Flash Flood Warning 15, NWS Sioux Falls, SD

145 
WGUS73 KFSD 070935
FFSFSD

Flash Flood Statement
National Weather Service Sioux Falls SD
435 AM CDT Mon Sep 7 2026

IAC149-193-071100-
/O.CON.KFSD.FF.W.0015.000000T0000Z-260907T1100Z/
/00000.0.ER.000000T0000Z.000000T0000Z.000000T0000Z.OO/
Plymouth IA-Woodbury IA-
435 AM CDT Mon Sep 7 2026

...FLASH FLOOD WARNING REMAINS IN EFFECT UNTIL 6 AM CDT EARLY THIS
MORNING FOR SOUTHEASTERN PLYMOUTH AND NORTHEASTERN WOODBURY
COUNTIES...

At 435 AM CDT, Doppler radar and automated rain gauges indicated
thunderstorms producing heavy rain across the warned area. Between 3
and 4.5 inches of rain have fallen. Additional rainfall amounts up to
0.5 inches are possible in the warned area. Flash flooding is ongoing
or expected to begin shortly.

HAZARD...Flash flooding caused by thunderstorms.

SOURCE...Radar and automated gauges.

IMPACT...Flash flooding of small creeks and streams, urban areas,
         highways, streets and underpasses as well as other poor
         drainage and low-lying areas.

Some locations that will experience flash flooding include...
  Kingsley, Correctionville and Pierson.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

Turn around, don't drown when encountering flooded roads. Most flood
deaths occur in vehicles.

Be especially cautious at night when it is harder to recognize the
dangers of flooding.

Please report observed flooding to local emergency services or law
enforcement and request they pass this information to the National
Weather Service when you can do so safely.

&&

LAT...LON 4257 9613 4267 9606 4258 9586 4256 9586
      4256 9583 4252 9574 4243 9579 4249 9599

FLASH FLOOD...RADAR AND GAUGE INDICATED

$$

SG

921 
WGUS73 KFSD 071051
FFSFSD

Flash Flood Statement
National Weather Service Sioux Falls SD
551 AM CDT Mon Sep 7 2026

IAC149-193-071101-
/O.EXP.KFSD.FF.W.0015.000000T0000Z-260907T1100Z/
/00000.0.ER.000000T0000Z.000000T0000Z.000000T0000Z.OO/
Plymouth IA-Woodbury IA-
551 AM CDT Mon Sep 7 2026

...FLASH FLOOD WARNING WILL EXPIRE AT 6 AM CDT EARLY THIS MORNING FOR
SOUTHEASTERN PLYMOUTH AND NORTHEASTERN WOODBURY COUNTIES...

The heavy rain has ended. Flash flooding is no longer expected to
pose a threat. Please continue to heed remaining road closures.

&&

LAT...LON 4257 9613 4267 9606 4258 9586 4256 9586
      4256 9583 4252 9574 4243 9579 4249 9599

$$

SG

Area Forecast Discussion
National Weather Service Sioux Falls SD
1241 AM CDT Mon Sep 7 2026

.KEY MESSAGES...

- Scattered showers and thunderstorms will continue for areas of
  northwest Iowa through roughly 4 am Monday. Showers should
  remain below severe criteria.

- Isolated to scattered morning showers are possible along and
  north of I-90. An isolated storm could become strong to severe
  producing hail the size of a quarter and 60 mph gusts.

- Isolated strong to severe storms will again be possible late
  Monday afternoon into Tuesday morning. Large hail up to the
  size of a ping-pong ball, wind gusts of 60 mph, and heavy
  rain are all possible. Though the risk is low, an isolated
  tornado is also possible.

- Quiet but continued warm conditions are expected for the rest
  of the work week.

&&

.UPDATE...
Issued at 934 PM CDT Sun Sep 6 2026

An area of convergence along a surface boundary will keep
showers and thunderstorms in the forecast for much of the
overnight. Storms will most likely remain along and south of
I-90, but it is possible a few may sneak farther north,
especially along and east of I-29. These storms are not expected
to be severe, however they could still produce 40-50 mph wind
gusts and brief periods of heavy rain.

Through the overnight southerly 850 mb winds advect in a plume of
warm and moist air. By sunrise a marginally unstable environment
will be in place with a low amplitude wave approaching out of the
southwest and an increasing LLJ. This could trigger a morning
round of elevated showers and thunderstorms if a parcel can lift
off the 850-700 mb layer. Wide spread severe weather is not
anticipated, however one or two storms may become strong to
severe producing hail to the size of a quarter and wind gusts to
60 mph. These should stay along and north of I-90 as they
progress to the east. Timing is most likely between 10 am and 3
pm Monday.

Meanwhile, Theta-e continues to advect in forming a deep ridge over
central and eastern South Dakota north to the Canadian border.
Instability continues to increase ahead of deepening trough over the
eastern Rockies. This trough ejects out onto the Plains in the early
afternoon and a surface low pressure forms. The LLJ increases in
response to a tightening SPG. By mid-to-late afternoon the cap west
of the James River will have mixed out, and the low and associated
frontal boundary will be at our western CWA border. Models vary a
bit but in general expect 1500-2500 J/kg MUCAPE, 35-45 kts of Bulk
shear, marginal mid-level lapse rates, but steep 7+ C low level
lapse rates to be in place by this time. As the boundary sweeps
through the LLJ continues to increase to 50 kts. Initial convection
has the potential to be strong to severe producing large hail the
size of a half dollars up to ping-pong balls, heavy rainfall, and
wind gusts to 60 mph. A sharp clock wise curve to the hodograph and
enhanced stretching potential combined with surface to 1 km SRH of
195 m2/s2 and surface to 3 km SRH of 300+ m2/s2 all indicate the
potential that upon convection initiation a storm or two could
produce a tornado. This probability is low, but still needs to be
mentioned. Timing of greatest severe risk looks to be between 6pm
Monday and 1 am Tuesday. Storm mode looks to be initially super
cellular with deep layer shear vectors nearly perpendicular to the
front. Over time the vectors become orientated at a 45 degree
angle, which would likely result in supercells growing into a
broken line.

Certainty in the late afternoon/evening storms is low to medium, due
to the fact that morning convection may work over some of the
environment. Depending on when those storms and clouds clear out
will impact how well the atmosphere recovers.

&&

.DISCUSSION...
Issued at 1246 PM CDT Sun Sep 6 2026

THIS AFTERNOON: Strong convection continues to develop in areas
along but mostly east of I-29 this afternoon.  The area continues to
reside in a moderately unstable environment, but lacks any
meaningful shear and mid-lvl lapse rates.  However, climatologically
significant PWATs are in place, with enough DCAPE to suggest an
increased risk for wet microbursts over the next 1-3 hours in a
narrow corridor from Madison to Sioux Falls, Sioux Center, Cherokee,
Storm Lake, Le Mars, and Sioux City. Rainfall may be the most
impactful hazard this afternoon with 1-2 hr totals of 3-4" already
in portions of southern Minnesota.  Additional convection is likely
to continue to produce 2-4" totals in pockets of NW Iowa, though
cold pool development may help push storms off an area before
extreme values may be produced.  The stronger MCV will also nudge
it's way northward later this afternoon, and may work to produce
additional northward/northeastward convection with it.

TONIGHT: This shortwave/MCV will drag it's feet eastward this
evening, with some expected weakening of convection with the lost of
daytime heating and general stabilization of the environment nearby.
However, an increased LLJ may begin to develop after 02Z which
could bring returning elevated showers and isolated
thunderstorms to the CWA late this evening and continuing into
the overnight hours. This activity will likely not be severe,
but any shower could still bring brief heavy rain.

MONDAY-TUESDAY: Elevated shower activity diminishes towards daybreak
as shortwave ridging arrives. An ejecting trough out of the Rockies
will bring additional elevated convection into central and northern
South Dakota into Monday afternoon, perhaps grazing the western CWA.
As the EML gradually erodes into south central SD late in the
afternoon, a few strong to severe storms may develop and track
east southeast into the early overnight hours. As the LLJ
increases after dark, elevated convection may expand in coverage
with continued low- end severe weather risks for Tuesday
morning.

More uncertainty for Tuesday, which is focused moreso on the
location of the surface front and impacts of overnight elevated
convection.  While the primary surface front settles in northwest
Iowa, a secondary stronger dew point gradient arrives later in the
afternoon. Should sufficient clearing develop, then a storm or two
could become stronger by mid-afternoon with activity quickly
tracking southeast by the evening.

EXTENDED: With ongoing convection, no forecast changes made to the
extended as a temporarily reprieve from active weather
develops. Temperatures may still remain near to above normal
for the rest of the work week.

&&

.AVIATION /06Z TAFS THROUGH 06Z TUESDAY/...
Issued at 1238 AM CDT Mon Sep 7 2026

Scattered light showers are ongoing over northwest Iowa. These will
taper off over the next few hours. VFR conditions for the overnight
except for areas in southwest Minnesota where winds have become
light enough for patchy fog to form. Fog will dissipate as southeast
winds increase through 07.12Z. Gusts west of I-29 will be 20-27 kts,
while gusts to the east will be 20 kts or less. Breezy winds expand
east through the afternoon with gusts of 20-30 kts with a few gusts
to 33 kts possible over south central South Dakota. Winds remain
gusty through the end of the period.

We'll be watching for two rounds of showers and thunderstorms
tomorrow. The first round looks to begin around 07.15Z through the
late morning to early afternoon. This round looks to remain mostly
north of I-90. Widespread severe weather is not expected, however an
isolated storm may become strong to severe producing 1 inch hail and
52 kt winds. The second round looks to begin in the late afternoon
to early evening. This round brings increased potential for severe
weather. Initial storm mode over central South Dakota will most
likely be supercells. These could produce hail up to the size of a
ping-pong ball, heavy rain, and wind gusts to 60 mph. Though the
risk is low an isolated tornado is also possible. As storms approach
the I-29 corridor they may begin to grow upscale into a broken line
to a QLCS. Hail threats diminish to marginally severe but wind risks
increase. Isolated brief spin ups and weak tornadoes are also
possible along bowing sections. Storms will be ongoing east of I-29
through the end of the period.

&&

.FSD WATCHES/WARNINGS/ADVISORIES...
SD...None.
MN...None.
IA...None.
NE...None.

&&

UPDATE...AJP
DISCUSSION...Dux
AVIATION...AJP

Hazardous Weather Outlook
National Weather Service Sioux Falls SD
944 PM CDT Sun Sep 6 2026

IAZ001>003-012>014-020>022-031-032-MNZ071-072-080-081-089-090-097-
098-NEZ013-014-SDZ038>040-050-052>071-080245-
Lyon-Osceola-Dickinson-Sioux-O'Brien-Clay-Plymouth-Cherokee-
Buena Vista-Woodbury-Ida-Lincoln-Murray-Cottonwood-Nobles-Jackson-
Pipestone-Rock-Dixon-Dakota-Beadle-Kingsbury-Brookings-Gregory-
Jerauld-Sanborn-Miner-Lake-Moody-Brule-Aurora-Davison-Hanson-McCook-
Minnehaha-Charles Mix-Douglas-Hutchinson-Turner-Bon Homme-Yankton-
Union-
944 PM CDT Sun Sep 6 2026

This Hazardous Weather Outlook is for northwest Iowa, west central
Iowa, southwest Minnesota, northeast Nebraska, central South
Dakota, east central South Dakota, south central South Dakota and
southeast South Dakota.

.DAY ONE...Tonight.

Scattered showers and a few thunderstorms will continue into the
overnight hours.  Severe storms are not expected.

.DAYS TWO THROUGH SEVEN...Monday through Saturday.

Tuesday morning a round of showers and thunderstorms are possible
along and north of I-90. While the risk is low, an isolated storm may
become strong to severe producing hail to the size of a quarter and
60 mph.

A second round of strong to severe storms is possible again Monday
late afternoon continuing into the overnight. Some of these may be
severe producing hail up to the size of a ping-pong ball, 60 mph wind
gusts, and heavy rainfall. Though the risk is low, an isolated
tornado is also possible.

.SPOTTER INFORMATION STATEMENT...

Spotter activation may be needed.