With ‘Deep Slab Instabilities and Persistent Weak Layers,’ Know Your Snowpack
TELLURIDE – Telluride has been on the receiving end of a series of vigorous storms this fall – an early holiday gift for local ski enthusiasts itching to reap the first spoils of early ski season. Yet a considerable conundrum has also arrived, along with the unusually heavy snowfall, in the form of an active avalanche season with impacts that could persist into the months ahead.
“Overall, we have much more snow coverage than we typically do this time of year,” says the Colorado Avalanche Information Center’s Josh Hirschberg, the backcountry forecaster for the Northern San Juans. The series of strong storms over the last month, combined with dry and cold periods in between, have led to the development of weak layers and slabs within the regional snowpack, he says, noting that the combination of weak layers and slabs is generally a good recipe for avalanches.
“On the whole, we’re dealing with more snow, but also a more complex snowpack than what we would normally see this time of year,” says Hirschberg, adding that with the additional snow has come an above-average amount of avalanches.
Winter arrived early this year, blanketing the high country with the first storm of the season on Sept. 27, continuing into October with above-average snowfall for that month. November boasted a total of 41 inches of snowfall, according to snow reporters at the Telluride Ski Resort, 6 inches above the 30-year average. November’s snows carried 3.7 inches of water, however, dishing up more moisture than what the Northern San Juans typically see.
The Telluride Ski Resort is currently reporting a 43-in. base, nearly double its base at this time last year.
While great for ski conditions, the abundance of heavier-than-normal snow could create a false sense of security for those venturing out into the backcountry. “Right now we have what appears to be fairly supportable snow,” says Telluride Ski Resort Snow Safety Director Craig Sterbenz, “but that’s because you’re skiing just in the upper layers of the snowpack.
“Yet that ‘supportable’ snow is overlying weaker layers,” he adds. Because of the underlying weakness, “We’re looking at persistent deep slab instabilities and persistent weak layers, which has created two distinct avalanche problems.”
The CAIC’s Hirschberg reports that northwest through northeast aspects are especially suspect right now, since forecasters have seen the most development of persistent slabs in those areas. Those slabs are often sitting on top of old layers of snow that fell early in the fall, and have over time become less supportable through the process of faceting, with the large temperature gradient typical with the San Juans’ cold nights and warm days allowing the formation of angular grains, which bond relatively poorly to one another and other grains, creating a snowpack that generally cannot support a heavy load.
While the formation of facets within the snowpack is fairly typical for the San Juans, this winter’s weather has also given rise to a phenomenon relatively rare to the area that may create an additional avalanche problem later in the season.
Forecasters noted the formation of very large surface facets – or surface hoar, the large, feather-shaped crystals that grow on the snow’s surface – at high elevations in late November. Then a Dec. 4 storm dropped 18 inches across the high country over the course of just one day, burying those large, faceted crystals that tend to bond poorly with new layers of snow.
“The good thing is that by the time that layer was buried, it wasn’t everywhere,” Hirschberg says of the surface hoar, “but the trick now is figuring out where it still is, because we know it’s out there, and could be something that will continue to cause problems for weeks to come.”
The avalanche hazard, as of Wednesday, Dec. 11, was rated “considerable” near and above treeline, and “moderate” below treeline, with triggered slides most likely to occur on northwest, north, northeast and east aspects, according to the CAIC. Hirschberg notes, however, that every new weather event, whether it’s sun, snow or wind – will change the snowpack considerably. Staying abreast of the most recent conditions is necessary when planning a backcountry tour.
Avalanche Forum Monday Night
Coinciding with the ever-evolving avalanche conditions is the start of the Monday Night Avalanche Awareness Forum Series, which kicks off Monday, Dec. 16, at 7 p.m. at Rebekah Hall, with a discussion about “Venturing into Avalanche Terrain in the San Juan Mountains.”
The forum will include presentations and community discussion about the nature of the San Juan snowpack, and about staying alive in avalanche country. Ski area boundaries and terrain management, the history of skiing in Bear Creek and the issue of uphill travel on the Ski Resort will be covered. Guest speakers include Peter Inglis of the San Juan Field School and Telluride Ski Patrol, Scott Spielman of the U.S. Forest Service, Jon Tukman of the Telluride Ski Area and local Search and Rescue members.
“It’s a good intro to newcomers who aren’t familiar with this terrain or snowpack, but also good for seasoned locals as a reminder to combat complacency,” Inglis says of the forum, which is the first in the winter-long series of avalanche education discussions hosted by the San Juan Field School and Telluride Ski Patrol.
The event is free. Colorado Outdoor Recreation Search and Rescue Cards will be on sale; they cost $15, and are good for five years.