444
FXUS63 KARX 221925
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
225 PM CDT Sun Sep 22 2024

.KEY MESSAGES...

- Chilly start to the week, but trending warmer for the latter
  half of the work week.

- Low rain chances (10-20%) for far southwest WI Monday
  night/Tue. Otherwise, a drier forecast for much of the week.

&&

.DISCUSSION...
Issued at 225 PM CDT Sun Sep 22 2024

Overview:

Early afternoon water vapor satellite imagery, heights, and
lightning showed a trough over Quebec with an open wave of 500mb low
pressure from Nebraska into Colorado with a ridge over the Gulf
States.  The surface front had pushed through the local area but was
near Rockford IL (70T/70Td).  North of this area, winds were from
the northwest at 10 to 20 mph with temperatures in the 60s and
dewpoints in the 40s and 50s.  Visible satellite imagery show some
high clouds continuing to affect parts of the area with the open
wave lifting northeast.  Some patchy cumulus were developing over
parts of MN/northern WI.

Tonight through Monday:

Through Monday, the 500mb open wave will track toward the Mid-
Mississippi River Valley as another mid-tropospheric trough will drop
southward from Canada into the Northern Plains.  Forecast concerns
center on how chilly temperatures will be tonight, if there will be
fog, and how far north precipitation will make it. Have some
scattered to broken high clouds affecting the area through Monday.
Farther north with the clearing skies and diminishing  winds,
temperature should drop into the upper 30s to lower 40s.  With the
clouds toward DBQ, temperatures may be held up in the mid 40s. The
HREF probabilities for temperatures 36 or colder are 0-10% for
typically cold areas, thus in those local spots, cannot rule out mid
30s. The record low Monday morning is 32 (1974) at La Crosse and 24
(1974) at Rochester, so it definitely can get cold this time of
year! The low level flow becomes more easterly and light at the
surface. Some hints at fog due to the low dewpoint depressions.
925mb winds remain elevated, so this should hinder fog development,
but could see some in the valleys  HREF probabilities for light
precipitation increase toward DBQ Monday, but we still have a dry
forecast. For now have PoPs at 10% or less toward DBQ.

Monday night through the week:

As we move into Monday night/Tuesday, model guidance continues to
show the aforementioned wave lifting towards the area while another
shortwave trough is forecast to drop down through the Northern
Plains. There remains some variability between model solutions on
how far northward rain may reach, but probs from GEFS/ECMWF suggest
some low end chances cannot be completely ruled out for far parts of
southwest WI. However, the ECMWF ens do still seem to be the more
excited of the two. Although it is still a bit out for the hi-res
guidance, have noted some models developing light echoes across the
Dakotas and bringing them down through MN. There is lower confidence
in how much further echoes may progress towards the area, and with
it being towards the end of the hi-res guidance, have not made any
significant adjustments to the forecast. As more guidance comes in,
this may be something to evaluate further. Otherwise, the cooler air
remains through the start of the work week, with highs forecast in
the upper 60s to lower 70s and lows in the 40s to around 50.

The overall upper level pattern for mid to late week continues to be
a challenge. Latest GEFS and ECMWF ens solutions have been
suggesting the potential for the aforementioned system to develop
into a closed low over the south. However, there remains lower
confidence in finer details surrounding this overall flow pattern as
solutions vary in how this system further evolves, especially with
the potential interaction of a tropical system. With all this said,
there is not much signal for precipitation among GEFS/ECMWF ens
solutions until the end of the week into the weekend. With the
aforementioned uncertainties and variability in guidance,
details will need to be refined over the coming days. For now
will keep with the resulting low chances (~20-30%) from the
blended model guidance. A gradual warming trend is forecast for
the later half of the work week, with highs into the mid to
upper 70s.

&&

.AVIATION /18Z TAFS THROUGH 18Z MONDAY/...
Issued at 1241 PM CDT Sun Sep 22 2024

VFR conditions with variable high cloudiness. A storm system is
bringing in high cloudiness aloft, but surface high pressure
and north winds are ushering in much drier air. Dewpoint
spreads remain large for Monday morning despite surface winds
decoupling, thus fog is not expected outside of valley areas in
southwest Wisconsin. Brisk north winds become light and variable
overnight. Easterly 900mb winds remain somewhat elevated 10 to
20kts through 12Z Monday; limiting fog potential.

&&

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

&&

$$

DISCUSSION...EMS/Zapotocny
AVIATION...Zapotocny