848
FXUS63 KARX 251040
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
540 AM CDT Sat May 25 2024

.KEY MESSAGES...

- Limited Reprieve In Precipitation & Storms Through This
  Afternoon, Returning Tonight, Lasting Into Monday.

- Elevated Storm Chances Increase Late Tonight Into Early Sunday
  Morning, Small Hail & Heavy Rain Remain Primary Hazards

&&

.DISCUSSION...
Issued at 349 AM CDT Sat May 25 2024

Cold Morning, Normal Temperatures Today:

The upper level low responsible for recent storms and
precipitation is becoming well wrapped on GOES Water Vapor
imagery loops, lifting through the canadian province of
Manitoba. The accompanying dry slot can be seen tightening over
the Upper Mississippi River Valley on GOES infrared satellite
imagery loops. Low level RAOBs (25.00Z) had the accompanying
low level cold airmass sinking southeast from the Northern
Plains into the Upper Mississippi River Valley, resulting in
slightly below normal temperatures early this morning in the
40s; primarily along our northwestern periphery from southeast
Minnesota into central Wisconsin. Although, upstream shortwave
ridging through the Central Plains has been shunting the west-
northwest winds responsible for this colder airmass with the
accompanying surface high pressure moving through Iowa. These
higher heights and accompanying anticyclonic flow will recover
temperatures back to normal today, in the 70s across the Upper
Mississippi River Valley. The respite in precipitation chances
is short lived however, as the active pattern returns tonight
lasting through much of the weekend.

Synoptic Setup This Morning & Potential Forecast Impacts:

The combination and struggle between the shortwave ridge, the slowly
exiting/lingering lifting closed low, and upstream jet streak(s)
over the Rocky Mountain West and southern Great Basin are resulting
in forecast discrepancies for storm chances tonight into Sunday
morning. The lifting low has been filling (i.e., weakening) on PV
analysis through the early morning hours. Interesting tidbit, 0.5
PV analysis late Friday night exhibited pressure falls as low
as 850 mb near the low center, translating into 50 kt winds
near 825 mb at International Falls, MN and Chanhassen, MN 25.00Z
RAOBs. More importantly, this lows accompanying PV folds
support and suggest its surrounding jet streaks persisting
through today.

Low Confidence In Precipitation Potential This Evening:

Ability of the northern jet streak shunting the shortwave ridge
through today will determine the trajectory of the 100+ kt
southern jet streak, and local precipitation and storm chances
tonight through Sunday. Initial precipitation potential this
evening will be dependent on overall influence of anticyclonic
flow and subsequent extent of the associated appendage. The
result is a low level pressure saddle with an accompanying
shrinking axis this evening. Current high  convective allowing
models place this initial band of west-southwest to east-
northeast precipitation from central South Dakota to north-
central Minnesota. Given the dependency on the outcome of this
skirmish between the aforementioned synoptic forcing, exact
location of this narrow band remains slightly challenging. May
see these initial, relatively minimal precipitation chances
graze our northwest periphery tonight. Given the lower
confidence from the limited width of the band and local dry
air, haven`t introduced PoPs tonight, but will be something to
initially keep an eye on.

Increased Storm and Preciptiation Potential Tonight:

Higher impact of the synoptic struggle will be extent of higher
moisture through the forecast area tonight ahead of increased
forcing from the subsequent jet streak. Peak forcing appears
late tonight within the polar exit region of the jet streak
providing 20^-5/s upper level divergence and 25 m/s + irrotational
upper level winds. While best forcing and moisture is expected
to remain to our south, the phasing of the low as it progresses
northeast provides more widespread precipitation and storm
chances locally along the warm frontal boundary. More exuberant
model solution suggest the 55 degree isodrosotherm quickly
advecting northeast through the forecast area while pessimistic
models keep sub 50 degree dewpoints. Highest confidence will be
in our south-southwest counties from northeast Iowa into
southwest Wisconsin.

Potential Storm Impacts & Hazards Into Sunday Morning:

The northern extent of the higher dewpoint temperatures also impact
amount of available instability. The strong upper level
divergence provides strong mid level ascent with steeper mid
level lapse rates, potentially allowing some pulse storms to
come to fruition. While instability is questionable,
combination of low freezing level and hail growth zones of
100-150 mb, small hail cannot be ruled out. Given the frontal
boundaries, shear profiles remain stout although strongest at
the surface which most likely will not be realized. Therefore,
hodograph profiles become quite messy within any available,
transient instability. Again, small hail and heavy rain remain
most likely hazards.

Storm & Precipitation Potential Through Sunday:

As the surface low closes, it bifurcates and lingers across the
southern half of the local forecast area. The 1" precipitable
water isohyet advects through the area, providing perpetual
moisture for heavier precipitation potential. The accompanying
surface warm front drapes west to east from the Iowa-Minnesota
border and points east, locally resulting in higher
precipitation potential initially Sunday morning along our
south. Precipitation shifts north through the late morning as
the low slowly trudges east, with subsequent precipitation
chances falling from northwest to southeast into the evening
hours from an additional pressure saddle and shrinking axis.
Amount of available moisture with this latter precipitation band
becomes a forecast discrepancy as highest moisture is expected
to follow the slowly trudging east surface low.

Storm chances will be highest late morning through the early
afternoon along the zonally draped warm front before storms outrun
this quasi-stationary boundary. High resolution convective
allowing models mostly disagree on phase of the low as select
models suggest occlusion and a narrow filament instability while
others drape a widespread 1000 J/kg+ of SBCAPE along and south
of the warm front. Potential impacts include locally heavy
rainfall near 1" through Sunday and small hail. Low confidence
at this time and will need to be ironed out further in coming
forecasts.

Storm & Precipitation Potential Into New Work Week:

These precipitation chances linger through Monday due to very
slow shift of the surface low. While precipitation chances do
return Tuesday, confidence remains low due to, again, the high
dependency on the exit of the low pressure. As a result, quite a
discrepancy between long term global ensembles (GEFS/EPS) for
0.01"+ of 24 hour rainfall. GEFS exhibits 70-100% confidence while
the EPS broad brushes 40-60% across most of the central CONUS.
Therefore, have continued with National Blend precipitation
chances, constrained mostly along our east in central
Wisconsin, aligning with one of the 100% confidence GEFS bands.


Through Midweek:

Otherwise, the trough passage results in temperatures 5 to 10
degrees below normal for Wednesday (80-100%; GEFS/EPS) before an
omega resembling block causes an amplified ridge to build
through the end of the week. Discrepancies in long term models
(GEFS/EPS) relate, again, to the exit behavior of this
weekend`s low(s). Will be subsequent forecast detail with
precipitation potential into next weekend.

&&

.AVIATION /12Z TAFS THROUGH 12Z SUNDAY/...
Issued at 503 AM CDT Sat May 25 2024

Passing high pressure to the south provides VFR conditions
across the forecast area from northeast Iowa, southeast
Minnesota, and western into central Wisconsin through the 25.12Z
TAF period. As the high pressure passes into this evening, a
surface low churns northeast through the Central Plains into
tonight. As a result, winds turn counter clockwise
(cyclonically) this evening, becoming out of the south as warm
air advects north. Initial precipitation impacts and
accompanying potential flight restrictions expected to remain
northwest of the local forecast area from south-central
Minnesota to west-central Wisconsin this evening. Near the end
of this (25.12Z) TAF period, precipitation and storm chances
spread from west-southwest to east-northeast, bringing flight
restrictions along with.

&&

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

&&

$$

DISCUSSION...JAR
AVIATION...JAR