While most of the destruction from the Aug. 10 derecho was caused by straight-line wind, the National Weather Service now says several tornadoes occurred as well.
In a damage survey released Sunday, the National Weather Service confirmed four brief tornadoes occurred in Marshall and Tama Counties between 11:30 am and 12:03 pm on Aug. 10.
One tornado just northwest of Marshalltown produced EF-1 winds as high as 100 mph. The other three were rated EF-U (EF-Unknown) because these did not strike any structures. Wind speeds on the EF scale are estimated base on damage.
All four tornadoes were only on the ground for three minutes or less.
The National Weather Service discovered these tornadoes among the straight-line wind damage via high resolution satellite imagery and UAS overflights.
27 tornadoes have occurred in Iowa this year. The state averages about 50 tornadoes per year.
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