, Light Showers in Northeast Iowa

The main forecast concern for tonight revolves around whether
valley fog will develop after midnight. A 10 kft layer of winds
< 10 kts in the RAP/HRRR profiles and lower visibility output
from MOS-driven guidance certainly catches ones attention for
fog development, but concerns linger over the thickness of
overnight cirrostratus blowoff from convection over South Dakota
and southwest Minnesota. While confidence is very high we will
have some form of cirrus cloud deck move through, the main
uncertainty is the thickness of the cloud shield.

If the cloud shield is too thin, it may have minimal impacts on
radiational cooling and allow for fog to form. It will be a
delicate balancing game and we may see fog in some locales and
not others depending on the degree of insulation from the
clouds. For now, did add some mention of fog in the Mississippi
River and tributaries north of La Crosse, but held off for
points south where the cloud shield may be thicker. Cloud trends
will certainly need to be monitored this evening.

A compact shortwave noted upstream in water vapor imagery in
southeastern Montana slides to the ESE over the next 24 hours
and gradually weakens during this time. The main forcing for
ascent and theta-e advection will be displaced south of the
region, but there are a few ensemble members that bring some
light precipitation to northeast Iowa late tonight into early
Monday morning. Did maintain some slight chance PoPs in the
forecast in these areas, but any impacts look to be quite low.

Monday - Wednesday Morning: Mostly Dry, Seasonal Temperatures

The filling shortwave axis migrates east of the region by mid
to late morning on Monday and takes the main precipitation
shield with it. Upper level ridging amplifies on Monday and
Tuesday over the High Plains, keeping the region in
northwesterly flow. Scattered diurnal showers are forecast to
develop north of I-94 in the afternoon on Monday and possibly
again Tuesday afternoon, though coverage and amounts look to be
low. The overall airmass slowly warms in the wake of our
anomalous cold snap at the end of last week, with highs
returning back into the upper 70s to low 80s for Tuesday and
Wednesday.

Wednesday Afternoon - Saturday: Wetter Pattern Arrives

The next appreciable chance for showers and storms arrives for
late in the week as a deeper shortwave arrives in the wake of
our early week ridge. The medium to longer range guidance differ
on how the low will evolve, with some solutions cutting off and
slowing the system while others are more progressive with the
upper trough. There isn`t a really clear cut trend in the
ensemble guidance at this range in the forecast as to which
scenario has the better probability, so the going forecast has a
smattering of 20-60 percent PoPs for Wednesday through Sunday.
The highest confidence in impactful precipitation looks to be
from Wednesday night into Thursday, with lesser confidence in
there being rain during the day on Wednesday and for Friday into
the weekend.

&&

.AVIATION /18Z TAFS THROUGH 18Z MONDAY/...
Issued at 1225 PM CDT Sun Aug 11 2024

Predominantly VFR conditions are expected for the TAF period
with a low (20%) risk for light showers south of a AEL to DBQ
line early in the morning Monday. Confidence in valley fog
formation tonight remains on the low side given that high clouds
will be passing through during this time. Most other variables
do point towards fog, so cloud coverage and thickness will need
to be monitored this evening. Winds this afternoon will be 5-10
kts from the W/NW, becoming light and variable this evening
into Monday morning.

&&

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

&&

$$

DISCUSSION...Skow
AVIATION...Skow