941
FXUS63 KARX 210806
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
305 AM CDT Sat Sep 21 2024

.KEY MESSAGES...

- Showers and storms today into tonight. A few could be strong later
this afternoon/evening. Pcpn chances shift southward later tonight
and Sunday.

- One more warm day today then much cooler (below normal) temps flow
in for Sunday, continuing for the first part of the new week.

&&

.DISCUSSION...
Issued at 305 AM CDT Sat Sep 21 2024

* CURRENTLY

Cluster of showers/storms continue to spark over northern MN, mostly
in response to evaluated Fgen with impingement of low level moisture
transport. This activity should continue through the overnight, but
RAP suggests a weakening in the lift after sunrise, so could see
that area of showers wane. Locally small chances could graze far
northern parts of the forecast area.


* TODAY/SUNDAY: shower & storm chances. Warm today, turning much
  cooler for Sun.

A trough in the northern branch of the upper level flow is on track
to slide west-east along the Canadian/U.S. border today. While the
bulk of its energy will hold north, short term models all suggest a
piece will stray farther south, moving across MN into central WI
tonight. To the south, bits of shortwave energy are progged to eject
northeast out of trough over the desert southwest, sliding across IA
and southern WI/northern IL by this evening. This train of ripples
in the flow are expected to persist through Sunday as the parent
shortwave lifts over the mid Mississippi river valley but gains more
of a positive tilt (some weakening). At the sfc, a cold front will
drop southeast across the region this afternoon/evening,
accompanying the northern shortwave wave. Ample forcing, although
fairly disjointed with no phasing of northern/southern features
evident.

A broad swath of low level moisture transport will slide ahead of
the sfc front with PWs of 1 1/2+". RAP SBCAPE builds to 2+K J/kg in
the afternoon, but some low level capping ahead of the front to
contend with (per RAP bufkit soundings). The deep layer shear lies
westward of the cold front (and west of the instability axis), but
soundings suggest enough to help foster updraft development. CAMS
models depict a mixed bag when it comes to how/where the convection
will spark/develop. Some develop shower/storms by mid/late morning
over southern MN, in response to a ripple in the upper level flow
pushing out of the northern plains. This results in pcpn chances
moving in by late morning, persisting through the afternoon.
Meanwhile the HRRR and RAP focus their chances on the cold front,
which would delay the showers/storms until late afternoon. However,
both models are doing poorly with ongoing convection. It`s a messy
setup. Confidence is high for at least scattered convection today
into tonight but lower with where/timing for where initiation is
more likely. Will lean into the model blend for the moment but
expect revisions to the forecast as we move through the day. A few
stronger storms will be possible and cannot rule out an isolated
severe.

Later tonight into Sunday the focus for pcpn shifts to the south and
the train of shortwave energy riding along the exiting cold front.
Mostly showers as the instability axis will be well south, with
chances generally along/south of I-90. Expect the chances to
continue to shift south Sun night with the exit of the front and
upper level forcing.


* NEW WORk WEEK: cool start, return to normal temps toward the
  weekend? Trending dry.

Long range guidance suggests a bit of upper level energy will shift
eastward, just south of the local forecast area Tue, while a
shortwave trough in northern branch of the flow transitions west-east
across southern Canada/northern parts of the region. Locally,
saturation is mostly confined to the mid/upper levels, negatively
impacting pcpn potential. The Grand Ensemble of models only paints a
0-15% chance for either bit of energy to drop measurable rain across
the forecast area.

That said, some recent deterministic runs of the GFS take that
northern shortwave and dig it over the mid mississippi river valley,
developing a closed 500 mb low that would then meander back
northeast, impacting the region with widespread rains and more cool
air. However, when compared with the GEFS suite of ensemble members
there are only 2 to 3 that harbor similar outcomes. EPS members also
have only a few in this bucket, with overwhelming support for a
drier outcome. WPC cluster analysis - the 4 clusters don`t show
much/if any potential for that closed low. For now, will view that
possible outcome as an outlier. Going to keep an eye on it, but not
putting much stock in it (yet).

For temps, GEFS/EPS favoring bringing in the west coast ridge moving
into the latter half of the new week. While the spread in
temperature outcomes grows, the upper 75% of both models` ensemble
members return highs into the 70s - back to the seasonable normals.


&&

.AVIATION /06Z TAFS THROUGH 06Z SUNDAY/...
Issued at 1041 PM CDT Fri Sep 20 2024

Skies will be mainly clear tonight and then high and mid clouds will
move into the area on Saturday morning and early afternoon.
As a cold front moves toward the area during the mid- and late
afternoon, the CAMs are showing scattered showers and thunderstorms
possibly impacting the TAF sites. Since confidence is not overly
high, just introduced a PROB30 at KRST starting at 21.21z and ending
at 22.01z and at KLSE starting at 21.22z and ending at 22.02z.

&&

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

&&

$$

DISCUSSION...Rieck
AVIATION.....Boyne