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. Residents and even weather forecasters were taken by surprise Thursday 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 Thursday morning 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.