406
FXUS63 KARX 261114 AAA
AFDARX

Area Forecast Discussion...UPDATED
National Weather Service La Crosse WI
614 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 storms pass 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 overall 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
location, strength, and quickness of deepening 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 by cold air advection increases
confidence in storm and small hail potential. Shear profiles
remain weak, resulting in storms pulsing up/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 provide a final round of
potential precipitation Monday night into Tuesday. Available
moisture will be further limited as northwest flow will be quite
dry. Cold air advection behind this final trough will introduce
slightly below normal temperatures from Tuesday into Wednesday
night.

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, currently extending along the Atlantic Coast on
GOES water vapor imagery, is expected to close along its
northern periphery, eventually regressing through Central
Canada. While the immediate ramification will be assisting in
the departure of the aforementioned area of everlasting closed
upper level heights, higher impact 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%) for 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, leaving 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 (+10) 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 is exhibited in the EPS from 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. In
collaboration with the neighborhood offices.

&&

.AVIATION /12Z TAFS THROUGH 12Z MONDAY/...
Issued at 615 AM CDT Sun May 26 2024

Precipitation can be seen spreading northeast through the Upper
Mississippi River Valley on radar imagery early this morning.
This rainfall will continue spreading northeast into the
afternoon hours as the surface low pressure progresses from
northeast Iowa into western and central Wisconsin. Highest,
albeit limited, thunder chances graze locally southern counties
from northeast Iowa into southwest Wisconsin this morning and
afternoon. South of both TAF sites (KRST & KLSE) where there are
very limited thunder chances. Otherwise, heavy rainfall
resulting in lower visibilities are the near term forecast
detail. Heaviest rainfall amounts today expected southeast
nearing 1.5" in spots.

Otherwise, ceilings reduce near IFR later this morning and are
expected to linger that way into MVFR through the 26.12Z TAF
period.


&&

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

&&

$$

DISCUSSION...JAR
AVIATION...JAR