738
FXUS63 KFSD 290942
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
442 AM CDT Tue Sep 29 2026

.KEY MESSAGES...

- A band of showers and isolated thunderstorms will spread east
  across the region through today. Much of the region could see a
  quarter to half inch of rain by this evening, with highest
  amounts near to south of I-90.

- The forecast has trended drier for tonight into early Wednesday,
  but a second round of rain may lift back into areas south of a
  Vermillion SD to Spencer IA line Wednesday afternoon into early
  Thursday. If this round of heavy rain lifts far enough north, it
  will bring a potential for areal flooding and rises on local
  rivers in northwest Iowa.

- Impacts from heavy rain over the next few days are still focused
  across portions of northwest Iowa and northeast Nebraska. Rain
  totals of 1 to 2 inches are most likely by Thursday morning, but
  there is still a low (20%) probability for localized amounts up
  to 3 to 4 inches. A Flood Watch remains in effect.

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

&&

.UPDATE...
Issued at 440 AM CDT Tue Sep 29 2026

Heavy rain potential remains the focus over the next 1-3 days, and
the latest models have trended toward a drier solution for most of
our forecast area. Today into early this evening is still on track
to bring a band of showers and embedded non-severe thunderstorms
east across the region. This portion of the system has trended a
bit faster than was seen in previous days, with the rain moving
east of Yankton to Sioux Falls to Marshall by early-mid afternoon.
Still a chance that far southeast portions of the forecast area
(south of Vermillion SD - Spencer IA) could see lingering showers
tonight into early Wednesday. However, latest models have shifted
the greater potential for heavy rain during this period closer to
I-80 (in contrast to near Highway 20 as was seen at this time
yesterday), which has decreased expected rainfall totals in our
far southeast by an inch or more.

This band of rain is projected to return northward into the far
southeast portions of our area Wednesday afternoon through early
Thursday morning, though low confidence in whether it will remain
south of Sioux City to Storm Lake, or lift farther north into our
northwest Iowa counties. Given this uncertainty, decided to leave
the existing Flood Watch as is for now. If the southern shift in
the storm track persists, the risks of river flooding in our area
will likewise decrease and the watch may be cancelled or adjusted
accordingly.

&&

.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 440 AM CDT Tue Sep 29 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.

Projected rainfall amounts through Thursday have trended downward,
with the most likely solution now remaining less than 2 inches.
This has resulted in a lower probability for Minor flooding to
less than 20% in most locations. However, uncertainty remains in
where a secondary band of heavy rainfall will be focused late
Wednesday into early Thursday, with a high-end rainfall potential
(10% chance of exceedance) still in the 3 to 4 inch range across
the existing Flood Watch area. With this in mind, will keep the
Flood Watch in place for now. However, the potential for river
flooding will ultimately depend on where this axis of heavier
rain falls, with the more likely scenario resulting in ponding of
water in low lying and poor drainage areas.

&&

.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...JH
DISCUSSION...BP
AVIATION...05
HYDROLOGY...JH