In the wake of excessive overnight rainfall, another round of thunderstorms developed along a cold frontal boundary across northwest Iowa. Deep unidirectional wind shear of 50-60 knots and up to 2000 J/kg MLCAPE with strong frontal forcing yielded a linear structure.  Large DCAPE contributed to outflow development, and as storm orientation evolved, a localized QLCS tornado developed near Superior.

Thunderstorm Wind in O'BRIEN, IOWA
From 20-SEP-18 15:56:00 to 20-SEP-18 15:59:00 (CST-6)
Location: HARTLEY
Magnitude: 61.00 EG
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Emergency Manager

Extensive tree damage occurred at a property on the south side of Hartley, with numerous large limbs in excess of 6 inches in diameter broken and a few trees split due to strong winds. A two-story home experienced broken windows and some minor roof and siding damage.

Thunderstorm Wind in CLAY, IOWA
From 20-SEP-18 16:15:00 to 20-SEP-18 16:17:00 (CST-6)
Location: WEBB
Magnitude: 56.00 EG
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Public

A large tree was toppled by thunderstorm winds.

Thunderstorm Wind in IDA, IOWA
From 20-SEP-18 16:15:00 to 20-SEP-18 16:18:00 (CST-6)
Location: IDA GROVE
Magnitude: 56.00 EG
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Public

Several large trees were blown down around town.

Thunderstorm Wind in DICKINSON, IOWA
From 20-SEP-18 16:20:00 to 20-SEP-18 16:23:00 (CST-6)
Location: TERRIL
Magnitude: 61.00 EG
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Emergency Manager

The roof was blown off the VFW building. A large tree was also broken in half.

Thunderstorm Wind in DICKINSON, IOWA
From 20-SEP-18 16:21:00 to 20-SEP-18 16:23:00 (CST-6)
Location: FULLER FLD ARPT
Magnitude: 56.00 EG
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Emergency Manager

A large tree was down blocking the southbound lane of U.S. 71.

Thunderstorm Wind in DICKINSON, IOWA
From 20-SEP-18 16:22:00 to 20-SEP-18 16:25:00 (CST-6)
Location: FULLER FLD ARPT
Magnitude: 61.00 EG
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Numerous 8 to 10 inch diameter limbs down.

Thunderstorm Wind in CLAY, IOWA
From 20-SEP-18 16:15:00 to 20-SEP-18 16:17:00 (CST-6)
Location: (3SE)SPENCER MUNI AR
Magnitude: 50.00 MG
Property damage: 0.00K
Crop damage: 0.00K
Reported by: ASOS

Thunderstorm Wind in CLAY, IOWA
From 20-SEP-18 16:26:00 to 20-SEP-18 16:28:00 (CST-6)
Location: GILLETT GROVE
Magnitude: 52.00 EG
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Public

Tree limbs down.

Tornado in DICKINSON, IOWA
From 20-SEP-18 16:21:00 to 20-SEP-18 16:26:00 (CST-6)
Location: SUPERIOR
Tornado F-scale: EF1
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Emergency Manager

A tornado spun up near the apex of a QLCS segment southwest of Superior, touching down with minor crop damage south of Iowa Highway 9 east of 300th Avenue.  The tornado tracked quickly to the east-northeast, crossing Highway 9 just west of Superior and causing its most significant damage (rated EF1) to an office building at an ethanol plant. Winds were measured to 99 mph at a nearby weather station at the plant, which displaced walls and partially collapsed some of the second story of the office building, as well as removing sections of the roof.  The tornado then turned to a slightly more eastward track across the far northern edge of Superior where it continued to cause damage to trees and damaged or destroyed several small outbuildings.  The tornado then tracked east of Superior causing damage to larger trees and crops until crossing over into Emmet county around one-half mile north of 150th Street.