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Sunday, December 12, 2010
Avalanche alert: a look at the first layer

A rain induced wet slab avalanche in Heather Canyon at Mt. Hood Meadows.

What's not to love about waking up to the first snowfall of the winter? Even if you paid royally for bald tires and crashed your car on the way to work, the sensation of fresh snow between your fingers is a tremendous feeling after a long hot summer. That three inches of October fluff/slush on your doorstep is a welcome sign of good days ahead.

The only problem is that those first innocent inches can wreak havoc on the overall snowpack stability later in the winter. Early season snowfall has a tendency to morph into faceted snow -- a dangerous and rotten breed of snow crystal that is responsible for dozens of avalanche accidents each winter. When faceted snow sets up below a crust layer it's a recipe for disaster, because when the crust layer gets dumped on, the facets collapse underneath. This basically turns the crust layer into a snow toboggan that takes all the snow sitting on top of it down the mountain on a great big ride in what's commonly known as an avalanche

Last winter, faceted snow from early season storms plagued the Continental ranges of the U.S. and Canada. Major avalanches were reported everywhere from Silverton, Colo. to Revelstoke B.C. as the evil facets caused persistent weak layers in the snowpack that caused major slides with such regularity that an interior-wide avalanche alert was posted in Canada for most of the season.

Launching into another winter the best defense to avoid avalanche accidents involving hidden instabilities is awareness. Do you know what's lurking at the bottom of your snowpack? Well, let's take a look at how the first layer is shaping up so far.

This is what faceted snow (or "surface hoar") looks like. Put a crust layer on these babies, watch them collapse and take everything on top of them for a ride -- you included, if you're not careful.

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Jackson Hole

"It's looking a lot better than last year," says Bob Comey, Director of the Teton National Forest Avalanche Center in the first week of December. "The Teton snowpack is pretty stable overall as we haven't seen many reactive crust layers. Down near Togwetee Pass there are a few problem signs, however. The snow is shallower and a faceted snow layer underneath a melt/freeze crust from late October has developed."

Despite the possible time bomb on Togwetee Pass, Comey's report bodes well overall for a region commonly beset with a stack of early season facet layers. Only time will tell how things settle out so keep up the snow dance. Any prolonged cold and clear periods will exacerbate issues. And of course, if/when it does dump, don't get casual about the killer potential of fresh snowfall and wind deposited snow. Direct action avalanches (aka the top layer) still claim the most lives in the Tetons.


"The Wasatch are looking pretty good right now. No instances of persistent weak layers," says Bruce Tremper, director of the Utah Avalanche Center. "The Uinta Mountains are a different story though. There is a fairly wide spread nasty layer in the Uinta that's already caused one fatality."

Utah's first avalanche accident of the year took the life of an experienced snowmobiler from Evanston, Wyoming on November 26th. The incident occurred in the Hump Creek drainage of the Western Uinta Mountains. The Utah Avalanche Center stated in their accident report that the three-to-five-feet deep and 600-foot wide slide cracked on a freshly wind loaded slope sitting on a Nov. 6 crust layer with faceted snow below -- classic conditions for disastrous results.

Tremper also noted that he hoped continued snowfall might heal the layer in the next couple weeks. "When you pile more snow on top you take away the temperature gradient that causes persistent weak layers," said Tremper. "When the snowpack is all one uniform temperature it just keeps getting stronger."


Some regions just have all the luck, or bad luck in this case...

"We have seen persistent weak layers in all of our zones," said forecaster Brian Lazar of the Colorado Avalanche Center in Boulder, CO. "There are several suspect layers including facets below an ice crust from early October snow and a mix of facets from a clear period in mid-November."

The October melt/freeze layer was blamed for the death of Wolf Creek Ski Patrol Director Scott Kay on November 22. Kay was buried in a terrain trap by a three-feet-deep slide consisting of both old and new snow. Recent mild and dry weather in Colorado has partially mitigated the danger of these weak layers but only because there is no new snow on top. The snowpack structure in the Rockies is weak overall and avalanche danger will skyrocket if the next storm system deposits a thick slab of fresh snow.

Interior Canada

"I'm not convinced we have a recipe for long term weak layers," said forecaster Ilya Storm of the Canadian Avalanche Center in Revelstoke, B.C. "What crust layers we have seen have been discontinuous, patchy and primarily contained to Northerly, high-elevation smooth terrain like glaciers and talus slopes."

No doubt these words are joyous news for the shredders of the interior, but don't get all giddy in Golden yet. Storm noted that a few big slides had released on a rain crust from Nov. 8 that sits less than foot off the ground. Storm also stressed the importance of community observations in avalanche forecasting. If you're out and about in the interior and observe a slide, or dig a pit, the Canadian Avalanche Center wants to know about it. You can send all observations to:

Pacific Northwest

Just like the climate, judging the danger of persistent weak layers present in the Pacific Northwest snowpack is a cloudy call.

"We may develop some issues depending on how the snow load comes in over the next few months," said Mark Moore, Director of the Northwest Weather and Avalanche Center. "We haven't had a lot of documentation about it yet but we're expecting a weak layer at the bottom of the snowpack below an October rain crust."

Layers of faceted snow often form below early season rain crusts in the Cascades, but the thick crusts and subsequent heavy snowfalls typically do a pretty good job of insulating the bad layers and strengthening the bottom of the snowpack mid-season. These rotten layers can rear their ugly head again in the spring, however, when the snow becomes isothermal and the entire pack is still sitting on the facets.

Cascade hardcores should keep an ear open for further developments about these lingering layers while they also keep a close eye on surface instabilities. Avalanche Danger in the Cascades is rated as considerable in many areas right now due to recent dumps.


Deep snowpack instabilities in the Sierra are being watched but don't yet pose much of an issue. In his December 5 Avalanche Advisory, Sierra Avalanche Center forecaster Andy Anderson mentioned a near ground-level faceted layer observed in the Mt. Rose area of North Tahoe, but he reasoned that an avalanche would be very difficult to crack off on this slowly strengthening and well-bridged layer.

Farther south in the High Sierra near Mammoth Mountain, Eastern Sierra Avalanche Center forecaster Josh Feinburg reported similar findings. Feinburg noted in his Dec. 5 advisory that, "Secondary avalanche concern continues to be a ground level faceted layer found in some high elevation North to East aspects. Although hard and unlikely to trigger, propagation is still possible."

These observations from forecasters at each end of the range speak to the fact that conditions will always change in the Sierra as you gain elevation. Both noted high elevation areas as potential danger zones so definitely be on your guard when you leave the balmy slopes of most Tahoe resorts and venture out into higher reaches via splitboard, snowmobile or bootpack.

For future updates about deep layer issues in your region check your local avalanche advisory regularly. Links to most advisories can be found here:

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