355
FXUS63 KARX 082338
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
638 PM CDT Fri Aug 8 2025

.KEY MESSAGES...

- The risk for scattered severe thunderstorms continues for much
  of the area on Saturday, though the greatest threat area is
  starting to shift eastward into western Wisconsin for the
  afternoon.

- Repeated rounds of rainfall are expected Saturday evening
  through Monday night across Iowa into southern Wisconsin, but
  exactly which areas see the heaviest amounts is dependent on
  how each day`s storms evolve. Localized rainfall amounts of
  3-5+ inches are possible (20- 40% chance).

- After today`s heat and humidity, temperatures fall back to
  near seasonal values for the weekend and through at least mid-
  week.

&&

.DISCUSSION...
Issued at 200 PM CDT Fri Aug 8 2025

Saturday: Thunderstorm/Severe Weather Trends

A line of storm clusters is progged to develop this evening
across western Minnesota and quickly track to the northeast,
laying out an outflow boundary across west-central Wisconsin
into southeastern Minnesota after 06Z. This outflow should cool
the 850-700-mb warm nose to the point of supporting convective
development along the boundary after the 07-09Z period. While
there is amble MUCAPE (2000- 2500 J/kg) above this level,
effective shear values are quite low and hodographs are rather
compact and complex. Therefore, there exists a narrow window in
which storms could produce hail of 1-2 inches in diameter before
the cells grow upscale and the hail threat transitions to wind.
With the outflow boundary laying out parallel to the mean mid
to upper level flow, it will only slowly meander eastward--
driven in part by the strength of the convective cold pools.
Given the slower storm motion, there is a risk for repeated
rounds of rainfall along the front and localized heavy rainfall
around 1-2 inches.

There is strong agreement in the 12Z CAMs that the storms will
linger along the front and shunt the effective warm sector
further to the east for the afternoon (mainly east of a Black
River Falls, WI to Charles City, IA line). Additional convective
cells should develop along and ahead of this boundary towards
midday and move eastward through the afternoon, exiting the
forecast area by the late afternoon. Once again, there is a
narrow window for hail with these storms before the transition
to linear segments and become more of a wind threat. How far
south these cell clusters can shove the surface outflow boundary
during the afternoon will play directly into which areas in
northeast Iowa and southwestern Wisconsin see heavy rainfall
from Saturday night into Monday.

Saturday Evening - Monday Night: Multiple Rounds of Heavy Rain

The convective outflow boundary slows and even retrogrades
northward during the day on Sunday across northeastern Iowa and
southwest Wisconsin as the mid to upper level flow becomes more
parallel to the boundary orientation and several undulations
ripple through the flow. Multiple overnight/early morning MCSs
are progged to lift over this front over a 48 hour window from
Saturday night through Monday morning before the pattern shifts
to the southeast, with the exact axis of heavier rain each night
dictated by how storms the previous day unfold. The HREF
probability matched-mean QPF progs have a corridor of 3 to 5
inches over far southwestern Wisconsin into east-central Iowa
through Sunday morning, with at least one more round of heavy
rain lingering just outside of this time window Sunday night
into Monday.

The good news is that we are going into this event dry--thus
the first round of rain on Saturday afternoon will likely result
in minimal impacts. However, as we go through the next
successive days of rainfall, the impacts increase along with the
threat for flash flooding and river flooding. Will need to
watch local rivers such as the Kickapoo in WI and the Turkey in
Iowa, where such rainfall amounts--should they occur in these
basins--would bring these rivers above flood stage.

Temperature/Precipitation Outlook into Next Week

The clouds and rainfall attendant with our weekend system will
naturally abate temperatures after the heat of this afternoon.
Given the earlier arrival of storms on Saturday, some areas west
of a Decorah, IA to Black River Falls, WI line may struggle to
reach 80 degrees as hinted at by multiple convective models. The
degree of clearing for Sunday and Monday will drive the highs
to some extent areawide, though the medium range solutions are
clustered tightly in the low to mid-80s. The heat may return for
the end of the week with a number of EPS/GEFS members trending
back towards 90 degrees for this period. Given that there is
little change in the overall airmass and we are at the peak of
evapotranspiration, it will still feel quite like summer for
this next week as dewpoints remain in the mid-60s to mid-70s.

&&

.AVIATION /00Z TAFS THROUGH 00Z SUNDAY/...
Issued at 637 PM CDT Fri Aug 8 2025

VFR conditions continue into the overnight hours. Showers and
thunderstorms are expected to develop between 07Z and 10Z on a
line roughly from KAEL to KEAU and gradually shift southeastward
throughout the day. MVFR to IFR flight categories will be
possible in the storms that produce the heavier downpours.
Predominantly southerly winds through the TAF period with gusts
between 20 and 30 mph this evening. When storms pass through,
winds may become gusty and erratic at times.

&&

.HYDROLOGY...
Issued at 200 PM CDT Fri Aug 8 2025

There is an increasing threat for localized flash flooding and
river flooding Saturday night through Monday--with river
flooding possibly lingering into the week. Contingency
forecasts show local rivers such as the Turkey and Kickapoo
reaching flood stage if some the highest projected rainfall
amounts occur in these basins. However, it remains too early to
nail down exactly how the rivers will respond to these repeated
rainfall events.

&&

.ARX WATCHES/WARNINGS/ADVISORIES...
WI...Flood Watch from Saturday afternoon through Monday morning for
     WIZ054-055-061.
MN...None.
IA...Flood Watch from Saturday afternoon through Monday morning for
     IAZ030.

&&

$$

DISCUSSION...Skow
AVIATION...Cecava
HYDROLOGY...Skow