301
FXUS63 KARX 260905
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
405 AM CDT Sun May 26 2024

.KEY MESSAGES...

- Heavy Rain & Low Storm Chances Move Northeast Across The
  Forecast Area Today, Rainfall Amounts Above 1.5" In Spots.

- Storm & Rain Chances Abate Tonight, Only To Return Monday Into
  Monday Night

- Slightly Below Normal Temperatures Through Midweek Become
  Slightly Above Normal By The End Of The Week

&&

.DISCUSSION...
Issued at 339 AM CDT Sun May 26 2024

Early Morning Storm Chances:

A surface low seen lifting through the Central Plains towards
the Upper Mississippi River Valley early this morning is
providing preciptiation chances from southwest to northeast.
The responsible upper level perturbation can be seen on GOES
Water Vapor imagery, collocated with upper level divergence of
10-20 ^-5/s through the Central Plains in RAP analysis. A
general weakening trend in forcing has resulted in the surface
low slowly filling as it lifts northeast. As a result, storm
potential has been on a weakening trend as it reaches the local
area. Primary hazards will be heavy rainfall and small hail as
storms have a limited amount of effective shear, pulsing up then
weakening shortly thereafter.

Rain & Storm Chances Through Today:

Rain and storm chances spread northeast through the morning hours as
the surface low traverses our southern peripheral counties from
northeast Iowa into southwest Wisconsin. Local instability as
the storm passes will be limited to our far south where the
moistest airmass can penetrate. Additionally, little to no
recovery in temperatures will limit any lower level instability
building, limiting instability to 500 J/kg of MUCAPE. Shear
profiles become increasingly messy, further decreasing any
confidence or potential for stronger or sustained storms.
Therefore, as higher precipitable water values near 1.25" push
northwest, heavy rainfall becomes the primary concern through
the afternoon. Current confidence has the 1.25" PWAT isohyet
bifurcating the southeastern half of the forecast area as the
low trudges east-northeast. With dry air wrapping northwest of
the surface low, a sharp cutoff in heaviest rainfall is
expected. High resolution model guidance slightly disagrees on
exact location of rainfall gradient due to differences in timing
of dry air from variability in location, strength, and
quickness in deepening of surface low.

Overnight Preciptiation Potential:

The surface low further occludes tonight, pivoting through east-
central Wisconsin during the overnight hours. As a result, an
accompanying low level Fgen band northwest of the low center
will increase precipitation potential through central
Wisconsin. Current confidence keeps the heavier rainfall
amounts to the east in central Wisconsin due to the progressive
pattern preventing the low from lingering. Will be important to
monitor exact location of surface low and subsequent Fgen band
on our doorstep for additional heavy rain potential.

Storm & Preciptiation POtential Monday:

The progressive pattern also provides a subsequent perturbation
Monday morning into the afternoon. The main moisture source
will be wraparound leftovers from today`s low pressure,
limiting overall heavy rainfall potential. Instead, steeper
lapse rates being worked on from cold air advection increases
confidence in storm and small hail potential. Shear profiles
remain weak, resulting in storms pulsing up and down. While
storm life will be limited, a low freezing level near 5k ft and
thick hail growth zone could allow a pulse storm to provide
small hail.

Monday Night Storm Chances & Colder Temperatures:

While these storm and precipitation chances will be short lived
primarily through Monday afternoon, a final appendage of the
wobbling upper level low will perpetuate a final round of
potential precipitation Monday night into Tuesday. Available
moisture will be further limited as northwest flow remains very
dry. Cold air advection behind this final trough will introduce
slightly below normal temperatures Tuesday into Wednesday night
As a result.

These below normal temperatures don`t sustain through the work
week as long term global models (GFS/GEFS/ECMWF/EPS) exhibit
similar solutions in amplifying an upper level ridge across the
Central CONUS.

Synoptic Pattern Through Midweek:

However, it will be quite an peculiar upper level synoptic
pattern through the work week. An extremely amplified upper
level ridge, that currently extends along the Atlantic Coast on
GOES water vapor imagery, is expected to close along its
northern periphery which eventually regresses through Central
Canada. While immediate ramification will be assisting the
southeast advection of the aforementioned area of everlasting
closed upper level heights, higher impacts will be influence and
amplification the subsequent upper level ridge. The initial
synergization of this anticyclonic flow with the mean pattern is
expected through Monday with rapid amplification of upper level
heights through Wednesday across the Northern Plains.

As a result, a quasi-blocking omega synoptic pattern is expected to
setup across the Upper Mississippi River Valley through the week.
Global models (GFS/GEFS/ECMWF/EPS/GEM/GEPS) all exhibit this quasi-
blocking pattern through midweek whilst differentiating on location
and intensity of the ridge axis. Additionally, mostly all intra
(ensemble) model dProg/dT has exhibited increasing confidence (+10-
20%) in higher heights over the Upper Mississippi River Valley
through the week. In other words, potential for a slower, longer
residing blocking pattern is increasing. Certainty (100%
confidence) in the 576 dam isoheight at 500 mb brushes the
IA/MN/WI border in the GEPS while enveloping the local area in
the EPS with the GEFS somewhere in between.

Slightly Above Normal Temperatures, Slower Precipitation Return:

As a result, the most boisterous EPS solution is the most
optimistic (60-80% confidence) for slightly above normal
temperatures by the end of the week while GEFS & GEPS remain
less keen. All models are certain of highest temperature
anomalies in southern Canada, along the ridge axis` northern
periphery. Ensemble meteograms vary from highs in the 60s (GEFS)
to 70s (EPS). Complementary impact will be a potentially dry
forecast into the weekend. Again, as one would expect, highest
confidence exhibited in n the EPS with the strongest ridge with
lower confidence in the GEFS & GEPS. This also carries into the
weekend, with the slowest solution for precipitation onset in
the EPS into Saturday afternoon. Given the confidence, have
slightly decreased precipitation potential from National Blend
Thursday Night through Friday.

&&

.AVIATION /06Z TAFS THROUGH 06Z MONDAY/...
Issued at 1050 PM CDT Sat May 25 2024

VFR conditions overnight will deteriorate to low-end MVFR to
high-end IFR late Sunday morning as a broad area of rain and
associated low pressure system lift into the area. There`s a 70
to 90% of IFR conditions at RST, with a 40 to 60% chance of IFR
at LSE. Opted for a OVC010 at LSE as conditions there are
expected to be on either side of the MVFR/IFR thresholds.
There`s a slight chance (10-20%) of an embedded thunderstorm
with these showers, but will leave out of the TAFs at this time.

&&

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

&&

$$

DISCUSSION...JAR
AVIATION...JAW