122
FXUS63 KARX 241706
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
1206 PM CDT Tue Sep 24 2024

.KEY MESSAGES...

- After today, mild temps settle in for the rest of the week
  with highs mostly mid to upper 70s (5 to 10 degrees above
  normal)

- Next shot for measurable rain holds off until Friday, through
  the weekend. Uncertainties with movement/development of a
  tropical system (and its interaction with a low pressure
  system south of the region) lowers confidence on areal
  coverage/potential amounts for the local area. Should get some
  clarity over the next few days.

&&

.DISCUSSION...
Issued at 215 AM CDT Tue Sep 24 2024

> Overview: a shortwave trough is dropping southeast across the
northern plains/western MN early this morning, per latest water
vapor imagery. Areas of light showers were resulting. Meanwhile,
another weaker ripple in the flow was moving northeast out of MO and
across IL. Both features have been well handled by the models over
the past few days, and both are expected to continue on their same
track today, more-or-less skirting to the west and east of the local
forecast area.

The northern shortwave will continue to dive south/southeast with
medium/long range guidance all developing a cutoff low over the
lower mississippi river valley by 06z Thu. An upper level ridge over
the plains "bends" northeast across the region, becoming the primary
influence on the region through Thu.

Moving into the weekend the GEFS and EPS both take the developing
tropical system tracking into the gulf northward, pinwheeling
counter-clockwise around the cutoff low, eventually merging -
lifting northeast - weakening as it slides over the eastern great
lakes for the start of next week. A whole lot of uncertainty here
with strength of the tropical storm/likely hurricane, its ultimate
track, and just how it interacts with the closed low. Most members
of the GEFS and EPS ensemble suites suggest at least southern
portions of the forecast area will be brushed by the northern extent
of the merged low pressure system. Current favored timing is from
Friday night through the weekend. 24 hour probabilities for over
1/4" are fairly low in the GEFS and EPS at this time, mostly 10 to
30% through the weekend (of note the Canadian ensemble is more
enthusiastic with chances and amounts, skewing the grand ensemble of
models with higher chcs/qpf). The tropical system muddies the water
quite a bit and lowers confidence in how the weekend will ultimately
play out. It will probably be a few more days before this evolution
gets some clarity.


> Rain Chances: while the showers to the east/southeast look to
continue to shift northeast, away from the local area, the light
showers to the west could reach portions of the southeast
MN/northeast IA. Trends in the radar imagery show some eastward push
- although the parent shortwave has more of a southeast tack. Rain
is coming out of mid level clouds (10kft) and Bufkit soundings point
to ample dry air sub cloud. Sfc obs show that pcpn is reaching the
ground, but light with trace to maybe a couple hundreths for
amounts. CAMS models aren`t enthused that even small chances will
reach the forecast area. Will continue to keep an eye on trends, but
for now will lean into a "dry" forecast, adjust if it looks more
likely that a few sprinkles will be realized. Any rainfall would be
confined to the early morning hours.

Moving into the weekend, as mentioned in the "overview", a lot of
uncertainty with how the cutoff upper level low/remains of the
tropical system interact and move. Significant impacts on the rain
chances and potential amounts for the local area. The bulk of the
members of both the EPS and GEFS paint at least light QPF for the
southern half of the forecast area and the NBM covers this with 20-
40% for rain chances. Given the uncertainties, this look reasonable.


> Temperatures: a very steady signal in the long range guidance with
a return of milder air as the region comes under the influence of
the upper level ridge. The upper 75% of the EPS and GEFS members
continue to favor above normal temperatures for Wed through the
weekend - but generally just by 5 to 10 degrees. Confidence remains
high with mid to upper 70s for highs (a few locations could push
80).

&&

.AVIATION /18Z TAFS THROUGH 18Z WEDNESDAY/...
Issued at 1206 PM CDT Tue Sep 24 2024

Light winds and mainly VFR conditions are likely through the TAF
period. River valley fog once again could impact KLSE for a few
hours early Wednesday morning. Surface winds will be light, but
winds aloft may increase somewhat approaching sunrise. Did trend
the TAF towards at least some impact at KLSE, but fine-tuning
likely will be needed through the evening as trends become more
clear.

&&

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

&&

$$

DISCUSSION...Rieck
AVIATION...JM