By Laura Zuckerman
SALMON, Idaho (Reuters) - A dust storm churned by winds of up to 50 miles per hour forced a 20-mile closure of an interstate in Idaho on Thursday, and highway officials scrambled to divert thousands of motorists near Idaho Falls.
Dirt blowing off cropland drastically reduced visibility for much of the day along a portion of Interstate 15, the north-south route that runs from California to Montana and intersects Salt Lake City and Las Vegas.
The early morning closure ordered by the Idaho Transportation Department was expected to extend into the evening hours and go into effect again on Friday night, when a second Pacific storm front was forecast to unleash high winds.
State highway officials said it was rare for a storm this time of year to sweep large clouds of dirt across the Upper Snake River Plain in the high desert of eastern Idaho.
"This may be the first time we have ever closed the interstate for a dust storm in December," spokesman Bruce King said.
The closure affected a segment of the interstate north of Idaho Falls that serves an average of 21,500 vehicles a day. Motorists were detoured along lesser-trafficked federal highways east of the Snake River.
Blowing dirt occasionally sweeps across the area in the spring and fall when crop fields are tilled. But snow typically prevents a dust-up in the winter.
Thursday's dust storm followed two similar but somewhat less unusual events in November that caused a closure of I-15 near Idaho Falls.
Whirlwinds of dirt the Friday after Thanksgiving turned day into night along that stretch of interstate, causing multiple accidents. The state had to remove abandoned vehicles and clear piles of sediment before reopening the route.
A high-pressure ridge parked for weeks over the Northern Rockies has shielded the region from Pacific storms that normally blanket the ground with snow, said Mike Huston, meteorologist with the National Weather Service in Pocatello.
That ridge has since weakened, opening the way for fronts bringing the high winds behind Thursday's dust storm and one predicted for Friday, he said.
(Editing by Steve Gorman and Cynthia Johnston)