926
FXUS63 KARX 170942
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
442 AM CDT Sun Aug 17 2025

.KEY MESSAGES...

- Heavy rain band this morning weakens through the late morning
  with a mostly dry forecast until tonight.

- Widespread heavy rain progresses from west to east overnight
  through Monday. An additional 1" to 1.5"+ is expected for much
  of the area.

&&

.DISCUSSION...
Issued at 304 AM CDT Sun Aug 17 2025

A narrow corridor of ongoing heavy rainfall is expected to
continue through the early morning hours, situated along an axis
of dilatation and pressure saddle visible on VWPs across the
Upper Midwest. Additional storm initiation upstream over
southwestern Minnesota early this morning on the nose of the low
level jet is expected to remain west of the local forecast area
as precipitation chances return tonight, becoming widespread
overnight.

Break In Heavier Precipitation This Afternoon:

Slight regression in the upper level synoptic high off the
southern CONUS will pass a weakening ridge axis across the Upper
Midwest today and subsequently point mid-upper level southwest
flow directly from the southern California coast through the
Upper Mississippi River Valley tonight. A slight break in
precipitation through the early afternoon locally will
accompany this synoptic progression before perturbations along
the southwest flow and associated ageostrophic ascent deepen and
advect a surface low northeast through the Northern Plains
perpetuating heavy rain concerns locally tonight through Monday.

Initial Rainfall Tonight:

As a low strengthens across the Northern Plains today,
invigoration of a southwest low level jet tightens the
anomalous (SPC climatology) ribbon of moisture (1.5"-2" PWATs
17.00Z RAOBs) lingering across the forecast area for tonight.
Resultant initial precipitation chances (20-40%) expected in a
narrow northwest to southeast oriented band primarily across our
western peripheral counties from northeast Iowa into southeast
Minnesota. Highest confidence in precipitation will remain
northwest of the local forecast area tied to forcing from the
surface low while frontal forcing remains superfluous,
inherently dependent on exact location of the front. Eventually,
precipitation potential (30-50%) increases from west to east
within this band as isentropic ascent increases tonight.

Widespread Precipitation Overnight Through Monday:

Subsequent strengthening to the surface low overnight
from a shortwave providing more westerly mid level flow results
in an easterly advection to the low near the International
Border and increased precipitation chances locally through
Monday. Current confidence keeps highest precipitation
potential constrained to the northern 2/3 of the forecast area,
closer to the surface low. Given the higher confidence, have
collaborated with WPC and neighboring WFOs to include a Slight
ERO through Day 2 in Wisconsin counties where heaviest
precipitation has been realized over the last 48 hours.

Additional cyclogenesis along a potential triple point near
SD/MN/IA could also increase heavy precipitation and storm
impacts farther south, across the local forecast area.

Cooler & Drier Through Midweek:

Today`s regressive synoptic upper level high sets up over the
Desert Southwest into midweek ushering in upper level diffluence
and back door cooler air locally with northeast winds keeping
the pattern drier and cooler with daytime highs mostly in the
70s. As this weekend`s moisture becomes trapped against activity
in the Atlantic, return flow ramps up into the Northern Plains,
resulting in the local forecast area surrounded by warmth and
weakening precipitation chances into next weekend.

&&

.AVIATION /06Z TAFS THROUGH 06Z MONDAY/...
Issued at 1236 AM CDT Sun Aug 17 2025

Line of storms from KMML in southwest Minnesota through KEFT in
southern Wisconsin causing VLIFR-IFR visibilities from heavy
preciptiation across the southwestern half of the local forecast
area in southeast Minnesota, northeast Iowa, and southwest
Wisconsin. Expect this line to continue through the morning
hours, remaining relatively stationary. Heavy rain causes main
threat of flash flooding.

The accompanying cloud deck expected to stick around through the
rest of the day. Storm chances increase again tonight,
progressing from west-northwest to east-southeast through Monday.
Similarly expect VLIFR-IFR visibilities due to heavy rain with
passing storms. Widespread aviation impacts will be possible.

Frontal boundary passage Monday night limits confidence for
aviation impacts Tuesday and after.

&&

.HYDROLOGY...
Issued at 405 AM CDT Sun Aug 17 2025

Besides an ongoing band of heavy rainfall from southwest
Minnesota through southwest Wisconsin early this morning,
additional more widespread heavy rainfall with 1"+ per hour
rainfall rates is expected overnight through Monday. The
combination of these high rates with many areas having already
experienced heavy rainfall over the last 48 hours, flash
flooding concerns persist for much of the area through Monday.

This morning`s narrow band of heavy rain produced 1" to 2"
rainfall rates and 3"+ accumulation maxima overnight locally
from Fillmore County, MN through Winneshiek County, IA,
Allamakee County, IA, and Grant County, WI. The initial tight
gradient of stronger storms that caused flash flooding has since
dissipated through the early morning hours, lessening overall
flash flooding concerns. Expect this line of heavy rain to
continue through the morning hours, providing another 1"+ in
some spots.

Friday`s heavy rain observations of 1" to 3"+ occurred in
northeast Iowa, southeast Minnesota, and western through central
Wisconsin.

An additional 1" to 2"+ of rainfall is expected overnight
through Monday mostly north of Interstate 90 in southeast
Minnesota and western into central Wisconsin. An initial NW-SE
oriented band of rainfall may form along the western half of the
forecast area tonight, potentially precipitating on already saturated
soils in southeast Minnesota, northeast Iowa, and far southwest
Wisconsin. Low confidence (20%) scenario but will be subsequent
detail to keep an eye on. Additional heavy rain overnight
through Monday will be more widespread. Precipitation exit may
linger in central Wisconsin through Monday night, further
exacerbating river rises locally being realized on the Yellow
River at Necedah, WI. Black River hasn`t realized as high rain
accumulations and therefore remains 3` below Action Stage at
Black River Falls, WI as of 17.09Z.

&&

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

&&

$$

DISCUSSION...JAR
AVIATION...JAR
HYDROLOGY...JAR