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Deadly Lake Tahoe avalanche: Snow drought likely contributed, scientists say
Some scientists said a snow drought likely contributed to a deadly avalanche near Lake Tahoe. They said the risk remains high for another avalanche to hit the Sierra.
LAKE TAHOE, Calif. - A snow scientist says a snow drought likely contributed to this week’s deadly avalanche in the Sierra Nevada, where eight people died and one is still missing, and warned the risk remains high for another slide.
Craig Clements, a meteorology professor at San Jose State University who has conducted avalanche research, said California had gone more than a month without snowfall, leaving the snowpack firm.
"In California, we have not had any snowfall for the last month plus, OK? And so the snowpack was very firm," Clements said on Thursday.
What creates an avalanche?
He said new snow falling on top of that hardened layer can create unstable conditions.
"And so that snow, new fallen snow, sits on a very firm layer. And it doesn’t have the time to bond well. And so it is already unstable," he said.
He added that the risk of an avalanche is highest in the first two days after a snow drought ends.
Authorities have not yet said what triggered this week’s deadly avalanche in the Sierra Nevada.
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Deadly trip
Fifteen people embarked on a three-day backcountry trip to Castle Peak near Lake Tahoe in the Sierra Nevada on Sunday. They skied to remote huts at 7,600 feet in Tahoe National Forest, carrying their own food and supplies. At 6:49 a.m. that day, the Sierra Avalanche Center issued an avalanche watch for the area, indicating that large slides were likely in the next 24 to 48 hours.
The group decided to return home on Tuesday, when an avalanche fell at 11:30 a.m. Eight were killed. One is still missing. Six people survived.
What made conditions so dangerous?
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When the weather is dry and clear, as it has been in the Sierra Nevada since January, snow crystals change and can become angular or round over time, Clements said.
If heavy new snow falls on the crystals, the layers often can’t bond and the new snow forms what is called a storm slab over a weaker layer.
"Because it’s on a mountain, it will slide," when it’s triggered by any change in the tension above or below, sometimes naturally but also because of people traversing the area, Clements said.
If there had been more consistent snowfall throughout the winter, different layers could have bonded more easily, Clements said. But even when a snow slab forms, the danger often only lasts a couple of days until the new snow stabilizes, he said.
Was climate change a factor?
Climate change can lead to weather extremes that include both drought and heavier precipitation, and scientists are studying how it might affect avalanches and where they occur.
Warmer-than-usual temperatures contributed to a snow drought throughout the West. But Clements said this week’s avalanche is fairly typical for California’s Sierra Nevada, and he doesn’t believe it can be linked to climate change.
Avalanches are a mechanism of how much snow falls on weak or stable layers, and this one was "a meteorological phenomenon, not a climate phenomenon," he said.
About 3 feet to 6 feet of snow has fallen since Sunday, when the group started its trip. The area was also hit by subfreezing temperatures and gale force winds. The Sierra Avalanche Center said the threat of more avalanches remained and left the snowpack unstable and unpredictable.