As a strong upper level wave pushed into the western Dakotas during the evening hours, isolated supercellular storms developed across central South Dakota and moved through areas west of the James River. With 2000-2500 J/kg of MLCAPE and deep layer shear of 35 to 45 knots, the isolated storms produced hail up to golf ball size. Soon, an increasing amount of DCAPE of 1000-1250 J/kg and increase in 0-3 km shear with development of a strong low-level jet aided in the transition to enhanced linear structures, which then developed a series of mesovortices as the line moved into far eastern South Dakota, southwest Minnesota and northwest Iowa. Several brief tornadoes developed with mesovortices in southwest Minnesota and northwest Iowa, along with areas of enhanced wind damage.

Hail in DICKINSON, IOWA
From 17-AUG-19 22:24:00 to 17-AUG-19 22:26:00 (CST-6)
Location: LAKE PARK
Magnitude: 0.88 
Reported by: Emergency Manager

Tornado in LYON, IOWA
From 17-AUG-19 21:34:00 to 17-AUG-19 21:43:00 (CST-6)
Location: LOSTER
Tornado F-scale: EF1
Property damage: 200.00K
Reported by: NWS Storm Survey

A tornado touched down 5 miles west of Rock Rapids, just east of Fig Avenue north of 160th Street, marked by damage to a corn field. The tornado traveled in a general east-southeast direction, striking a farmstead and flattening a silo within the first half-mile of its track.  Within another minute, the tornado slashed through another farmstead, causing significant damage to a grain bin, large trees, and removed roofing from a home and a livestock shed.  Around three miles west-southwest of Rock Rapids, several power transmission poles were broken.  As the tornado passed just south of Rock Rapids, damage was mainly to trees, with several uprooted.  The final structural damage occurred south of Rock Rapids just east of U.S. Highway 75 with roof panels torn off another livestock shed.  The tornado lifted just over 2 miles southeast of Rock Rapids after crossing County Road K52.  The average path width was 50 yards.

Thunderstorm Wind in OSCEOLA, IOWA
From 17-AUG-19 22:01:00 to 17-AUG-19 22:03:00 (CST-6)
Location: SIBLEY
Magnitude: 65.00 EG
Property damage: 75.00K
Reported by: Public

Several grain bins were damaged, and large road signs were bent over on the south exit to Sibley from Iowa Highway 60.

Thunderstorm Wind in OSCEOLA, IOWA
From 17-AUG-19 22:01:00 to 17-AUG-19 22:03:00 (CST-6)
Location: SIBLEY
Magnitude: 61.00 EG
Property damage: 50.00K
Reported by: Social Media

A metal shed was damaged, along with numerous trees down.

Thunderstorm Wind in OSCEOLA, IOWA
From 17-AUG-19 22:02:00 to 17-AUG-19 22:04:00 (CST-6)
Location: SIBLEY MUNI ARPT
Magnitude: 74.00 EG
Property damage: 100.00K
Reported by: Public

Severe damage done to several grain bins. The bolts holding the empty bins in place were broken off and the bins were lifted or shifted from their original position.  The top collapsed on the northwest side of the empty bins.

Thunderstorm Wind in DICKINSON, IOWA
From 17-AUG-19 22:30:00 to 17-AUG-19 22:32:00 (CST-6)
Location: WAHPETON
Magnitude: 65.00 EG
Property damage: 10.00K
Reported by: NWS Storm Survey

Winds caused roof damage to a small barn.

Thunderstorm Wind in DICKINSON, IOWA
From 17-AUG-19 22:32:00 to 17-AUG-19 22:34:00 (CST-6)
Location: WAHPETON
Magnitude: 56.00 EG
Reported by: NWS Storm Survey

Large branches, up to 3 inches in diameter, were snapped from trees.

Thunderstorm Wind in DICKINSON, IOWA
From 17-AUG-19 22:34:00 to 17-AUG-19 22:37:00 (CST-6)
Location: OKOBOJI
Magnitude: 81.00 EG
Property damage: 250.00K
Reported by: NWS Storm Survey

A widespread swath of wind damage occurred from the eastern shores of West Okoboji Lake to the eastern shore of East Okoboji Lake.  Areas near West Lake Okoboji featured limb and minor building damage with wind speeds of 65 to 75 mph. However, more significant damage occurred east of Highway 71 where large trees were snapped and an office building severely damaged with wind gusts from 75 to 93 mph.