369
FXUS63 KARX 212023
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
323 PM CDT Sat Sep 21 2024

.KEY MESSAGES...

- Showers and storms into tonight, few could be strong to
  severe. Rain chances linger for parts of northeast IA/southern
  or west central WI into Sunday

- Below normal temps into early next week.

&&

.DISCUSSION...
Issued at 315 PM CDT Sat Sep 21 2024

Overview:

Water vapor satellite imagery, heights, and lightning show a
trough over southern Canada with another area of closed 500mb
low pressure approaching the Four Corners region with a ridge
over the Southern Plains. Lightning was occurring with scattered
convection over parts of central and eastern WI and along a cold
front across MN as well as ahead of the storm system in the
southwest U.S. With the warm front lifting north, we did have a
few strong to severe storms move through parts of northeast Iowa
and western WI. At 20Z...an outflow boundary was pushing
through parts of southwest WI with the surface warm front
between PDC and DBQ. The surface cold front had pushed to south
central MN, however there were showers continuing west of this
area.

Rest of today through Sunday night:

The Canadian trough will continue to swing through the
Mississippi Valley through Sunday morning. The will drag a cold
front through the forecast area. Meanwhile, the closed low over
the Four Corners region will open up and shift eastward through
the Central Plains. Moisture transport will continue through the
evening hours across the forecast area with MUCAPE of 2000J/kg
per the SPC Mesoscale page, however the SBCAPE is around
2500J/kg across northern Iowa. Scattered showers and
thunderstorms will continue ahead of the cold front with strong
to severe storms possible due to the CAPE and effective shear
around 45kts. The convection and outflow boundary could temper
the severe potential farther north, however the instability and
downdraft CAPE of 1200J/kg or higher could lead to some strong
to severe storms. The winds will veer with time, however the
instability lingers into the evening. The CAMs were dry for the
early day/early afternoon convection, thus not doing well with
the pre-frontal storms. Hopefully, the RAP/HRRR has a handle on
increasing convection through the afternoon with the
showers/storms gradually pushing east through 07Z. The CAMs do
hint at re-developing across the southern part of the forecast
area for after 07Z into Sunday morning. Showers should gradually
exit the area during the afternoon with cooler air for the area.
Highs Sunday should top out in the mid 60s and lower 70s, more
seasonable for this time of year.

Monday through the week:

Behind the weekend cold frontal system, cooler conditions look to
continue for the start the week with highs in the upper 60s to low
70s and lows in the 40s to low 50s. Model guidance continues to
suggest a shortwave trough in the northern flow while shortwave
energy slides just south of the local area for the early part of the
work week. There remain some differences between the individual
GEFS/ECMWF ens solutions on precipitation potential. However, have
noted a very slight trend towards a drier solution in the most
recent blended model guidance, which currently keeps 15-20% chances
in far southwest WI for late Monday night into Tuesday.

Behind the upper level trough, guidance looks to favor upper level
ridging building eastward. Still some variability, but GEFS/EPS ens
solutions do not show a strong/definitive QPF signal through much of
the rest of the week. Otherwise, temperatures are forecast to
gradually increase back into the mid/upper 70s by the end of the
week.

&&

.AVIATION /18Z TAFS THROUGH 18Z SUNDAY/...
Issued at 1255 PM CDT Sat Sep 21 2024

A warm front is lifting into the area today followed by a cold front
tonight.  These weather systems will bring scattered showers and
thunderstorms to the area.  The hi-res CAMs are going to have to
catch of with the showers developing with the warm front at the
models were dry.   Will continue to refine the timing of
showers/storms until the drier and cooler air arrives.  It appears
the showers/storms exit RST by around 06 or 07Z and 09Z at KLSE,
however re-development will be possible until the mid-level trough
pushes through Sunday morning. South to southeast surface winds 5 to
15kts with some stronger gusts at KRST shift to the southwest and
northwest with the frontal passage this afternoon and early this
evening.

&&

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

&&

$$

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