907
FXUS63 KARX 171741
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
1241 PM CDT Tue Sep 17 2024

.KEY MESSAGES...

- Above normal temperatures continue through the work week
  with considerable uncertainty in temperatures from Sunday
  onward. Today and Wednesday look to also feature drier
  conditions compared to previous days.

- Next shower and thunderstorm potential arrives Thursday
  and Thursday night. If thunderstorms develop during the late
  afternoon and evening - a big if - a severe storm cannot be
  ruled out.

- Additional periods will have rain potential from Saturday
  onward but there is considerable uncertainty surrounding the
  exact timing of these periods.

&&

.DISCUSSION...
Issued at 359 AM CDT Tue Sep 17 2024

TODAY THROUGH SATURDAY: Warmth Continues, Dry Tuesday and Wednesday

Overall pattern through this weekend continues to be dominated by
the influence of an upper ridge centered over a Texas to western
Great Lakes axis. 17.00z guidance is in good agreement that this
ridge will be maintained either overhead or just to our east through
Wednesday. With an upper low favored to advance from the Great Basin
to the northern High Plains, surface pressure gradient should
increase as a surface cyclone deepens in MT/ND/SD, driving a light
south-southeasterly breeze each of the next two afternoons while
these south- should also advect drier air over the CWA. Given
the warm temperatures aloft and expected mixing, temperatures at
the surface should reach the mid to upper 80s with a few 90s
readings Tuesday and Wednesday while RH values drop below 40
percent in some areas east of the MS River. Thursday through
Saturday, warm temperatures look to continue but with a slow
decreasing trend as heights aloft moderate. Additionally, the
turn toward dry conditions looks to end as low level moist
advection increases Wednesday night into Thursday as the
northern High Plains upper low begins to churn eastward over the
southern Canadian Prairies.

THURSDAY AND THURSDAY NIGHT: Potential for Showers, Thunderstorms

Starting early Thursday, as the aforementioned upper low moves
eastward, multiple shortwaves will advance downstream in the south-
southwesterly flow aloft over the Upper Midwest. Given the influence
of the upper ridge discussed above, primary limiting factor for
precip from early Thursday morning through the nighttime hours will
be capping. While multiple shortwaves do eject over SE MN during the
morning and early afternoon hours, appreciable 850mb moisture looks
to hold off until the latter part of the afternoon, so am doubtful
any parcels are able to overcome the warm temperatures aloft and
have attempted to move PoPs toward the low end of consensus blends
during the daytime hours Thursday. For Thursday evening, have
largely kept NBM likely PoPs as temperatures aloft slowly moderate
and low-level moisture ramps up. Progged 17.00z GFS MLCAPE values
reach 1500 J/kg west of the MS River as sfc-6km shear values
increase to around 40 knots, so should convection succeed in
overcoming the cap, a severe storm or two cannot be ruled out. The
broader universe of guidance seems to point to this low end
potential as well as reflected in 17.00z CSU-GEFS ML output and LREF
joint probabilities. Plenty of details still to be worked out, but
will need to keep an eye on late Thursday afternoon and evening.

SATURDAY THROUGH MONDAY: High Uncertainty

Wide spread is seen across operational and ensemble guidance as
zonal flow aloft Friday and Saturday gives way to large scale
troughing over the central CONUS. Uncertainty largely surrounds the
timing of any shortwaves in the zonal flow aloft and then how an
upper low, initially over southern NV early Saturday, rejoins the
larger scale flow pattern. Difficult to latch on to any solution at
this time, so have retained NBM PoPs which are spread over every day
Saturday through Monday. Forecast users are cautioned that, while
precip may be mentioned in the forecast for 78 hours, the end result
will likely see precip focusing on much shorter periods within this
larger time frame. Hopefully details become clearer soon.

&&

.AVIATION /18Z TAFS THROUGH 18Z WEDNESDAY/...
Issued at 1235 PM CDT Tue Sep 17 2024

Mid and high clouds will be more prevalent across western half
of the forecast area as shortwaves eject out of the longwave
trough. With diurnal mixing, winds will gust into the 15 to 25
knots for this afternoon. This will primarily impact KRST.

&&

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

&&

$$

DISCUSSION...JM
AVIATION...Boyne