By Laura Zuckerman
Sept 6 (Reuters) - Idaho highway crews worked Friday evening to clean up debris that forced the closure of a major highway near Boise after a powerful thunderstorm the day before brought flash floods, mudslides and golf ball-sized hail.
A "supercell" storm that slammed into southwestern Idaho late Thursday afternoon unleashed wind gusts up to 58 miles per hour (93 kph) near Boise, the capital, and triggered a flood in nearby Owyhee County that swept away a vehicle whose driver was later rescued, said National Weather Service meteorologist Valerie Mills.
"It's the strongest storm I've seen in my 14 years here," said Mills, who is based in Boise.
Supercells are considered the most dangerous of four categories of storms because of the extreme weather they generate. Their massive size and a powerful internal rotation that creates fierce updrafts can spawn high winds, tornadoes, large hail and downpours producing flash floods and mudslides, Mills said.
Landslides northeast of Boise temporarily trapped two vehicles even as downed trees, boulders and torrents of mud pounded a key highway that connects the state capital to the mountain communities of central Idaho, keeping it closed amid clean-up operations expected to wrap up Saturday morning.
The supercell dumped 1.5 inches (3.8 cm) of rain in 30 minutes in Nampa, just west of Boise, and nearly 1 inch (2.5 cm) of rain in 20 minutes on a nearby resort town, Mills said. At its peak, golf ball-sized hail hammered traffic on the only interstate that links Boise to eastern Idaho and Salt Lake City.
The storm, which erupted at about 4:30 p.m. local time on Thursday, smashed records with 7,781 lightning strikes in the 14 counties of southwestern Idaho and southeastern Oregon monitored by the Boise weather service office, she said.
Mills said the weather service on Thursday received two unconfirmed sightings of tornadoes in Idaho, a state that has averaged fewer than a dozen twisters a year since 1953. (Editing by Steve Gorman and Eric Walsh)