A flurry of unexpected tornadoes swept through central Iowa, injuring at least 17 people, flattening buildings in three cities and forcing the evacuation of a hospital yesterday.

Residents - and even weather forecasters - were taken by surprise as the tornadoes hit Marshalltown, Pella and Bondurant. Ten people were injured in Marshalltown and seven at a factory near Pella, but no deaths were reported.

Alex Krull, a meteorologist with the National Weather Service in Des Moines, said forecasting models produced yesterday showed only a slight chance of strong thunderstorms later in the day.

"This morning, it didn't look like tornadic supercells were possible," Krull said. "If anything, we were expecting we could get some large hail, if strong storms developed."

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Marshalltown, a city of 27,000 people about 80km northeast of Des Moines, appeared to have been hit the hardest. Brick walls collapsed in the streets, roofs were blown off buildings and the cupola of the historic courthouse tumbled 55m to the ground.

National Weather Service meteorologist Rod Donavon said two primary storms spawned the series of tornadoes. One developed in the Marshalltown area, causing damage there, while the other started east of Des Moines and travelled through Bondurant and into Pella.

- AP