513
FXUS63 KARX 181046
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
546 AM CDT Fri Sep 18 2026

.KEY MESSAGES...

- Widespread rainfall is expected Friday into Saturday, heavy at times,
  with totals around 1-4 inches expected, amounts of 5+ inches
  possible (5-15%, mainly in NE IA), and amounts up to around 7"
  cannot be ruled out. Localized flooding and river rises are
  expected where the heaviest rainfall occurs. While the
  location of the heaviest rainfall remains uncertain,
  confidence in occurrence was high enough (50%) to issue a
  Flood Watch including areas along and south of I-90 and north
  of US Highway 18.

- Additional rainfall chances continue into the new week
  (20-50%) with cooler than normal temperatures expected.

&&

.DISCUSSION...
Issued at 1223 AM CDT Fri Sep 18 2026

Widespread rain Friday into Saturday

WV satellite shows and upper high over the ArkLaTex region, a low
moving over northern CA, and a longwave trough advancing over
eastern Canada. Multiple shortwaves are seen within southwesterly
flow aloft from the Four Corners to the northern Plains. At the
surface, lee pressure falls are occurring in NE CO with southerly
return flow in the southern Plains to the east of this feature, a
high is building in western Ontario in the wake of the Canadian
upper trough, and a quasi-stationary front is present roughly along
Interstate 70 where the northeasterly winds associated with the
Ontario high meet the northern extent of the return flow.

Friday into Saturday, aforementioned front will tighten as it is
forced northward by the strong return flow across the central CONUS.
Broad lift across the area looks to occur by mid-morning as a
disturbance ejects over MN so could see a few lighter showers early.
As time goes on, strong return flow will lead to PWATs approaching
2", near the top of climatology. This return flow will also serve to
both force the front northward and tighten the front. As 850/700mb
frontogenesis increases over the CWA in the afternoon, additional
showers will occur. Over time, the front should stall out over the
forecast area and, as a LLJ ramps up during the evening, rounds of
moderate to heavy rain will occur with a few rumbles of thunder as
elevated instability increases. Overall, 17.19z NBM QPF values
suggest most areas have a 50% chance for final rain totals to be
within the 1-4" range and a small (5-20%) chance for more than 5",
mainly in northeast Iowa north of US Highway 18. Taking a look at
the 17.12z HREF and 17.18z REFS to get a sense of high end
possibilities, probability-matched mean values of 5-7+" are seen
between I-90 and a latitude defined by the IA/MN border. Needless to
say, this would lead to flooding issues for affected basins and
affected paved areas, as discussed further in the Hydrology section
below.

Cooler with additional shots at rain next week

The front that fills the focus of all our rain problems Friday
into Saturday looks to be shunted back southward as the Ontario
surface high continues to build. This will lead to cooler
temperatures through next week. Quasi-zonal flow aloft favors
periodic potential for rain as disturbances translate
downstream. With the aforementioned front favored to be to our
south, best chance for additional rain during each of these
periods looks to be in northeast Iowa and southwest Wisconsin.
No periods stand out as having a higher amount of hazard but,
depending on where heaviest rain falls Friday/Saturday, any
additional rain in NE IA/SW WI would be unwelcome and delay
falls in rivers.

&&

.AVIATION /12Z TAFS THROUGH 12Z SATURDAY/...
Issued at 543 AM CDT Fri Sep 18 2026

Low clouds will remain in place, leading to MVFR or lower
flight categories. Rain showers will move in over the region for
much of this 24 hour period, resulting in lower visibility with
the heavier rainfall. Isolated thunderstorms are possible, but
the potential and confidence in a storm being near a terminal
are too low to mention in the TAF at this time. IFR to LIFR
ceilings are likely towards the later half of this TAF cycle.
The wind is forecast to be out of the east with the potential
for gusts to reach 20-25 kts this afternoon into this evening.

&&

.HYDROLOGY...
Issued at 1223 AM CDT Fri Sep 18 2026

With highly moist profiles expected, efficient rain rates
could lead to flooding issues, particularly in paved areas with
known runoff problems, such as Austin MN. As for river flooding,
just a few miles will make all the difference with regards to which
basins are most affected but ensemble river forecasts suggest minor
to moderate flooding would occur in basins where the higher end
amounts discussed above occur with a small (5% or less) chance for
major if high end amounts affect multiple areas within one of the
quicker responding basins (i.e. Trempealeau, Black, or Kickapoo).

Given the slowing movement of the front and the potential for repeat
generation of showers and thunderstorms along it, flooding concerns
in urban areas and within any basins are high enough to warrant a
Flood Watch for Friday evening into early Saturday afternoon. Have
endeavored to include counties with the best signal for greater than
5" of rainfall per 17.12z HREF and 17.18z REFS PMM QPF output,
mainly along and south of I-90 and north US Highway 18. This may
need to be adjusted in space and time as more shorter term guidance
arrives with exact locations remaining uncertain.

&&

.ARX WATCHES/WARNINGS/ADVISORIES...
WI...Flood Watch from this evening through Saturday afternoon for
     WIZ041-042-053>055-061.
MN...Flood Watch from this evening through Saturday afternoon for
     MNZ086>088-094>096.
IA...Flood Watch from this evening through Saturday afternoon for
     IAZ008>011.

&&

$$

DISCUSSION...Ferguson
AVIATION...Muscha
HYDROLOGY...Ferguson