697
FXUS63 KARX 161917
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
217 PM CDT Wed Sep 16 2026

.KEY MESSAGES...

- Shower and storm chances (40-70%) return tonight into
  Thursday morning with minimal impacts expected.

- Widespread rainfall on Friday and Saturday with rainfall
  amounts of 1-3" trending more likely with locally higher
  amounts possible. Flooding and river rises may occur where the
  heaviest rainfall amounts fall.

- Additional rain chances (20-50%) into early next week with
  cooler than normal temperatures.

&&

.DISCUSSION...
Issued at 217 PM CDT Wed Sep 16 2026

Tonight - Thursday: Rainfall Potential Tonight

Broadly anticyclonic flow continues to move east, allowing for
modest moisture return to the region, even as the frontal boundary
remains shunted well to our south across Central Iowa. This will
continue to build north through the afternoon hours, increasing
pWats, especially along and north of I-90, where they currently sit
at less than 1 inch. A shortwave impulse in the entrance region of a
passing upper level jet streak will likely create a vorticity lobe
and an area of mid-level frontogenesis across Central and South
Central Minnesota this evening. As this interacts with increasing
moisture building from the south, an area of light to moderate
showers is expected to develop/move into the region after 03Z and
into the overnight hours. This should mainly impact areas along and
north of the I-90 corridor. Areas further south across portions of
Northeast Iowa and Southwest Wisconsin may also see some shower and
weak thunderstorm development along a residual outflow boundary from
showers and storms that move through this afternoon and evening. The
bulk of the activity is expected to remain to our south across East
Central Iowa but depending on where this boundary sets up, far
southern portions of our area could see some thunderstorms with this
activity given the modest amount of elevated instability (~500 J/kg)
still expected to be present. Rain looks to be over for most by 12Z
tomorrow morning with dry conditions expected for much of the day on
Thursday. With temperatures in the low to mid 70s, enjoy those dry
conditions because they won`t be lasting for much longer...

Friday - Saturday: Increasing Risk for Heavy Rainfall, Localized
Flooding Possible

A fairly substantial setup for widespread rainfall across the local
area is in store beginning Friday as a shortwave trough pushes off
of the Rockies and into the Great Plains. As this occurs, rapid
surface cyclogenesis occurs in much of deterministic guidance with a
deepening surface low pushing into the Upper Midwest. As this low
approaches, it will be interacting with an already present quasi-
stationary boundary which will allow for a frontal Maddox type
setup to occur with 850mb moisture transport overriding the
front and sloping 600-800mb frontogenesis northwest of the
surface low and just north of the quasi-stationary boundary. In
the presence of this forcing, weak but notable instability in
the GFS/NAM atop this frontogenetic layer will allow for some
convection to develop in this forced regime. Consequently, model
guidance continues to hint at a few county wide corridor of
heavy rainfall where this frontogenesis aloft is maximized north
of the quasi-stationary front. Currently, the main forecast
uncertainty with this conceptual model is the exact position of
the surface low as it tracks eastward and into the region. Given
there is a fairly pronounced high pressure center near the
Hudson Bay that will aid in advecting some dry air into the
northern periphery of this system, the northern extent of how
far heavy rainfall will get remains in question. That being
said, the bulk of clusters in the grand ensemble
(GEFS/EC/Canadian ensemble) keep the axis of the heaviest rain
within the local area, with a tendency for areas near and north
of I-90 at this time with this system.

While exact location for the heaviest rainfall amounts remains
somewhat uncertain, confidence is increasing for heavy rainfall
where the strongest forcing manifests. Probabilities within the
grand ensemble (GEFS/EC/Canadian ensemble) are already very high (60-
80%) for rainfall amounts over 1" within this axis of heavy rain and
modest probabilities for over 2" (50-70%). When looking at the
RRFS which remains the only CAM guidance available at this
point in the forecast cycle, this follows this sentiment with a
broad swath of 1-3" of rain and pockets of localized amounts
over 4", which is to be expected in a convective setup like
this. These locally higher pockets are also illustrated in the
EC Extreme Forecast Index which has a shift of tails of 1 across
the region suggesting some members have rainfall totals
approaching the 99th percentile of model climatology. This is
shown in some EC members which have amounts of 4-7"+ where the
heaviest rain occurs. Consequently, where these higher amounts
do fall, river rises into flood stage will be possible along
with flash flooding where storms frequent the same locations.

Overall, the main state of the forecast at this point is confidence
is increasing for a corridor of heavy rainfall across the region
where 1 to 3 inches of rainfall will be common with locally higher
rainfall amounts of 4 inches or greater possible. This will likely
result in some flooding concerns where this occurs with uncertainty
on the exact location of heaviest amounts. An additional thing to
keep in mind, is that this rainfall will likely be heaviest from
Friday evening and into early Saturday morning, so any flooding
concerns that do take place will likely manifest after dark so
be sure to plan in advance to have multiple ways to receive
warnings during this time period.

Sunday - Next Week: Cooler, Additional Rain Chances

While rainfall from Friday/Saturday exits the area, yet another area
of rainfall looks to fill its footsteps Sunday into Monday. A weak
cutoff low is expected to trek east through through the North
Central CONUS Sunday through early Tuesday. Accelerated
southeasterly flow out ahead of this will provide upper level lift
across portions of the Mid and Upper Mississippi River Valley. This
lift may be enough for surface cyclogenesis to take place with a
weak surface low developing across the Central Plains late Sunday.
While there won`t be as much deep moisture as Friday/Saturday, pWats
still look to be in the 1.25-1.75 inch range for Sunday into Monday.
There is plenty of uncertainty and differing opinions between the
ensemble systems as to how the cutoff low will manifest itself and
what that will mean for rainfall into early next week but for now
will stick with a a broadbrushed 20-55% PoPs Sunday through Monday.
Any additional rainfall after what`s expected Friday into Saturday,
even if it`s not expected to be as heavy or as long-lived, may
exacerbate ongoing areal/river concerns so this is something that
will need to be closely monitored. Ingredients look to be there but
details remain a bit fuzzy. With the cutoff low and expected rain
chances, highs for the first half of next week will be on the lower
side in the 60s, slightly below normal for this time of year.

&&

.AVIATION /18Z TAFS THROUGH 18Z THURSDAY/...
Issued at 1229 PM CDT Wed Sep 16 2026

VFR conditions this afternoon and early evening will give way to
MVFR and eventual IFR conditions overnight as showers and low-level
moisture lower cigs and bring some fog potential to the region.
Showers will likely move into the region near or just after midnight
with MVFR to IFR reductions likely. Cannot rule out a few
thunderstorms given the weak MUCAPE present, however confidence (10-
20%) was too low to include in the TAF. Eventually rain departs
towards daybreak with lingering low-level moisture bringing some
potential for both IFR cigs and vsby reductions during the morning
hours. Winds will generally remain light and out of the east
throughout much of the TAF period.

&&

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

&&

$$

DISCUSSION...Barendse/Naylor
AVIATION...Naylor