Alert archive / Iowa / Flash Flood Warnings in 2026

Flash Flood Warning 2, NWS Sioux Falls, SD

May 17, 2026Plymouth County, Sioux County, Iowa

NWS Sioux Falls, SD issued this flash flood warning at May 17, 2026, 8:03 PM CDT for Plymouth County, Sioux County, Iowa. It was in force for 3 hr. 2 reports came from inside the area while it was in force.

The area

The outline as issued, with what was reported inside it.

The facts

Issued: May 17, 2026, 8:03 PM CDT (01:03 UTC)
Expired: May 17, 2026, 11:15 PM CDT
In force: 3 hr
Issued by: NWS Sioux Falls, SD
Area: Storm-based polygon
Flood severity: None

Counties Plymouth County, IA, Sioux County, IA

The radar

The Sioux Falls radar (KFSD), 90 km from the middle of the area, began a volume at 01:03 UTC, the scan nearest the moment this went out.

Open the radar archive at this moment

The archived warnings, watches and discussions in force at that minute are drawn over the scan, this one among them.

What was reported inside it

Time	Report	Where	Remark
May 17, 8:16 PM CDT	FLASH FLOOD	1 Wsw Le Mars, IA	911 dispatch reports a couple of different flooded roads in Le Mars including portions of Holton Dr as well as 5th Ave.
May 17, 8:16 PM CDT	FLASH FLOOD	1 Wsw Le Mars, IA	911 dispatch reports a couple of different flooded roads in Le Mars including portions of Holton Dr as well as 5th Ave.

Local storm reports filed while the alert was in force, from inside its polygon or its counties.

Did it verify

Verified. The Iowa Environmental Mesonet's Cow matched one storm report to this flash flood warning; the first came 13 minutes after it went out.

Cow counts a report within 15 km of the polygon while the warning was in force: a tornado for a tornado warning; hail of 1 inch or more, a gust of 58 mph or more or wind damage for a severe thunderstorm warning; a flash flood report for a flash flood warning.

The nearest sounding before it

Omaha, NE (KOAX), launched 00:00Z, 1.1 hours before the alert went out and 174 km from the middle of its area. This is the routine balloon launch nearest before it, the air the forecaster had in hand, not a measurement taken for the alert.

Skew-T log-p

Temperature in red, dew point in green, the mixed-layer parcel dashed.

Hodograph

The wind turning with height, colored by layer: ground to 1 km, 1 to 3 km, 3 to 6 km, 6 to 10 km.

Mixed-layer CAPE: 2478 J/kg
Mixed-layer CIN: -15 J/kg
Mixed-layer LCL: 1258 m above ground
0-6 km bulk shear: 34 kt
0-1 km storm-relative helicity: 178 m²/s²
0-3 km storm-relative helicity: 298 m²/s²
Significant tornado parameter: 2.0
Supercell composite: 17.9

Open this sounding in Storm Lab

The text

As transmitted.

255 
WGUS53 KFSD 180103
FFWFSD
IAC149-167-180415-
/O.NEW.KFSD.FF.W.0002.260518T0103Z-260518T0415Z/
/00000.0.ER.000000T0000Z.000000T0000Z.000000T0000Z.OO/

BULLETIN - EAS ACTIVATION REQUESTED
Flash Flood Warning
National Weather Service Sioux Falls SD
803 PM CDT Sun May 17 2026

The National Weather Service in Sioux Falls has issued a

* Flash Flood Warning for...
  North Central Plymouth County in northwestern Iowa...
  South Central Sioux County in northwestern Iowa...

* Until 1115 PM CDT.

* At 803 PM CDT, Doppler radar indicated thunderstorms producing 
  heavy rain across the warned area. Between 2 and 3 inches of rain 
  have fallen. Additional rainfall amounts up to 1 inch are possible 
  in the warned area. Flash flooding is ongoing or expected to begin 
  shortly.

  HAZARD...Flash flooding caused by thunderstorms.

  SOURCE...Radar.

  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...
  Le Mars, Maurice, Struble and Brunsville.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

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

&&

LAT...LON 4300 9617 4295 9608 4288 9608 4281 9611
      4277 9618 4275 9626 4280 9627 4289 9621
      4297 9622

FLASH FLOOD...RADAR INDICATED

$$

Kalin

2 follow-up statements

994 
WGUS73 KFSD 180204
FFSFSD

Flash Flood Statement
National Weather Service Sioux Falls SD
904 PM CDT Sun May 17 2026

IAC149-167-180415-
/O.CON.KFSD.FF.W.0002.000000T0000Z-260518T0415Z/
/00000.0.ER.000000T0000Z.000000T0000Z.000000T0000Z.OO/
Plymouth IA-Sioux IA-
904 PM CDT Sun May 17 2026

...FLASH FLOOD WARNING REMAINS IN EFFECT UNTIL 1115 PM CDT THIS 
EVENING FOR NORTH CENTRAL PLYMOUTH AND SOUTH CENTRAL SIOUX 
COUNTIES...

At 904 PM 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. Minimal additional rainfall is 
expected.

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...
  Le Mars, Maurice, Struble and Brunsville.

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.

&&

LAT...LON 4300 9617 4295 9608 4288 9608 4281 9611
      4277 9618 4275 9626 4280 9627 4289 9621
      4297 9622

FLASH FLOOD...RADAR AND GAUGE INDICATED

$$

Kalin

428 
WGUS73 KFSD 180411
FFSFSD

Flash Flood Statement
National Weather Service Sioux Falls SD
1111 PM CDT Sun May 17 2026

IAC149-167-180421-
/O.EXP.KFSD.FF.W.0002.000000T0000Z-260518T0415Z/
/00000.0.ER.000000T0000Z.000000T0000Z.000000T0000Z.OO/
Plymouth IA-Sioux IA-
1111 PM CDT Sun May 17 2026

...FLASH FLOOD WARNING WILL EXPIRE AT 1115 PM CDT THIS EVENING FOR 
NORTH CENTRAL PLYMOUTH AND SOUTH CENTRAL SIOUX COUNTIES...

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

&&

LAT...LON 4300 9617 4295 9608 4288 9608 4281 9611
      4277 9618 4275 9626 4280 9627 4289 9621
      4297 9622

$$

Meyers

What forecasters said that day

The local office's own words before this flash flood warning went out, as it sent them, from the Iowa Environmental Mesonet's archive of National Weather Service text.

NWS Sioux Falls, SD wrote its last area forecast discussion before this flash flood warning went out at May 17, 6:53 PM CDT, 1 hour 10 minutes earlier. Its key messages:

There is high confidence that severe thunderstorms will develop this afternoon and evening, producing large hail, destructive straight-line winds, and a few tornadoes. An Enhanced Risk (level 3 of 5) of severe storms is in place.
A transitioning storm mode will shift hazards from large hail and tornadoes initially, to widespread wind damage as an intense line of storms tracks east of Interstate 29 this evening.
A lower risk of severe weather exists late Monday afternoon and evening, mainly over northwest IA, before quieter weather moves in for the mid-week period.

Area forecast discussion, May 17, 6:53 PM CDT, 1 hour 10 minutes before

Area Forecast Discussion
National Weather Service Sioux Falls SD
Issued by National Weather Service Rapid City SD
653 PM CDT Sun May 17 2026

.KEY MESSAGES...

- There is high confidence that severe thunderstorms will
  develop this afternoon and evening, producing large hail,
  destructive straight-line winds, and a few tornadoes. An
  Enhanced Risk (level 3 of 5) of severe storms is in place.

- A transitioning storm mode will shift hazards from large hail
  and tornadoes initially, to widespread wind damage as an
  intense line of storms tracks east of Interstate 29 this
  evening.

- A lower risk of severe weather exists late Monday afternoon
  and evening, mainly over northwest IA, before quieter weather
  moves in for the mid-week period.

&&

.DISCUSSION...
Issued at 120 PM CDT Sun May 17 2026

The short-term convective outlook presents a highly volatile
spring severe weather setup. Latest surface analysis shows a
complex pattern, anchored by a strong 994 mb low over northwest
KS. An inverted trough extended north of the low through
central SD, while a well-defined warm front stretched northeast
of the low into east central NE. A weaker low was also noted in
north central NE. Visible satellite imagery revealed stable
wave clouds advancing northeast over far northeast NE and
extreme SE SD, ahead of the advancing warm front. Regional
radar imagery showed a complex of strong to severe storms over
north central NE. This storm complex is forecast by short range
guidance to continue tracking east this afternoon, intensifying
as interacts with the richer low- level moisture pool near and
south of the warm front.

Moderate to strong instability will be in place over the
southern to eastern CWA by late afternoon with MLCAPE nearing
3000 J/kg. In addition, deep layer shear will increase to around
50 kt, coincident with a mid level speed max ejecting from the
deep upper trough to our west. As the system evolves over the
next several hours, it appears the ingredients for a significant
severe weather event will be most favored over the southern to
eastern CWA. Ahead of the main convective line, some CAMs show
supercell development along the northward advancing warm front.
These storms would present a risk of very large hail (2"+) and
tornadoes. Given the amount of low level SRH/curved hodographs
present, a couple strong tornadoes are possible. The convective
cluster currently entering our far southwest CWA should
strengthen as it shifts east, with an initial hail and tornado
threat transitioning to a damaging straight line wind threat
(70+ MPH) as it grows upscale into a squall line/QLCS. Some
line-embedded QLCS tornadoes are possible, especially for any
line segments that become oriented north/south or
northwest/southeast, given southwesterly 0-3 km shear vectors.
Timing from latest high-res guidance suggests this activity
exits to the east of our CWA late this evening.

Precipitation amounts will vary significantly based on convective
tracks, though NBM guidance and HREF probability match mean
fields project a widespread 0.50 to 1.50 inches of rainfall,
particularly across northwest Iowa and southwest Minnesota. On
Monday, the cold front finally pushes into the area. Instability
and shear are forecast to be much lower than today, however,
there will be a conditional risk of severe storms over the
eastern CWA during the late afternoon to evening, if the airmass
can destabilize sufficiently ahead of the front. SPC's Day 2
outlook indicates as Marginal Risk (level 1 of 5), for our MN
and IA counties.

Behind the departing upper wave, quasi-zonal flow sets up for
Tuesday and Wednesday, yielding quiet conditions and seasonally
cool high temperatures moderating from the upper 50s Tuesday to
mid-upper 60s Wednesday.

The next upper wave moves in for the Thursday to Friday period,
however Gulf moisture remains cutoff ahead of this system.
Evaluating the ECMWF Extreme Forecast Index (EFI) reveals no
significant climatological anomalies for precipitation or wind
across the upper Midwest during this period, reinforcing the
the idea of a low-impact system for our area. The current
forecast sticks closely to the NBM consensus, maintaining broad
20-30 percent probabilities for light rain for late Thursday
into Saturday.

By late this week and especially this weekend, temperatures
will embark on a steady upward trajectory, as the mid level flow
backs west, then southwest. The Climate Prediction Center (CPC)
6-10 day outlook favors above-normal temperatures across the
Northern Plains and Upper Midwest, which supports blended
guidance high temperatures reaching well into the 80s on
Sunday.

&&

.AVIATION /00Z TAFS THROUGH 00Z TUESDAY/...
Issued at 650 PM CDT Sun May 17 2026

Showers and storms will continue to move eastward into MN through
the evening hours, the forecast area should be largely clear of
shower/storm activity after ~05z. Intermittent MVFR/IFR conditions
in heavy rainfall as well as hail and strong winds will accompany
the strongest storms. Widespread MVFR/IFR cigs will set in overnight
and persist through much of Monday.

&&

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

&&

DISCUSSION...ILX/25
AVIATION...Wong

Hazardous weather outlook, May 17, 1:29 PM CDT, 6 hours 34 minutes before

Hazardous Weather Outlook
National Weather Service Sioux Falls SD
Issued by National Weather Service Lincoln IL
129 PM CDT Sun May 17 2026

IAZ001>003-012>014-020>022-031-032-MNZ071-072-080-081-089-090-097-
098-NEZ013-014-SDZ038>040-050-052>071-181830-
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-
129 PM CDT Sun May 17 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...This afternoon and tonight.

A significant severe weather event is possible this afternoon and
evening. Very large hail, damaging wind gusts, and tornadoes are all
possible. If you have outdoor plans know where your nearest shelter
is located. Please ensure you have multiple ways to receive warnings.
Have a plan to take shelter and act immediately if your area comes
under a severe weather warning.

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

Strong to severe storms are possible again Monday afternoon and
evening, particularly over northwest Iowa and southwest Minnesota.

.SPOTTER INFORMATION STATEMENT...

Spotter activation will likely be needed this afternoon and evening.

In the hours before

Local storm report, May 17, 7:36 PM CDT, 27 minutes before

Preliminary Local Storm Report
National Weather Service Sioux Falls SD
736 PM CDT Sun May 17 2026

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0731 PM     Tornado          Salix                   42.31N 96.29W
05/17/2026                   Woodbury           IA   Emergency Mngr

            Emergency manager reports tornado near
            Salix.

&&

ak

Local storm report, May 17, 7:39 PM CDT, 24 minutes before

Preliminary Local Storm Report
National Weather Service Sioux Falls SD
739 PM CDT Sun May 17 2026

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0729 PM     Tstm Wnd Gst     2 WSW Salix             42.30N 96.32W
05/17/2026  M69 MPH          Woodbury           IA   Public

            Via personal weather station. Exact time
            estimated via radar.

&&

ak

Local storm report, May 17, 7:45 PM CDT, 18 minutes before

Preliminary Local Storm Report
National Weather Service Sioux Falls SD
745 PM CDT Sun May 17 2026

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0643 PM     Hail             3 SW Craig              42.87N 96.35W
05/17/2026  M1.75 Inch       Plymouth           IA   Public

            Photo via social media. Time estimated via
            radar.

&&

ak

Local storm report, May 17, 7:47 PM CDT, 16 minutes before

Preliminary Local Storm Report
National Weather Service Sioux Falls SD
747 PM CDT Sun May 17 2026

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0736 PM     Tstm Wnd Gst     4 NNE Sergeant Bluff    42.46N 96.31W
05/17/2026  M66 MPH          Woodbury           IA   Public

            Via personal weather station. Exact time
            estimated via radar.

&&

ak

Local storm report, May 17, 7:55 PM CDT, 8 minutes before

Preliminary Local Storm Report
National Weather Service Sioux Falls SD
755 PM CDT Sun May 17 2026

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0748 PM     Tstm Wnd Gst     Lawton                  42.48N 96.19W
05/17/2026  M67 MPH          Woodbury           IA   Public

            Via personal weather station.

&&

ak

Local storm report, May 17, 7:56 PM CDT, 7 minutes before

Preliminary Local Storm Report
National Weather Service Sioux Falls SD
756 PM CDT Sun May 17 2026

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0749 PM     Tstm Wnd Gst     Moville                 42.49N 96.06W
05/17/2026  M63 MPH          Woodbury           IA   Public

            Via personal weather station.

&&

ak

The office that day

Everything else NWS Sioux Falls, SD issued on the same UTC day.

Issued	Product	Office	Counties	Lasted
May 17, 7:20 PM CDT	Severe Thunderstorm Warning	FSD	Plymouth, Sioux	28 min
May 17, 7:20 PM CDT	Severe Thunderstorm Warning	FSD	O'Brien, Osceola	26 min
May 17, 7:26 PM CDT	Tornado Warning	FSD	Woodbury	49 min
May 17, 7:32 PM CDT	Severe Thunderstorm Warning Considerable damage threat	FSD	Cherokee, Ida, Plymouth, Woodbury	58 min
May 17, 7:47 PM CDT	Tornado Warning	FSD	Cherokee, Plymouth, Woodbury	43 min
May 17, 8:00 PM CDT	Severe Thunderstorm Warning	FSD	O'Brien, Osceola, Plymouth, Sioux	45 min
May 17, 8:21 PM CDT	Severe Thunderstorm Warning Destructive damage threat	FSD	Buena Vista, Cherokee, Clay, Dickinson, Ida, O'Brien and 3 more	54 min
May 17, 8:26 PM CDT	Tornado Warning	FSD	Buena Vista, Cherokee, Clay, O'Brien	28 min
May 17, 8:48 PM CDT	Tornado Warning	FSD	Clay, Dickinson	27 min
May 17, 8:54 PM CDT	Tornado Warning	FSD	Clay, Dickinson	51 min
May 18, 5:11 PM CDT	Severe Thunderstorm Warning	FSD	Cherokee, Ida, Plymouth, Woodbury	49 min
May 18, 5:48 PM CDT	Severe Thunderstorm Warning Considerable damage threat	FSD	Buena Vista, Cherokee, Clay, O'Brien	42 min
May 18, 6:08 PM CDT	Severe Thunderstorm Warning	FSD	Plymouth, Sioux	37 min

Nearby

Flash Flood Warnings in Iowa, 2026 Every flash flood warning in 2026

Sources

The product and its follow-ups from the Iowa Environmental Mesonet's archive of National Weather Service and Storm Prediction Center text; local storm reports from the same archive; the sounding from the Mesonet's radiosonde archive; radar from the NEXRAD Level II archive.