(Adds tornado rating, damages, state of emergency in South Dakota)
By Kevin Murphy
KANSAS CITY, Mo. (Reuters) - A tornado that ripped through Nebraska, injuring 15 people and flattening buildings, may have reached a wind velocity of up to 200 miles per hour (320 kph), the National Weather Service said on Saturday.
It was part of an unusual early autumn storm system that also dropped 3 feet of snow (91 cm) in South Dakota and Wyoming, where a 380-mile (610-km) stretch of Interstate 90 was still closed on Saturday.
The twister struck the small town of Wayne in northeast Nebraska on Friday. It damaged about 10 homes and two dozen businesses in Wayne, according to Nebraska Emergency Management spokesperson Jodie Fawl.
"There are corn and soybean fields littered with debris all over the place from houses and buildings that were damaged," Fawl said.
The tornado produced EF2 to EF3 damage in Wayne and possibly EF4 damage at a business park there, the National Weather Service said. An EF4 tornado has wind speeds of 166 to 200 mph (267 to 320 kph), according to the weather service.
Damage in Wayne will run into the millions of dollars, Mayor Ken Chamberlin said on Saturday. The storm flattened some buildings at the airport and the business park.
"The rebuilding process with be lengthy," Chamberlin said in a statement. "It's really heart-wrenching to see all the damage, but I have all the confidence in this community."
Twisters also swept through parts of Iowa, destroying more than 20 homes and damaging 40 to 60 farms, said Woodbury County Emergency Management Director Gary Brown.
South Dakota Governor Dennis Daugaard declared a state of emergency on Saturday because of heavy snows that snapped tree branches and power lines and closed highways in southwestern South Dakota and northeastern Wyoming on Friday. Scores of motorists were temporarily stranded and thousands of homes without power.
National Guard bulldozers, blowers and crane-equipped vehicles were being deployed to clear snow and remove stuck vehicles, Daugaard said.
Interstate 90 was closed from western South Dakota to northeastern Wyoming, according to transportation departments in both states. Parts of the highway may remain closed until Sunday afternoon, South Dakota officials said.
Roads in Rapid City and other larger towns in southwestern South Dakota also were buried, said Darin Bergquist, the state's transportation secretary.
"Travel is all but impossible and will remain that way well into tomorrow," Bergquist said on Saturday. (Additional reporting by Noreen O'Donnell and Greg McCune; Editing by Vicki Allen and Peter Cooney)