642
FXUS63 KARX 171006
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
500 AM CDT Tue Jun 17 2025

.KEY MESSAGES...

- Moderate confidence for storms, some strong to severe, primarily
  affecting locally southern counties in northeast Iowa and
  southwest Wisconsin later this afternoon into tonight.

- Low confidence in scattered storm potential this afternoon
  through evening and northern extent of precipitation and/or
  storms into tonight.

- Heavy rain with potential storms primarily affect northeast
  Iowa and southwest Wisconsin through overnight through
  Wednesday.

- Storms primarily impact north of Interstate 90 Thursday
  evening through Friday night.

- Anomalously warm weekend on tap with temperatures 10 degrees
  above normal likely.

&&

.DISCUSSION...
Issued at 242 AM CDT Tue Jun 17 2025

The upper level wave responsible for Monday`s storms can be seen
lifting northeast on upper level water vapor imagery early this
morning with a longer wave trough extending into the Central Plains
and a minute wave embedded over North Dakota. The quick departure
to yesterday`s weak wave has once again stranded the everlasting
boundary near/across the forecast area early this morning.

Synoptic Setup For Today:

The overall story remains quite repetitive with previous
questions surrounding potential mesoscale influences abound.
Weakening synoptic flow will lift northeast through the morning
hours, spawning convection over the Upper Mississippi River
Valley. As has been the case, exact location of frontal boundary
will determine northern extent of storms and accompanying
precipitation and storms locally. Most recent high resolution
model forecasts suggest burbling initially along the zonally
oriented boundary to the north quickly followed by scattered
storms building south in the warm sector. An overall lack of
forcing will keep storms relatively scattered initially, driven
by -250 J/kg SBCIN and 1500+ J/kg SBCAPE in all high resolution
model guidance.

As previously hinted at, northern location of storms and
precipitation remains the most unknown forecast detail. While model
guidance has remained adamant that storms will primarily affect
the southern half of the forecast area from northeast Iowa into
southwest Wisconsin, additional forcing from strengthened cyclogenesis
along the southwest-northeast oriented boundary supplemented by
ageostrophic ascent more aligned to the forecast area could push
precipitation and storm impacts push farther north. Regardless,
as storms push north into the boundary expect rapid weakening
within the stable airmass.

Storm Timing & Evolution Today:

Expect storm coverage to fill in shortly after onset within the warm
sector given a quite unstable airmass. Higher shear values remain
north of the frontal boundary with 80% probabilities for 30+kt 0-6km
shear in HREF members south of the frontal boundary primarily
due to a passing mid level jet streak. Storms likely will be
responsible for their own increased shear from their
accompanying outflows. As previously mentioned, confidence for
strong to severe storms locally remains in the southern half of
the forecast area. Damaging wind will be the main concern, seen
in northeast trends for machine learning algorithms over the
last few runs. However, most recent HRRR (17.00Z) neural
network only paints a 5-10% probability grazing our southern
peripheral counties.

Precipitation & Storm Chances Overnight Through Wednesday:

Synoptic forcing eventually gets it`s act together overnight through
Wednesday providing ample forcing to finally shunt the everlasting
front and accompanying surface low east. Fortunately confidence
in frontal location keeps higher severe storm chances farther
southeast although hail and damaging winds can`t be ruled out
with the strengthening low pressure. Heavy rain will be the main
concern as current confidence places the frontal boundary
across the southern periphery of the forecast area and a spread
in PWATs from 1" to 1.75" in HREF members. Resultant rainfall
amounts vary in bands from 0.5" to 1.5". Subsequent forecast
detail to iron out.

Storm Chances Thursday:

A short reprieve in storm chances overnight Wednesday into Thursday
ceases Thursday afternoon as trough slides/digs through the Northern
Plains into the Great Lakes. Accompanying cold front sags southwest
from the Great Lakes. Cluster analysis shows a corridor of agreement
for instability and shear across the Upper Mississippi River
Valley pre cold front. Best forcing will remain tied to the
aforementioned synoptic forcing to the north, aligning with NBM
PoPs. Primary concern would be large hail based on quite
elongated deterministic GFS hodographs although ensemble
hodograph plumes exhibit little agreement.

Storm Chances Friday & Friday Night:

A subsequent rapid rise in upper level heights (LREF) ushers in
strong return flow overnight Thursday through Friday with 75% of
LREF members in agreement for a nose of increased low-level theta-e
advecting through west-central Wisconsin. Accompanying joint
probabilities for CAPE, CIN, & shear reach 40-60% over varying narrow
corridors of the Upper Mississippi River Valley in the EPS and
GEPS while staying <10% for the GEFS. The varying location in
agreement and the lack of agreement leaves much to desire in
the way of confidence in impacts at the current forecast hour.

Warm Weekend Into Next Week:

A rapid increase of upper level heights and associated strong
low level return flow in LREF cluster analysis Thursday night
through Friday results in anomalous warmth through the weekend.
LREF members show an plume of 850mb temperatures 12C above
normal spanning from lee of the Rockies through the Great Lakes.
While all LREF models (GEPS/EPS/GEFS) have exhibited a warmer
solution over previous runs, the GEPS solution is 10F warmer
than the GEFS and EPS. Although the opposite can be seen in
dewpoint temperatures, keeping all LREF members apparent
temperatures spread from 91 degrees in the 25th percentile to
103 degrees in the 75th percentile. Spread greatly widens into
early next week. Accompanying ECMWF extreme forecast index &
shift of tails reaches 0.7 to 0.8 in max Temps Sunday through
Tuesday and 0.8 to 0.9 for min temps Sat night. Many facets of
the forecast await before determining exact timing and strength
of synoptic Bermuda high.

&&

.AVIATION /12Z TAFS THROUGH 12Z WEDNESDAY/...
Issued at 500 AM CDT Tue Jun 17 2025

Messy weather pattern persists with a mix of small scale (micro) and
synoptic scale features that will drive pcpn chances/cloud coverage
today and tonight...and likely for the next few days.

A surface front continues to wobble west-east across IA, and could
nudge northward to the IA/MN by late afternoon. Moisture return is
mostly parallel and south of the front. However, a convectively
enhanced shortwave trough moving out of NE looks to run up to the
front - and with instability still pooled into and north of the
front - likely spark SHRA/TS. How widespread and when they get going
isn`t clear, but enough forcing mechanisms in the vicinity to
continue small chances for KRST/KLSE. Will hold with PROB30s for
now. Overall setup currently favors the bulk of the rain chances
south of I-90.

MOving into the overnight, showers and storms look to become more
widespread as an upper level shortwave trough lifts northeast out of
the southern plains. Much of Wed is trending wet with impacts to
cigs (likely MVFR) and vsby (from time to time).

&&

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

&&

$$

DISCUSSION...JAR
AVIATION...Rieck