902
FXUS63 KARX 181029
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
529 AM CDT Wed Sep 18 2024

.KEY MESSAGES...

- Unseasonably warm weather will continue through tomorrow
  with highs into the 80s.

- Showers and thunderstorms appear likely (60%) late Thursday afternoon
  through the nighttime hours. There will also be a low (5 to
  15%) chance for a severe storm, mainly near and west of the
  Mississippi River late Thursday afternoon and early evening.

- Gradual cooling is favored this weekend into early next week.
  Precip may (30 to 50%) occur at some point during Saturday
  through Tuesday, with Monday starting to appear to have a
  relatively higher chance for precip.

&&

.DISCUSSION...
Issued at 402 AM CDT Wed Sep 18 2024

TODAY AND TOMORROW: Warmth Continues, Dry Today

With the influence of an upper ridge continuing across the region,
expect temperatures to once again reach the 80s this afternoon, with
all but portions of Taylor County having an 80-90% chance of
reaching or exceeding 80. Expect dry conditions with RH values
dropping to around 30 percent in SW WI as well, although perhaps not
as dry as Tuesday as increasing cloudiness may inhibit mixing this
afternoon.

Tomorrow, aforementioned ridge should be displaced to the east as an
upper low advances over the southern Canadian prairies. While
temperatures aloft should take a small step downward, southerly warm
advection looks to ramp up ahead of an advancing cold front, so
highs in the 80s remain a good bet (70-95%). This southerly
advection should finally bring additional moisture to the CWA,
particularly west of the Mississippi River, so do not expect sub 30%
RH values to return. This moisture should also set the stage for
precip, as discussed below.

THURSDAY AND THURSDAY NIGHT: Potential for Showers, Thunderstorms

As the upper low advances from E MT to the Canadian prairies with an
associated cold front extending southward through the eastern
Dakotas, expect a narrow plume of low level moisture to be
transported northward to IA/MN by early Thursday with this axis
slowly shifting eastward through the day. Latest guidance suggests
showers and thunderstorms may develop both during the morning hours
ahead of a weak shortwave and again during the late afternoon and
evening when aforementioned moisture transport is maximized in our
forecast area and a stronger shortwave translates eastward.

For the morning window, am doubtful much in the way of precip
develops as best moisture transport will remain to our west and
updrafts will struggle to overcome inhibition from the still warm
temperatures aloft. Indeed, extended CAM guidance suggests only
spotty precip Thursday morning and have attempted to undercut
NBM PoPs, likely still influenced by parameterized guidance,
accordingly.

Moving ahead to the late afternoon and evening, MLCAPE looks to
increase to 1500-2000 J/kg west of the MS River as southerly moist
advection ramps up. With winds aloft out of the west on the increase
as well, sfc-6km bulk shear values could top 40 knots. Thus, while
some inhibition may remain courtesy of warm temperatures in the 700-
800mb layer, unless the morning precip overachieves and limits
surface heating, expect thunderstorms to develop west of the MS
River during the late afternoon and sweep east through the evening.
Given the aforementioned instability and shear, a window will exist
for a few severe thunderstorms in SE MN and NE IA. While shear
profiles and progged DCAPE suggest mainly a threat for hail and
damaging winds, the near surface profile also points at a very small
(2%) risk for a tornado.

SATURDAY THROUGH TUESDAY: Precip Possible, Moderate Timing Uncertainty

Operational guidance is now in good agreement that overall flow
should transition to quasi-zonal for Saturday and Sunday before an
upper low lifts northeastward from the Four Corners to the Upper
Midwest sometime Sunday night through Tuesday. With guidance
seemingly trying to converge on Monday being favorable for precip,
have attempted to focus higher PoPs on the 12z Monday through 12z
Tuesday time frame. Outside of this, uncertainty remains high, as
guidance is in typical disagreement on the timing and potency of
lead shortwaves for Saturday and Sunday. Thus, precip mentions
remain in the forecast for an extended period with this update.

&&

.AVIATION /12Z TAFS THROUGH 12Z THURSDAY/...
Issued at 529 AM CDT Wed Sep 18 2024

South to southeasterly winds expected for the TAF period. These
winds will pick up later this morning and afternoon with gusts
around 20kts west of the Mississippi. VFR conditions remain,
while some mid to high level clouds move into the area today.
These CIGS will remain between 20kft and 25kft.

&&

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

&&

$$

DISCUSSION...Ferguson
AVIATION...Cecava