773
FXUS63 KFSD 281906
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
206 PM CDT Mon Sep 28 2026

.KEY MESSAGES...

- Widespread showers and isolated thunderstorms will spread east
  through the region late tonight through Tuesday. While severe
  weather is not expected, much of the region will see a quarter
  to  half inch of rain by Tuesday evening.

- From Tuesday night through early Thursday, additional rounds
  of rain will impact areas near and southeast of Lake Andes SD
  to Madison SD to Pipestone MN line. Prolonged rainfall on top
  of recent rainfall has potential to bring areal flooding and
  rises on local rivers.

- Impacts from heavy rain over the next few days are poised to
  be greatest in portions of northwest Iowa and northeast
  Nebraska, where 2 to 4 inches of rain is likely by Thursday
  morning. A Flood Watch has been issued for this area.

- Drier weather prevails Thursday through Friday and again early
  next week.

&&

.DISCUSSION...
Issued at 145 PM CDT Mon Sep 28 2026

LATE THIS AFTERNOON: Stratus is showing some breaks and lifting
in the clouds through early this evening, more than previously
anticipated. More sunshine filtering through may allow temperatures
to rise into the low to mid 70s.

TONIGHT AND TUESDAY: Cloudy, mild, rainy weather takes hold tonight
through at least early Wednesday across the region. Redeveloping and
thickening evening cloud cover will keep temperatures mild
overnight, only dropping to around 60 degrees. Broad warm air
advection ahead of a northern stream shortwave will prompt scattered
showers and isolated storms to slowly spread east from central SD
toward the I-29 corridor overnight. A low level pool of moisture up
to about 3000ft may bring drizzle ahead of the main area of rain,
but it`ll take the sfc low and associated low level jet tracking
east overnight to saturate dry mid levels. Showers should hold off
near and east of the I-29 corridor until Tuesday morning, spreading
east through the day. Despite marginally supportive deep layer shear
(25-40 kts), weak instability (<500 J/kg) should inhibit severe
weather threat. Locally heavy rainfall is the main concern thanks to
our prolonged wet pattern. High confidence (>80%) of most sites
northwest of a Sioux City to Spencer line will see at least a
quarter inch of rain Tuesday. Most higher resolution model guidance,
including the HREF LPMM, suggests a few sites will see a 1+ inches
of rain by Tuesday evening.

TUESDAY NIGHT THROUGH WEDNESDAY NIGHT: Mid week brings increased
threat for heavy rain, localized ponding of water on roads and other
surfaces, and rises on local rivers as an additional few rounds of
rain progress through the region. Early Tuesday evening, rain should
sag southeast further into IA, providing some breaks in the rain.
Overnight through early Thursday, a broad region of widespread,
persistent showers and isolated thunderstorms is expected to
overspread much of NE, IA, and portions of far southeast SD and
southern MN. During this period, stationary front stalls amidst
broad ascent from the right entrance region of the upper jet and the
remnants of Hurricane Polo track into the Central U.S. Of particular
note is the low level jet feeding in moisture-rich, monsoonal air
with precipitable water values of 1.75-2.00, near the max of
climatology for this time of year.

From Tuesday through Thursday morning, an area near and south of a
Norfolk to Sioux City to Spencer line has a high (>70%+) chance of
receiving at least 2 inches of rain with potential for a few
locations to see 4 to 5 inches. Recent model runs continue to wobble
the heaviest axis of rain anywhere from the Hwy 20 to the I-80
corridor. This area with the heaviest, most persistent rains
near/south of a Sioux City to Spencer line has the greater potential
for any flooding impacts. There is definitely uncertainty in the
northern extent of the rain as well. While areas near and south of
Lake Andes to Madison and Pipestone line (including Yankton, Sioux
Falls, and Worthington) have a high (>60%) chance of receiving over
an inch of rain from tonight through Thursday morning, locations
further northwest will be mostly done with rain by Tuesday night,
missing the later rounds.

THURSDAY THROUGH THE WEEKEND: A strong northern stream cold front
tracking through late Wednesday night kicks the Desert Southwest
trough and associated stationary front eastward heading into
Thursday. Dry air filtering in and surface high pressure passing to
our north behind the departing system should bring a mostly dry
Thursday and Friday with increasing sunshine. Thursday brings a
breezy north wind with cooler air, but we`ll see highs hovering
around 60 degrees both days.

An early look at next weekend shows little to no notable weather
impacts. Expect mostly dry conditions and a slight warm up for
Saturday. A passing northern stream front on Saturday night brings a
very low chance of rain north of I-90, but the main impact looks to
be a cool down for Sunday.

&&

.AVIATION /18Z TAFS THROUGH 18Z TUESDAY/...
Issued at 1258 PM CDT Mon Sep 28 2026

Early this afternoon, 700 to 1500 ft stratocumulus lingers
across the region with growing breaks in the clouds. Most model
guidance favors some lifting of ceilings to a scattered MVFR or
low VFR deck late this afternoon but have low confidence in
exactly how quickly that will occur.

VFR conditions are likely to prevail this evening. Showers with
occasional thunder and MVFR to IFR stratus will spread eastward
into the region overnight, mainly impacting Huron and Sioux
Falls by 12z and lingering in the region through much of the
morning. South winds will prevail through the period, gusting
into the 20s through early this evening and weakening overnight.

&&

.HYDROLOGY...
Issued at 205 PM CDT Mon Sep 28 2026

Precipitation thus far through September has been as much as
150-400% of normal, largely erasing the previously existing drought
and resulting in wet soils across the broader region. With the
forecast for an additional 2+ inches from eastern Nebraska into
Iowa, at least some concerns for flooding begins to be introduced.
HEFS guidance suggests between a 10-40% chance of Minor flooding
along the lower Big Sioux, Floyd, and Little Sioux rivers. The NWM
guidance is supportive of this idea as well with Annual Exceedance
Probability (AEPs) of 20-50% (suggesting Action to Minor stage).
While the more common impacts will likely be ponding of water in low
lying and poor drainage areas, some potential for river flooding
also exists but will ultimately be dependent on where the axis of
heavier rain falls.

&&

.FSD WATCHES/WARNINGS/ADVISORIES...
SD...None.
MN...None.
IA...Flood Watch from Tuesday evening through Thursday evening for
     IAZ014-021-022-031-032.
NE...Flood Watch from Tuesday evening through Thursday evening for
     NEZ014.

&&

$$

DISCUSSION...BP
AVIATION...BP
HYDROLOGY...Kalin