At least five people dead after mass casualty pileup on Interstate 90 in Montana

At least five people died after a dust storm fueled by wind gusts topping 60mph caused a pileup on Friday evening on the Interstate 90 freeway in Montana, authorities said.

At least 20 vehicles crashed. Sgt Jay Nelson of Montana highway patrol said authorities believed weather was the cause.

“It appears as though there was heavy winds, causing a dust storm with zero visibility,” he said.

The highway patrol did not have an immediate count of the number of injuries, but Nelson said additional ambulances had to be called in from Billings to help.

The governor, Greg Gianforte, said on Twitter: “I’m deeply saddened by the news of a mass casualty crash near Hardin. Please join me in prayer to lift up the victims and their loved ones. We’re grateful to our first responders for their service.”

The incident happened three miles (5km) west of Hardin.

But the dust storm’s roots could be traced back several hours, when storms popped up in central southern Montana between 1-2pm and slowly began moving east, according to Nick Vertz, a National Weather Service meteorologist in Billings.

A so-called “outflow” or a surge of wind that is produced by storms but can travel faster than them flew east-south-east about 30 miles ahead of the storms, Vertz said.

A 40mph (64km/h) gust of wind was recorded at the nearby Big Horn County airport at 4.15pm. The crash was reported to the highway patrol at 4.28pm.

By the airport weather station’s next reading at 4.35pm, the gusts had picked up to 62mph. Another reading 20 minutes later recorded a gust of 64mph.

The wind easily picked up dust – a product of recent temperatures into the 90s Fahrenheit (30s Celsius) and triple digits over the past week – and reduced visibility to less than a quarter mile (0.4km).

“If they looked up in the sky while they’re in Hardin, they probably didn’t see much of what you’d think of for a thunderstorm cloud, maybe not even much at all,” Vertz said. “It was just a surge of wind that kind of appeared out of nowhere.”

As first responders attempt to clear the wreckage, the meteorologist said they could expect to be safe from additional winds and thunderstorm activity.

“It should be a relatively clear, calm night for them.”