152
FXUS63 KFSD 290550
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
1250 AM CDT Tue Sep 29 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.

&&

.UPDATE...
Issued at 852 PM CDT Mon Sep 28 2026

Taking a look across the area, we`re continuing to see showers and
thunderstorms develop across central SD this evening mainly in
response to increasing lift and a strengthening nocturnal LLJ (30-
40kts). While this activity will likely continue to lift across the
same areas for the next couple of hours, should see most of this
activity start to work its way into southeastern SD right around
midnight as surface high pressure pushes further into the Great
Lakes Region. Made some slight tweaks to short-term POPs to better
refine time of arrival into our western-most zones and southcentral
SD. From here, the forecast remains on track with light to moderate
showers and isolated thunderstorms gradually progressing through the
area with a weakening cold front for most of the day on Tuesday.

While additional rounds of shower and potentially thunderstorms will
be possible between Wednesday and Thursday; current trend continue
to point to our far southeastern zones being the main area for any
additional accumulations. With this in mind, our flood watch looks
to be in great shape. Lastly, overall accumulations should continue
to trend down with better dynamics going just to our south and
southeast with the front. Nonetheless, while most areas should
receive between 0.25" to 0.75" of an inch over the next few day;
can`t rule out a narrow pocket of 1-2+ inches somewhere in the area
outlined in our Flood watch.

&&

.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 /06Z TAFS THROUGH 06Z WEDNESDAY/...
Issued at 1242 AM CDT Tue Sep 29 2026

Mainly VFR conditions will transition to a mix of IFR to LIFR
towards the end of the TAF period. Taking a look at satellite
imagery, we`re continuing to see cloud cover increase over our
western-most counties mainly in response to the developing
showers across central SD. Should see this activity gradually
work its way eastwards ahead of a cold front through the
morning hours of Tuesday. Pockets of MVFR to IFR vsbys will be
possible at times as rainfall rates intensify. However, MVFR to
IFR ceilings are also expected behind the developing precipitation
shield and should persist into Tuesday night. Otherwise, increasing
southerly winds will become more light and variable by Tuesday
night leaving room for some widespread fog development. Locally
dense patches could lead to IFR to LIFR vsbys at times.

&&

.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 this evening through Thursday evening for
     IAZ014-021-022-031-032.
NE...Flood Watch from this evening through Thursday evening for
     NEZ014.

&&

$$

UPDATE...05
DISCUSSION...BP
AVIATION...05
HYDROLOGY...Kalin