522
FXUS63 KARX 141102
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
602 AM CDT Sat Sep 14 2024

.KEY MESSAGES...

- Showers continue to move in throughout the morning and into
  much of the day. Highest confidence (50-80%) for showers and
  isolated storms is west of the Mississippi River this morning,
  then shifting north of I-90 during the afternoon. Isolated
  lingering showers are possible (15-20%) for Sunday and
  Monday.

- Seasonably warm conditions are expected to persist this
  weekend and into next week with high temperatures generally in
  the 80s for much of the local area, roughly 5-10 degrees above
  average for this time of year.

&&

.DISCUSSION...
Issued at 331 AM CDT Sat Sep 14 2024

Today: Showers Move Into The Region, Highest Confidence For Showers
and Isolated Storms West of Mississippi River

The overall synoptic setup currently is dictated by a few key
features that can be noted in 500mb geopotential heights across
deterministic guidance this morning. Positioned to our south, the
remnant upper-low from what remains of Hurricane Francine is
meandering through the lower portions of the Mississippi River
Valley. Throughout the overnight hours and into the morning, a lobe
of vorticity was ejected northward caught in between an upper-level
ridge to the east and a broad trough further west. This piece of
vorticity combined with tropical moisture advection (precipitable
waters of 1.5-1.75" in the 14.03z RAP) from the southeast provided
by the remnants of Francine will continue to be the primary
instigating mechanism for any showers and isolated storms throughout
the day. As this vorticity lobe pushes north, showers west of
the Mississippi River this morning will shift north of I-90
during the afternoon.

With the main axis of moisture and forcing being focused generally
west of the Mississippi River, noting probabilistic guidance (14.00z
GEFS/EC ensemble) favoring high probabilities (60-90% chance) for
over 1/10" of precipitation across northeast IA and southeast MN
throughout today. Overall, the chance for any thunder is on the
lower side (20-30%) west of the Mississippi River with instability
generally being fairly skinny in model soundings at KRST with MLCAPE
values of up to 1000 J/kg during the afternoon hours with virtually
no instability present this morning. Furthermore, with the lower
convective threat and the overall forcing regime being on the weaker
side, noting fairly low probabilities (10-30% for over 0.5" in the
GEFS/EC ensemble/HREF) for higher rainfall amounts despite progged
precipitable waters being generally above the 90th percentile for
KDVN sounding climatology. However, if any storms manage to get
going some locally higher rainfall amounts cannot be ruled out.

Sunday - Monday: Small Chances For Showers & Storms

The forcing regime for any precipitation generally is a bit more
questionable with the aforementioned vorticity lobe ejecting north
of the region for Sunday/Monday. As a result, the primary
instigating mechanism for any showers would be embedded
shortwaves around the periphery of the ridge to our east in
conjunction with the aforementioned moisture rich airmass in
place. However, based on the state of current deterministic
guidance this currently is low confidence. With the 14.00z HREF
probability matched mean having very low QPF across our region and
the overall unimpressive synoptic setup, have kept minimal
precipitation chances (15-20%) for Sunday/Monday for now. With
slightly more favorable instability present in 14.03z RAP
soundings for Sunday (1500-2000 J/kg MLCAPE), would not be able
to rule out an isolated non-severe storm if a stronger updraft
can be realized with the overall questionable forcing regime.

Tuesday - Friday: Continued Warm Pattern

As we head into next week, upper-level ridging generally is favored
to build back into our region with a broad trough situated across
the Rockies. As a result, expecting this warmer pattern to persist
with 850mb temperatures in the 14.00z EC/GFS of 16 to 18 degrees C.
This in combination with the NAEFS percentiles generally keeping
850mb temperatures in the 90th percentile would suggest temperatures
will likely remain above average through next week. Consequently,
forecast high temperatures with the national blend through Friday
generally remain steady state with high temperatures generally in
the 80s through Friday with only around 2 to 3 degrees of inter-
quartile spread within the 13.12z grand ensemble.

Precipitation chances are dependent on if our region can eventually
be pushed into the southwesterly flow regime off to our west.
Different schools of thought on how that will transpire by the
second half of the week with the 14.00z EC/GFS trying to pivot
pieces of shortwave energy west of the region for the second half of
the week. However, respective ensemble members in the GEFS/EC
ensemble do not all jump on this bandwagon with medium confidence
(40-60% chance) for measurable precipitation in portions of
southeast MN/northeast IA. Consequently, still a fairly lower
confidence period that will likely need more adjustments in
upcoming forecast cycles.

&&

.AVIATION /12Z TAFS THROUGH 12Z SUNDAY/...
Issued at 602 AM CDT Sat Sep 14 2024

Showers, isolated storms and MVFR cigs will be the primary aviation
forecast concerns for the TAF period. As a weather disturbance
continues to move through the region, scattered showers are
currently moving through portions of southeast MN and northeast IA.
This is generally expected to continue through the rest of the
morning and into the afternoon with increasing instability allowing
for some potential for isolated storms to develop. However, with
instability generally on the weaker side, have opted to hold off on
any VCTS mention as any storms will be very isolated in nature. A
tough cig forecast remains for KRST with the stronger signal across
guidance for category reductions remaining just to the west of the
TAF site. Both the NBH and 14.06z HREF have medium probabilities (30-
50%) for MVFR cigs with deterministic soundings and guidance keeping
cigs right around 3kft. Consequently, thinking that likely will see
periods of sct to bkn low-level cigs with periods of MVFR certainly
possible at KRST and points westward later this morning. As a
result, have decided to include a short tempo group for when
confidence would be the greatest for any MVFR cigs.

&&

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

&&

$$

DISCUSSION...Naylor
AVIATION...Naylor