Powerful storms in Sierra prompt avalanche warning

A powerful series of storms that began over the weekend and continued to drop heavy powder on weak layers of the snowpack Tuesday could trigger avalanches in the Sierra backcountry, forecasters warned.

The likelihood of an avalanche was expected to increase from “considerable” to the most extreme level — “high” — by Tuesday night, according to an advisory released by the Sierra Avalanche Center.

The warning, which will last until at least Thursday, covers all elevations. The avalanche center only deals with backcountry territory, not ski resorts.

A snow covered cabin in the Sierras seen at Phillips, California, on Tuesday January 3, 2017. A snow covered cabin in the Sierras seen at Phillips, California, on Tuesday January 3, 2017. Photo: Michael Macor, The Chronicle Buy photo Photo: Michael Macor, The Chronicle Image 1 of / 10 Caption Close Powerful storms in Sierra prompt avalanche warning 1 / 10 Back to Gallery

Since Sunday, the Sierra has gotten 20 to 30 inches of snow, and another 2 to 4 feet were expected to fall between Tuesday and Thursday. Gusts above 100 mph have pounded ridge tops, and winds across the mountains were forecast to increase overnight Tuesday.

An expected temperature change in the storm also spelled trouble, said Steve Reynaud, a forecaster with the Sierra Avalanche Center.

“The best way for a storm to come in is to start off warm and become cold, to have the lighter snow on top of the heavier snow,” he said. “This is the opposite.”

Avalanche forecasters call this type of storm an “upside-down cake,” Reynaud said.

By Monday, the monitoring center had already recorded numerous human-triggered avalanches on Elephant’s Hump in the Carson Pass area. Many of the avalanches were caused by wind slabs, when gales deposit snow on a leeward slope.

A winter storm warning will be in effect until Thursday at 4 a.m. Travel through the mountains will be “extremely dangerous,” said Zach Tolby, a National Weather Service meteorologist.

On top of the storm, rainfall Dec. 15 froze and created a coat of ice — now buried 2 to 4 feet beneath the snowpack — which looms as a problem spot that could give way and trigger large and destructive avalanches, forecasters said. Additionally, weak snow fell over the so-called rain crust in the days after the storm, Reynaud said.

“There’s a structural weakness in the snowpack,” said Ethan Gicker of the U.S. Forest Service. “I would highly recommend people not to go into the backcountry right now. Everyone should stay home and stay safe.”

A 64-year-old man from Aptos (Santa Cruz County) was killed in an avalanche last month while skiing at Mt. Rose Ski Tahoe. His body was found Dec. 11 buried beneath 8 to 10 feet of snow, officials said.

In February, an avalanche near Alpine Meadows ski resort struck a bus and pushed it 30 feet into a house. Later that day, a driver going up to a nearby ski resort was struck by a smaller, secondary avalanche. No one was injured in either incident, but the house was damaged.

Kimberly Veklerov is a San Francisco Chronicle staff writer. Email: kveklerov@sfchronicle.com Twitter: @kveklerov