674
FXUS63 KARX 121126
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
626 AM CDT Mon Aug 12 2024

.KEY MESSAGES...

- Although a few showers could occur in spots today, chances for
  more widespread showers/storms continue to increase later this
  week, highest (60-80%) Wednesday night and Thursday.

- Temps will be warmer, near seasonable values, for most of this
  week.

&&

.DISCUSSION...
Issued at 246 AM CDT Mon Aug 12 2024

Today-Tuesday:

A few ripples in the flow were evident in GOES water vapor imagery
early today across the Dakotas into Iowa/Minnesota. Regional radar
mosaics showed scattered weak returns across southern Minnesota and
northern Iowa with a few surface reports of light rain.
However, the bulk of showers/storms should pass farther south
across central/southern Iowa into Missouri today where the warm
advection/moisture transport signal is focused. A few showers
or sprinkles could graze southeast Minnesota and northeast Iowa.
Otherwise, much like the past few days, isolated to scattered
showers/storms are possible diurnally across northern into
central Wisconsin this afternoon where weak destabilization is
expected to occur. Although the airmass remains dry, a favorably
deep light wind layer, relatively clear skies, and surface
ridge axis crossing the region may lead to some fog development
tonight, especially in valleys, but patchy fog could not be
ruled out area-wide in the scenario. Gradual warming will
continue into Tuesday as mid-level ridging spreads eastward
from the plains.

Wednesday-Friday:

Surface ridging will slide east into the Great Lakes on Wednesday
with return southerly flow developing and deep layer moisture
beginning to increase. Some showers/storms could develop
(20-40%) through the day with the increase in moisture
transport/warm advection, especially with any weak lead
shortwaves.

However, the higher shower/storm potential is expected for Wednesday
night and Thursday as a broad mid-level trough crosses the region.
NAEFS precipitable water values increase above the 90th percentile
relative to climatology during this time and among all
GEFS/EPS/CMC ensemble members, probabilities for at least 1/2
inch of rain through Thursday are generally 40-60% with 20-40%
chances for 1" or more. Thus, although there is uncertainty how
this system will evolve, especially with convection involved,
confidence in a more widespread precipitation event continues
to grow, with NBM rain chances continuing to increase (60-80%
chance). Global model spread in temps is larger on Thursday,
likely owing uncertainty in storm track and impacts of
clouds/rain. No strong signal for severe weather is evident
given the greater uncertainty in details of the trough/surface
pattern evolution and potential for clouds/showers to limit
destabilization. The trough may linger across the area on Friday
with some continued shower chances, but spread in potential
outcomes for the trough increases in time.

Next Weekend:

Heading into next weekend, guidance generally suggests ridging
expanding across the southern Rockies/plains downstream from a
larger trough across the Pacific Northwest. Our late week trough
will move east across the Great Lakes. Lower end rain chances
(10-30%) linger due to potential proximity to the larger trough,
although this evolution remains unclear. Overall, seasonable
temps are favored with global ensemble interquartile ranges for
high temps in the 70s to mid 80s.

&&

.AVIATION /12Z TAFS THROUGH 12Z TUESDAY/...
Issued at 626 AM CDT Mon Aug 12 2024

Valley fog will be the main TAF concern towards the end of the
forecast period with light surface winds, lessening cloud cover,
and a fairly deep light wind layer in the 11.09z RAP.
Consequently, already have a high degree of confidence that
valley fog will occur at KLSE for Tuesday morning to introduce a
BCFG beginning at 09z with category reductions potentially
required in upcoming forecast issuances. Another possibility
that will need to be considered with surface high pressure
directly overhead is the possibility of more widespread fog
outside of the river valleys which may bring visby reductions
at KRST for Tuesday morning. However, with mixed signals in
model visby guidance, have opted exclude any fog mention at KRST
at this time. Otherwise, VFR conditions are expected through
much of today with decreasing cloud cover and with winds
remaining light and variable.

&&

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

&&

$$

DISCUSSION...JM
AVIATION...Naylor