484
FXUS63 KARX 181920
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
220 PM CDT Fri Sep 18 2026

.KEY MESSAGES...

- Widespread rainfall persists through Saturday morning with
  rainfall totals of 1-3" likely for much of the local area.
  Locally higher amounts of 4-6"+ will be possible. Flash
  flooding and rivers rising into flood stage may occur where
  these heaviest amounts fall. A Flood Watch is in effect for
  much of the area south of I-94 to address this flooding
  potential.

- An additional round of storms will be possible (40-70% chance)
  in southwest WI and portions of northeast IA on Saturday
  afternoon/evening. Locally heavy rainfall will be possible
  with these storms which may result in further flooding
  potential.

- Rain chances (20-50%) persist through Monday before trending
  drier for the remainder of the upcoming week with cooler than
  normal temperatures expected.

&&

.DISCUSSION...
Issued at 220 PM CDT Fri Sep 18 2026

Rest of Today - Saturday Morning: Widespread Rain, Heavy Rain
Leading to Flooding Concerns Overnight

Rainfall has spread into portions of the region as stratiform
rainfall from the deepening surface low we have advertised over the
last couple of days is pushing eastward. With some dry air
intrusion through portions of Iowa, have noticed a lull in
precipitation generally near and south of I-90 this afternoon,
which much of the CAMs suggest will persist into the early
evening. The key mechanism we are watching for which will come
later this evening and into the overnight is a fairly profound
surge of 850mb moisture transport as the nocturnal low-level jet
enhances and interacts with a quasi-stationary boundary
situated over the region. As this occurs, the recent RAP
generally agrees on advecting some more favorable MUCAPE into
the region which in turn switches precipitation mode from
stratiform to convective as illustrated uniformly across all of
the CAMs. Consequently, heavy rain and flooding concerns begin
to increase across much of the area during the overnight as
precipitable waters approaching 2 inches and warm cloud depths
over 4km would suggest a substantially primed environment for
heavy rainfall with fairly strong sloping frontogenesis where
the instability can reside atop this. In addition, the moisture
transport vector may eventually turn more westerly which may
aid in allowing for some backbuilding convection overnight and
into the early morning hours, however the CAMs disagree to what
extent this occurs as the stronger 0-3km shear vector present
in the RAP would suggest some MCS organization which could keep
convection more progressive.

As far as amounts are concerned, the inter-quartile range in the NBM
for rainfall amounts over generally ranges between 1-3" across much
of the region. When looking at the 90th percentile to ensemble
maximum of the NBM, some pockets of 4-6"+ begin to show up. As a
result, would think that the general story with this event will be a
widespread 1-3" of rainfall but localized higher amounts where
storms manage to traverse the same locations. Consequently, have
expanded the Flood Watch to encompass the area where these higher
amounts may be possible. If a more northern solution does take place
with this system, portions of northeast IA may underachieve for
rainfall amounts overnight with a few members in the lower
percentiles of the NBM having amounts under 1". For hydrological
impacts across the region in light of this rainfall, refer to
the hydro section for additional details.

Aside from the rainfall impacts, cannot fully rule (under 10%
chance) out some stronger storms overnight as well as the
aforementioned instability pushes into the region. In general, shear
profiles in RAP hodographs are not overly impressive, however modest
mid-level lapse rates of around 6-6.5 C/km and MUCAPE values
approaching 2000 J/kg may allow for some small hail or gusty winds
with any descending cores, granted the heavy precipitation
environment here would likely limit hail size substantially. In
any case, storms will likely exit the region by early Saturday
morning with the heavy rain and strong storm threat quickly
diminishing after sunrise.

Saturday - Monday: Cooler, Additional Heavy Rain Possible
Saturday Afternoon/Evening in Southwest WI and Northeast IA

After we get past this system, continued rain chances remain across
portions of the area as an upper-level trough/low slowly pivots
through the Great Plains with the aforementioned boundary still in
place. Consequently, additional periods of rainfall remain possible
through Monday with zonal flow in place with some 850mb moisture
transport running overtop this. The most reasonable shot for
additional rainfall would likely be Saturday afternoon and into the
evening for portions of southwest Wisconsin as a more profound axis
of 850mb moisture transport pushes north into southwest Wisconsin as
shown in the NAM/GFS. Will need to watch this as convective
initiation with this forcing begins to develop in far southwest
Wisconsin and northeast Iowa by afternoon which could lead to
additional heavy rainfall. Overall, confidence is low in how
this would manifest, but some of the CAMs suggest some pockets
of additional amounts of around 1-2" could be possible where
storms form which could certainly result in flooding issues
considering tonight`s rainfall.

Otherwise, the quasi-stationary boundary responsible for all of
this will gradually sag south into Sunday and Monday so would
expect precipitation potential to gradually diminish into
Monday. The NBM suggests some uptick in precipitation
probabilities into Monday as the aforementioned main upper-level
trough/low pivots over the area on Monday. However,
probabilities in the grand ensemble (GEFS/EC/Canadian ensemble)
for amounts over 0.25" are fairly low (10-30%) during this
period, so not expecting much for impacts with this round of
precipitation.

Tuesday - Next Friday: Drier with Below Normal Temperatures

As we head into Tuesday, high pressure situated near the Hudson Bay
and northeasterly surface flow will aid in advecting some drier and
cooler air into the region throughout next week. Consequently, not
seeing any significant signals within the NBM and other
ensemble guidance for precipitation chances during this period.
Cloud cover will likely linger with the aforementioned departing
upper-level low/trough on Tuesday, however as some upper-level
ridging pushes into the region by Wednesday morning, cloud cover
will clear. Will be curious to see if we can manage to get some
frost on Wednesday morning, particularly in more typical cold
spots of west/north-central Wisconsin as the lower percentiles
of the NBM have lows falling into the middle to upper 30s in
these areas on Wednesday. However, as this ridging builds into
the region, would expect a gradual warming of temperatures
through the end of the work week.

&&

.AVIATION /18Z TAFS THROUGH 18Z SATURDAY/...
Issued at 1234 PM CDT Fri Sep 18 2026

A soggy TAF period is expected, characterized by showers,
storms, and MVFR-LIFR ceilings and visibilities. These impacts
are expected across much of the area, including the TAF sites,
with the greatest impacts occurring overnight, generally between
23z-13z. Ceiling heights are expected to be between 0.5-1kft
though some instances below 0.5kft are possible while IFR/LIFR
visibilities are expected in heavier rainfall overnight.
Rainfall largely moves out of the region late Saturday morning
though lingering showers and perhaps some storms should continue
through the afternoon hours (40-60%). Apart from these impacts,
winds will be primarily from the east around 10-15G20-25KT this
afternoon and evening, becoming lighter and more variable
through Saturday morning.

&&

.HYDROLOGY...
Issued at 220 PM CDT Fri Sep 18 2026

Widespread rainfall amounts of 1-3" will likely result in numerous
rivers to rise into action stage in addition to a few sites expected
to rise into minor flood stage such as the Gays Mills, WI and Viola,
WI along the Kickapoo River and Charles City, IA along the Cedar
River. The key question that remains where corridors of locally
heavy rainfall may fall which may result in more significant
flooding depending on the basins they fall into. Basins that see 4-
6"+ of rainfall may see rises in moderate or perhaps major flood
stage (under 10% chance), particularly the quicker responding basins
such as the Trempealeau, Black and Kickapoo River Basins.

As far as urban and flash flooding is concerned, the areas most
susceptible at this time would be along a line from Fillmore County,
MN to Adams County, WI as these areas received 2-3" of rain
earlier this week. In order to get flash flooding, repeat
thunderstorms would be needed given the somewhat progressive
nature of the complex overnight. However, given some tendency in
the HREF PMM to show rainfall amounts in the 4-6" range
overnight in some locations, this coupled with the fact that
6-hr RFC Flash Flood Guidance is around 2-3" would suggest that
flash flooding is feasible where these locally higher amounts
occur. Overall, there has remained a somewhat consistent signal
for the majority of the members to favor these higher amounts
near I-90 in southeast MN and east-southeastward into western
WI. However, given that various CAMs and deterministic guidance
has some deviation by a couple of counties from this
aforementioned axis, have opted to expand the Flood Watch to
include the entire local area near and south of I-94.

&&

.ARX WATCHES/WARNINGS/ADVISORIES...
WI...Flood Watch from 7 PM CDT this evening through Saturday
     afternoon for WIZ032>034-041>044-053>055-061.
MN...Flood Watch from 7 PM CDT this evening through Saturday
     afternoon for MNZ079-086>088-094>096.
IA...Flood Watch from 7 PM CDT this evening through Saturday
     afternoon for IAZ008>011-018-019-029-030.

&&

$$

DISCUSSION...Naylor
AVIATION...Falkinham
HYDROLOGY...Naylor