Flash Flood Warning 12, NWS Sioux Falls, SD

486 
WGUS73 KFSD 061837
FFSFSD

Flash Flood Statement
National Weather Service Sioux Falls SD
137 PM CDT Sun Sep 6 2026

IAC119-143-MNC105-062100-
/O.CON.KFSD.FF.W.0012.000000T0000Z-260906T2100Z/
/00000.0.ER.000000T0000Z.000000T0000Z.000000T0000Z.OO/
Lyon IA-Osceola IA-Nobles MN-
137 PM CDT Sun Sep 6 2026

...FLASH FLOOD WARNING REMAINS IN EFFECT UNTIL 4 PM CDT THIS
AFTERNOON FOR NORTHEASTERN LYON, NORTHWESTERN OSCEOLA AND SOUTHERN
NOBLES COUNTIES...

At 137 PM CDT, Doppler radar and automated rain gauges indicated
thunderstorms producing heavy rain moving back into the area from the
west, especially near the Minnesota-Iowa state line from Ellsworth to
Bigelow. Between 2 and 3.5 inches of rain have fallen across the
warned area. The expected rainfall rate is 1 to 3 inches in 1 hour.
Additional rainfall amounts of 1.5 to 3 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...
  Worthington, Ellsworth, Little Rock, Rushmore and Bigelow.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

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

Be aware of your surroundings and do not drive on flooded roads.

Flooding is occurring or is imminent. It is important to know where
you are relative to streams, rivers, or creeks which can become
killers in heavy rains. Campers and hikers should avoid streams or
creeks.

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 4350 9605 4360 9605 4367 9563 4367 9549
      4344 9564 4343 9586 4349 9605

FLASH FLOOD...RADAR AND GAUGE INDICATED
EXPECTED RAINFALL RATE...1-3 INCHES IN 1 HOUR

$$

JH

596 
WGUS73 KFSD 062056
FFSFSD

Flash Flood Statement
National Weather Service Sioux Falls SD
356 PM CDT Sun Sep 6 2026

IAC119-143-MNC105-062106-
/O.EXP.KFSD.FF.W.0012.000000T0000Z-260906T2100Z/
/00000.0.ER.000000T0000Z.000000T0000Z.000000T0000Z.OO/
Lyon IA-Osceola IA-Nobles MN-
356 PM CDT Sun Sep 6 2026

...FLASH FLOOD WARNING WILL EXPIRE AT 4 PM CDT THIS AFTERNOON FOR
NORTHEASTERN LYON, NORTHWESTERN OSCEOLA AND SOUTHERN NOBLES
COUNTIES...

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

A Flood Watch remains in effect until 1000 PM CDT Sunday for portions
of Iowa, southwest Minnesota, and southeast South Dakota.

&&

LAT...LON 4350 9605 4360 9605 4367 9563 4367 9549
      4344 9564 4343 9586 4349 9605

$$

JH

Area Forecast Discussion
National Weather Service Sioux Falls SD
1028 AM CDT Sun Sep 6 2026

.KEY MESSAGES...

- Thunderstorms increase in coverage through the morning and
  continuing through the late afternoon. An isolated stronger
  storm could produce ping pong ball size hail and 60-65 mph
  wind gusts.

- Higher risks for locally heavy rain develop by early
  afternoon Sunday, with potential for pockets of 2-4" of rain
  and localized amounts to 6". Localized flash flooding is
  possible into early evening.

- A shifting pattern may bring higher rain chances, slightly
  cooler temperatures, and a severe weather risk by Monday
  evening and early overnight. This risk could continue Tuesday
  afternoon south of I-90.

- Dry and continued above normal temperatures are favored for
  most of the upcoming week.

&&

.UPDATE...
Issued at 1026 AM CDT Sun Sep 6 2026

Ongoing storm development in a corridor of moderate instability
but poor shear will continue to produce very isolated hail and
brief gusty winds. A bigger threat developing will be locally
heavy rainfall with estimated hourly rates as high as 4 to 5"
per hour in a few locations.

Latest CAMS all indicate a corridor of increased potential in SW
Minnesota, NW Iowa, and adjacent areas of SD where rapid
developing and slow moving storms could lead to flash flooding.
For this reason, will issue a flood watch for flash flooding
into the evening.

Depending on cold pool development, this threat could end
earlier than the 03Z expiration time, however several CAMS all
hint at lingering elevated convection on the LLJ into the
evening in NW Iowa.

&&

.DISCUSSION...
Issued at 218 PM CDT Sat Sep 5 2026

THIS AFTERNOON: We're slowly recovering the airmass post convection
this afternoon, though continued overrunning of the convective
outflow to our south has persisted in developing additional
cloud cover in northeast Nebraska and northwest Iowa.
Temperatures will continue to rise and it appears we'll reach
into the upper 80s to lower 90s once again. Across NW Iowa
temperatures have been cooler, but a late day temperature surge
is still possible if clouds clear. Dew points will also recover
slowly and should end up into the upper 60s to 70s by this
evening. The resultant heat index values climb back to the 95 to
102 range in quite a few locations.

TONIGHT: We'll be watching modest theta-e advection kicking off
additional showers and thunderstorms over the western High Plains
later this afternoon and early evening. This activity will
likely struggle as it tries to reach central South Dakota later
this evening. Greater concerns begin to develop after midnight
as the LLJ intensifies. The majority of guidance is now showing
two areas of concern...one along the Minnesota river valley and
Buffalo Ridge where some increased convergence and influence of
mid-lvl vorticity could spark elevated storms. A second area
over western/central Nebraska and central South Dakota where a
potentially stronger mid- lvl area of vorticity enters the state
and begins to slowly drift east northeast. Elevated convection
in both of these areas would favor brief marginally severe hail
and a few 40-60 mph wind gusts. The biggest uncertainty is the
time of development which may range from 4am to 7am.

SUNDAY:  While there isn't a lot of certainty in meteorology, my
confidence is growing that we'll be dealing with some type of
impactful weather during the day.  Already ongoing convection at
daybreak may continue to spread both east (from western complex),
and southeast (from elevated Ridge convection)...an interesting
artifact of being stuck under the mid-lvl ridge axis.  Occasionally
strong updrafts may still produce isolated severe weather through
the morning as this activity meanders. Soundings by mid-day either
side of the I-29 corridor show a nearly uncapped environment which
could signal some potential for ongoing elevated storms to tap into
the surface layer and intensify.  Shear remains quite marginal, but
2000-2500 J/KG MLCAPE and decent mid-lvl lapse rate could suggest
some large hail and downdraft potential.  While extremely low
probabilities...the low LCL and chaotic SFC wind pattern could lead
to a few funnels.  Gradual cold pool development and continued
southeast shift in the upper vort max should push this convective
cluster southeast and out of the CWA by early evening.

Another concern, which may end up being the biggest concern will be
heavy rain.  As evident this morning, the environment overhead
remains rich for locally heavy rainfall with several areas of 4-6"
of rain.   A similar situation will be in place tomorrow with
another deep warm cloud layer, high PWAT, and slow storm motion (AOA
10-15 knots) event.    REFS/HREF 6 hr PMM through 00z all indicate
localized totals of 2-4"...however as shown by last night's
data...the many of the totals were closer to the ensemble max.  In
this case the potential shifts to higher localized values of 3 to
5"+.  All this to say that these high end amounts may be quite
spatially variable which makes pinning down a potential flood
watch more difficult. Areas from Mitchell to Tyndall are by far
the most susceptible to flooding, with 3 hr FFG in most other
areas between 2.5-3.25". Will allow next shift to take a closer
look and try to pin point any areas of concern as data focuses
in on the potential.

SUNDAY NIGHT: Any MCS that develops Sunday afternoon should exit the
area early in the evening leaving subsidence in it's wake. There
still remains a 30-40 knot LLJ that will develop and may produce
scattered convection into the James river valley into Monday
morning.

MONDAY-TUESDAY:  The mid-lvl ridge axis slides east by Monday, with
deeper troughing moving into the PACNW and Northern Rockies.  Lee-
side troughing develops a surface boundary over the western Dakotas
early on Monday with an increasing SPG bringing gusty winds to the
CWA Monday afternoon.  Temperatures may also warm back towards the
90s ahead of the front.  While we'll have to watch return flow and
boundary layer moisture along the boundary, there may be sufficient
pooling of moisture to overcome lingering capping by late in the
afternoon in central South Dakota.  Should storms develop, then a
narrow focus for severe convection may extend into the western half
of the CWA into the early overnight hours. Depending on the extent
of coverage of storms, the aforementioned front may dip into
northern Nebraska and Iowa by Tuesday. Convection may again
develop along this boundary with some increased severe weather
potential focused mostly in the southern third of the CWA.

WEDNEDAY-FRIDAY: The rest of the work week favors slightly cooler
temperatures and drier conditions as high pressure moves into the
Plains. Temperatures will remain slightly above seasonal normals in
the lower to middle 80s.

&&

.AVIATION /12Z TAFS THROUGH 12Z MONDAY/...
Issued at 628 AM CDT Sun Sep 6 2026

A few aviation concerns through the period. First, patchy
stratus and fog along/east of I-29 is leading to IFR and lower
conditions through mid morning. Second, convection is slowly
moving north out of NE into south central SD, with additional
convection trying to develop near the Buffalo Ridge into
northwestern IA. Showers/storms are expected to expand in
coverage through the morning, continuing over the area through
the afternoon. Storms may produce erratic winds and MVFR/IFR
conditions. Locally heavy rain is possible with thunderstorms.
Narrowed timing a bit at all sites, but still some uncertainty
on how quickly storms move in (and therefore out) of the the
area this evening.

Additional isolated to scattered storms possible toward daybreak
Monday, but confidence is too low to include at this time.

&&

.FSD WATCHES/WARNINGS/ADVISORIES...
SD...Flood Watch until 10 PM CDT this evening for SDZ062-067-071.
MN...Flood Watch until 10 PM CDT this evening for MNZ080-081-089-
     090-097-098.
IA...Flood Watch until 10 PM CDT this evening for IAZ001>003-
     012>014-020>022-031-032.
NE...None.

&&

UPDATE...Dux
DISCUSSION...Dux
AVIATION...SG

Hazardous Weather Outlook
National Weather Service Sioux Falls SD
347 AM 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>071-070900-
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-
347 AM 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...Today and tonight.

Patchy fog may reduce visibility below a mile east of I-29 this
morning.

Thunderstorms return to the area by late morning, continuing through
the late afternoon and evening. Isolated strong storms may produce
hail to the size of a ping pong ball and wind gusts to 65 mph.

Locally heavy rainfall is possible Sunday with isolated totals of 2
to 4 inches. Some locations with multiple rounds of thunderstorms
may exceed 4 inches of rainfall through this evening.

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

Hot and humid again for Monday with Heat Index values in the 90s to
near 100 for areas along the southern Missouri River Valley.

Periodic storm chances continue into the middle part of the week.
Isolated severe storms and locally heavy rainfall possible.

.SPOTTER INFORMATION STATEMENT...

Spotter activation may be needed Sunday and again at times early
next week.