February 24, 2024

Whereas we had been celebrating the top of a sunny New Yr’s Eve weekend, the snow within the Sierra Nevada all of the sudden turned as brittle and spiky because the outdated ice cream in your freezer.

That Jan. 1 phenomenon — when snow vaporized upward at evening, refreezing right into a layer of fragile crystals — is blamed for this month’s scary proliferation of avalanches which can be occurring throughout a broad swath of the Lake Tahoe panorama, together with one which killed a skier at Palisades Tahoe.

There have been greater than 50 to this point, and it could worsen: This weekend’s rain and heavy snow might add weight and stress to the buried crystals, often known as “floor hoar,” which act like weak stilts and simply collapse. Over time, the issue could ease. Or it might plague our snowpack for a number of weeks, a number of months, and even all the season.

“Simply one other reminder — issues aren’t the Tahoe norm,” wrote Jan Czyzewski of Blackbird Mountain Guides in a report final week to the Sierra Avalanche Heart, after witnessing indicators of 5 completely different avalanches, together with one which was 500 ft broad, whereas snowboarding in Ward Canyon on Tahoe’s West Shore.

Climate on the evening of Jan. 1 supplied the proper recipe for the creation of floor hoar: Clear and chilly skies, excessive relative humidity and calm winds, in keeping with Brandon Schwartz, director and lead avalanche forecaster for the Sierra Avalanche Heart in rugged Tahoe Nationwide Forest.

Then it snowed. On Jan. 2, the layer of delicate crystals was buried beneath recent powder. One other weak layer, far deeper, was created by frozen rain on Dec. 19.

On Thursday, the Sierra Avalanche Heart warned that “triggering a big avalanche stays attainable immediately the place weak snow is buried beneath the snow floor. “Indicators of unstable snow is probably not apparent,” it added. “With every passing storm, these weak layers turn into buried deeper within the snowpack.”

Since Jan. 2, 52 avalanches have been noticed within the Central Sierra, together with the lethal Palisades slide and quite a few much less publicized shut calls. This compares to 36 avalanches throughout the identical time interval final 12 months and 10 in 2022, though the Heart’s newly improved reporting system could have helped increase this 12 months’s depend.

On Monday, two avalanches had been reported within the wilderness past Kirkwood and a 3rd in Euer Valley, north of Truckee. A number of different avalanches, each pure and human-triggered, had been reported within the Relay Peak space close to Incline Village.

Twelve avalanches had been documented on Sunday, together with a skier-triggered slide in a bowl close to Soda Springs that despatched snow sliding 500 ft and a snowmobile-triggered, 200-foot broad break throughout a chute above Homewood.

“I’ve by no means felt whoomphing like I did immediately,” when the higher snowpack collapsed on Sunday because of a fractured buried snow layer, in keeping with a report filed to the Avalanche Heart by an unnamed skier at Highland Lakes, past Bear Valley. “A number of occasions we felt whole slopes whoomph, nearly reminding us of an earthquake because it looks like all the slope moved.”

One giant collapse close to Stevens Peak, east of Kirkwood, prompted a slide the dimensions of a soccer discipline, reported Travis Feist of the Avalanche Heart.

Such risks are acquainted in Colorado and different Rocky Mountain states, in keeping with Clinton Alden of the College of Washington, who research snowpack evolution and ice layer formation in seasonal snow cowl.

In comparison with the maritime mountains alongside the West Coast, the Rockies have a colder and shallower snowpack, which is extra prone to develop layers of instability, mentioned Alden. Colorado suffered 64 avalanche fatalities between 2013-14 and 2022-23, in comparison with 9 in California.

Floor hoar types commonly within the Sierra, mentioned Schwartz. However it’s often destroyed by wind and warming air temperatures previous to new snowfall.

How do clear chilly nights and shallow snow improve threat?

Floor hoar is created when water vapor is deposited on the snow floor, like dew. The vapor shortly turns into ice, skipping the liquid part utterly, mentioned Alden. It’s an atmospheric occasion, reworking the floor of outdated snow right into a crystalline layer that lacks cohesion and energy.

“It’s analogous to the feather-like crystals that develop in your freezer,” mentioned Neil Lareau, a professor of atmospheric science on the College of Nevada in Reno. If shortly buried by the brand new snow, these crystals turn into a hidden peril.

A secondary and lesser wrongdoer was the creation of free, dry and sugary “faceted” crystals in clear and chilly climate, additionally buried by snow on Jan. 2, mentioned Schwartz. These fragile “aspects” are made when molecules of water vapor transfer by the snow, re-aligning themselves,and are largely prone to kind in shallow snow.

as a result of there’s a dramatic temperature gradient within the quick distance between the bottom and air, in keeping with Alden. The bottom shortly loses warmth at evening — and if skies are cloudless, it cools nonetheless quicker. This course of is way slower when snowpack is deep.”]

Our famed “Sierra cement” can set up a protecting basis for all subsequent winter storms. It’s heavy and considerable, with six-armed stellar flakes which can be giant, sturdy, interlocked and cohesive.