Utah winter storm brought rare ‘thunder snow’: What is it, and how does it happen?

Many areas got hit with a foot or more of snow, prompting school closures and delays.

(Chris Samuels | The Salt Lake Tribune) A downed tree branch takes down a power line from heavy snow in Salt Lake City, Wednesday, Dec. 15, 2021.

A snowstorm that dumped up to a foot of snow in Utah’s valleys overnight and about twice that in the mountains created hazardous road conditions and closed several schools on Wednesday morning.

The storm, which had been forecast for days, arrived as expected. The surprise came when lightning flashed across the sky and thunder rolled as snow fell.

It’s called thunder snow — a weird but not unprecedented phenomenon, according to the National Weather Service.

“Thunder snow is not common here at all,” said Jon Wilson, a National Weather Service meteorologist in Salt Lake City. “Sometimes we’ll go a whole winter without any. I would say, on average, it happens less than one time per winter.”

“It’s very rare,” Wilson continued, “but it’s the same process that results in thunder and lightning when we’re used to it.”

That process involves a clash of competing weather systems, Wilson said. Temperatures reached the mid-50s in Utah on Tuesday afternoon, and “when that front came through, it dropped us down into the 20s and 30s really quickly,” he continued.

When it’s warmer, there’s more moisture in the air, Wilson said, creating the potential for cloud development. The stronger the clouds build up, the more potential there is for thunder, lightning, heavy rain and, in some cases, heavy snow this time of year.

“Normally, we don’t have the ingredients in the wintertime that we do in the summer,” Wilson said. “That’s why lightning and thunder happen way less often in the winter.”

The overnight storm prompted several school districts to close campuses, switching instead to virtual classes, while others saw one- and two-hour delayed start times.

Utah Highway Patrol troopers also responded to more than 150 wrecks and about 500 instances of vehicles sliding off the road Wednesday or needing other help.

Two Weber State police officers were hospitalized Wednesday after they stopped on Interstate 15 to help a car that had wrecked in snowy conditions near 600 South in Salt Lake City. An oncoming car struck a vehicle behind the car that officers were assisting, causing a chain reaction that resulted in serious but survivable injuries for the officers.

Road conditions improved throughout the day Wednesday. But more snow is expected Thursday — a 90% chance, with accumulations of 1-3 inches possible. Highs in the upper 20s are expected.

Dozens of flights also were delayed or canceled at Salt Lake International Airport. Officials urged passengers to check the status of their flights before they head to the airport this week.

Snowfall totals:

According to the weather service, snow totals around the state Wednesday included:

• Snowbird — 21 inches.

• Alta — 19 inches.

• Brighton — 16 inches.

• Cottonwood Heights — 15 inches.

• Tooele — 15 inches.

• Solitude — 13 inches.

• South Jordan — 12.5 inches.

• Bountiful Bench — 12 inches.

• Salt Lake City — 12 inches.

• University of Utah — 12 inches.

• Taylorsville — 10.9 inches.

• Payson — 9.5 inches

• Kearns — 9.5 inches.

• Sandy — 9.5 inches.

• West Valley City — 9.2 inches.

• Hill Air Force Base — 8 inches.

• Logan — 8 inches.

• Beaver — 3.5 inches.

— Tribune reporters Kolbie Peterson and Connor Sanders contributed to this story.