Monday, March 31, 2014

Oh no! Dust on snow

Yesterday, we got a brief and silty taste of the Dirty Thirties when a major dust storm blew over Crested Butte.  Scarp Ridge recorded wind speeds of 110 mph (the highest of the season), and these winds brought with them a sizeable chunk of Moab.
Dust-ageddon in Crested Butte on March 30, 2014.  Photo credit: Matt Hogan

How will this dust on snow event affect snow stability and avalanches?   Lets start with a key concept: albedo.   Albedo is how reflective a surface is.  White colors have a high albedo -- they reflect a high amount of radiation energy rather than absorb it. That is why you reach for a light colored T-shirt if you're going outside on a hot sunny day instead of a black T-shirt.  Fresh snow has a very high albedo and it reflects most incoming solar energy. (So thats why my mom always made me double up on sunscreen when I went skiing!)  On the other hand, the dark-colored dust more readily absorbs solar energy and heat, and also retains that heat longer.

Pristine snow reflects radiation more effectively than dusty snow.  Courtesy of Jeff Deems.
Dusty snow can absorb two or three times the solar energy of a clean snowpack.  Whenever dust is near the surface of the snow (even when its buried up to a foot deep), it amplifies the rate of surface warming, increasing the amount of snowmelt and weakening the snow around it.   This leads to more frequent loose wet avalanches.  Intense melt rates send freewater deeper into the snowpack, which can also compromise the strength of deeply buried weak layers, causing an increased likelihood of wet slab avalanches.  Both of these problems, with or without dust, can be avoided by monitoring how well the snowpack refreezes at night and getting off of slopes before they thaw too much.  Dust on the snow simply shortens that window of stable snow and may prevent good overnight refreezes in some situations.
Snirt.  Snow+Dirt.   Photo taken March 31, 2014 near Crested Butte.

The second avalanche concern is that dust can do weird things when its buried by a slab of snow, and it can behave like a persistent weak layer.  Because they absorb and retain heat longer, dust layers can cause wet grains above or below them to remain unfrozen and unstable longer. Dust can also cause tremendous temperature gradients in the surrounding layers, which causes the bordering snow around it to decay and facet. This is not always the case, but it is worth checking on how reactive dust layers are after they get buried by spring storms.  It will be the easiest layer you will ever identify in a snowpit!

The crown of a large slab avalanche that was remotely triggered from hundreds of yards away last spring.  It failed on a dust layer.
Sadly, dust on snow has negative impacts on the timing of spring runoff and water resources in the West.  The Center for Snow and Avalanche Studies in Silverton, CO tracks and studies the effects of dust-on-snow events, and is a great resource for more information on this topic.  On the bright side of things, your favorite mountain bike trails will melt out sooner now, and business is booming if you own a car-wash business.

Zach Guy
CBAC Forecaster

Wednesday, March 19, 2014

A Canary in the Elk Mountains?

Canaries were once used in coal mining as an early warning system for toxic gases leaking into the mine.  Signs of distress, or worse, a dead bird, would caution the miners that it was time to retreat from the mine.

Last week, a warm airmass brought our first real taste of spring to the Elk Mountains.  Nothing out of the ordinary as far as Crested Butte weather goes, but enough that pale white skin made its spring debut on Elk Street before hastily retreating to aloe vera treatments.  On Sunday and Monday (March 9 and 10), temperatures rose to just above freezing at 12,000 feet under an intense March sun.   Days later, following a few inches of snow and cooler temperatures, we observed the results of some very unnerving deep slab avalanches.  Sometime after Monday evening, a huge slab tore off of the south face of Mt. Owen.  This appeared to be triggered by a cornice falling onto a shallow part of the slope, but it propagated to parts of the slab that were 12 feet deep.   

Deep slab on Mt. Owen, first spotted 3/12/14

Looking down the crown

On Wednesday night, another monster ripped off of a southwest facing ridge near Avery Peak.  This one raised the hairs on the back of my neck.  Two deep slabs back to back, under relatively mild and stagnant weather.  The Avery slide did not appear to be cornice-fall triggered, and has raised a lot of questions and spurred a lot of speculation on the failure mechanics among local and statewide avalanche professionals.  Although meltwater on high elevation terrain has been pretty minimal to this point, it seems plausible that a hot spot on the slope, such as a sunbaked rock or simply an oven-like part of the slope, could have channeled heat and meltwater into a shallow part of the snowpack.  Once meltwater percolates to a buried weak layer, it can compromise the layer’s strength causing a wide and destructive failure. The timing of these events can be unpredictable.  Maybe warming had subtle effects on the slab properties that added up just enough on this slope.  We're still not sure.
And another.  This one near Avery Peak.

With just a tease of spring under our belt and much more warm and sunny weather inevitably to come, this begs the question:  Are we seeing the tip of the iceberg right now?  Did the canary just faceplant into the bottom of the cage?  Warming and meltwater will continue progressing into the snowpack, first on southerly slopes, and eventually around the compass to north.  We know weak layers are at the bottom of the snowpack lurking and they are proving to be reactive.  Cornice falls will become more frequent as these overhanging blocks of snow continue to thaw and sag from their own massive amounts of weight.   We are not out of the woods yet when it comes to deep slab problems.  These last two slides should serve as a healthy reminder to use an extra dose of caution this spring in your backcountry travels. Be diligent in your terrain selection and in the attention you give to weather, snowpack, and avalanche patterns in the upcoming months.

Zach Guy
CBAC Forecaster