665
FXUS63 KARX 171845
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
145 PM CDT Thu Sep 17 2026

.KEY MESSAGES...

- Isolated light showers are possible through this afternoon
  (10-15%), mainly south of I-90.

- Widespread rainfall is expected Friday into Saturday, heavy at
  times, with totals of 1-3 inches expected (30-60%) and amounts
  of 4+ inches possible. Localized flooding and river rises are
  expected where the heaviest rainfall occurs.

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

&&

.DISCUSSION...
Issued at 143 PM CDT Thu Sep 17 2026

Today - Tonight: Mostly dry, Isolated Showers South of I-90

The wave and weak moisture transport responsible for this
morning`s rainfall have largely shifted eastward this afternoon
and along with them, more widespread rainfall. However, can`t
rule out some lingering isolated showers south of I-90 through
the afternoon as lingering low level moisture across the region
along with some weak isentropic ascent on the 305K surface
should be enough for said development. These forcings move east
overnight with cloudy conditions remaining as we head into
Friday morning.

Friday - Saturday: Widespread Rainfall and Flooding Potential

A 500hPa shortwave trough is expected to eject off of the
closed low currently situated across the Pacific Northwest this
afternoon, moving eastward tonight while an associated surface
low moves into the Upper Midwest. The warm front associated with
the surface low moves into the Upper Mississippi River Valley
Friday morning and stalls across the region which will be the
catalyst for a widespread rainfall event across the Upper
Mississippi River Valley Friday into Saturday.

South of this boundary, a very moist airmass is expected to be
in place, characterized by PWATs around 1.7-1.9 inches.
Moisture transport associated with a strengthening 850hPa low
level jet will also nudge into the boundary, ushering the
saturated airmass northward which combined with strong 850hPa
warm air advection and 800-700 frontogenesis should provide
ample forcing for rainfall development. This rainfall could be
fairly efficient at times Friday given the saturated atmosphere
though an overall lack of instability throughout the day should
hinder convectively enhanced rainfall. Heading into the
overnight hours Friday, the aforementioned moisture transport
strengthens further per the 17.15z RAP while MUCAPE increases to
around 250-500 J/kg at which time some convectively enhanced
rainfall and rain rates of 1+ inches per hour would become more
favorable. Storm motion vectors also parallel the boundary
during this time which leads to the potential for training
showers/storms as we head into Saturday morning. Given the
favorable set-up for widespread rainfall which could be heavy at
times, confidence is fairly high that amounts should be around
1-3 inches (30-60%) though locally higher amounts of 4+ inches
are possible, especially where convectively enhanced
precipitation occurs.

While confidence in efficient rainfall and a few inches of
rainfall is fairly high, the location of said heaviest rainfall
is less certain with a couple scenarios in place. The first and
higher confidence scenario (70-80%) is that the axis of
heaviest rainfall develops generally along and south of I-90
where the various CAMs, and by association the 17.12z HREF, are
currently in the best agreement as to where convectively
enhanced rainfall is most likely to develop overnight Friday
into Saturday. The second scenario brings the heaviest rainfall
a bit further north and a bit further south with those along
I-90 largely being spared the heaviest rainfall (20-30%). While
confidence is higher in the first scenario, some of the CAM
guidance does suggest higher rainfall rates are favored across
northeast Iowa which may result in this axis tending towards the
second scenario. Overall, it appears to be more of a question
of where will the axis of highest rainfall amounts occur rather
than a question of if it will occur.

From an impacts standpoint, areal flooding and river rises are
likely for areas that receive the greatest rainfall amounts and
for those areas that received more abundant rainfall amounts
earlier this week. Flash flooding is possible, although this
threat appears to be greatest overnight Friday into Saturday
when convectively enhanced rainfall and training storms are
favored. For additional information, please see the Hydrology
section.

Sunday - Early Next Week: Additional Rainfall and Cooler

The upper wave responsible for the widespread rainfall moves
east of the region Sunday while the aforementioned upper low
begins to shift eastward across the northern United States. The
quasi-stationary boundary responsible for the rainfall Friday-
Saturday is currently expected to remain stalled across
northeast Iowa into southwest Wisconsin which should result in
some additional showers and storms along the boundary as the
upper low continues eastward. The upper level pattern remains
fairly active through the beginning of the new week while the
aforementioned boundary remains situated across northeast Iowa
and southwest Wisconsin, resulting in additional chances for
showers and storms (20-50%).

Apart from the rainfall potential, cooler temperatures are on
the horizon with highs topping out in the upper 50s to low 60s
for the end of the weekend and into next week per the 17.13z
NBM, generally 5 to 10 degrees below normal for this time of
year.

&&

.AVIATION /18Z TAFS THROUGH 18Z FRIDAY/...
Issued at 1241 PM CDT Thu Sep 17 2026

MVFR to IFR cigs this afternoon will improve to VFR cigs by
evening with continued VFR conditions overnight. As we head into
the morning hours on Friday, rain will overspread the region
with ensuing MVFR cigs and vsbys. Based on the HREF and the
potential for some heavier rainfall rates, cannot rule out
(10-30% chance) for some IFR vsbys reductions towards 18z as
well. However, the bulk of IFR reductions likely will not occur
until after 18z on Friday. Winds will remain light from the
northeast for much of the TAF period before shifting east and
increasing to around 10-15 kts towards 18z on Friday.

&&

.HYDROLOGY...
Issued at 143 PM CDT Thu Sep 17 2026

A prolonged period of efficient rainfall is anticipated Friday
into Saturday, beginning Friday morning. While the exact axis of
where the heaviest rain will ultimately situate itself is
uncertain, confidence is medium to high that a widespread 1-3
inches of rain will occur across the Upper Mississippi River
Valley (30-60%) though areas of 4+ inches are possible where
storms frequent which may lead to some instances of areal
flooding.

Rises on area rivers are expected given the most likely amounts
of 1-3 inches, some of which may approach action to minor flood
stage. River flooding would become much more likely where
showers/storms frequent and higher totals are realized.

Flash flooding is somewhat less of a concern as rain rates
should be on the lower side, especially early in the event.
However, any convectively enhanced areas of heavy rain will
certainly pose a flash flood risk, primarily overnight Friday
into Saturday when rainfall rates of 1+ inch per hour are
favored. If the current trends continue, a Flood Watch will
likely be needed.

&&

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

&&

$$

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