Thursday, April 11, 2013

Spring Skiing and Wet Slabs

Spring is a great time to get into the backcountry and enjoy the expansive mountain ranges surrounding town.  You can safely pass through steep, avalanche terrain on most days, but there are still a few hazards that need to be on your radar, one of which is wet slabs. 
Presently, our high peaks either have a dry, winter snowpack (on north aspects), or are in some transitional stage as you move towards southerly aspects.   Many of these slopes have weak layers at the bottom of the snowpack that formed early this winter.   As the days get warmer and the sun angle gets higher, meltwater at the surface begins to percolate through the dry snow. We call this the “wetting front.”  As water moves through the snow, it changes the snow’s properties, causing it to weaken.  When it refreezes at night, frozen ice bonds form which make for a very strong and very stable snowpack.  When the wetting front advances to a weak layer, that layer can lose all of its strength and fail, resulting in a huge, wet slab avalanche. These are one of the most destructive types of avalanches we see.
Every winter we have weak layers near the ground, and every spring meltwater percolates through them.  Yet in most years, we only see a handful of wet slabs.  These are very hard to predict on a slope by slope basis, but you can eliminate your risk with a few spring skiing/riding practices. 
(1) Wet slabs are most common after several consecutive nights without a good refreeze.  Avoid avalanche terrain if it didn’t refreeze well overnight.  Clear, cold nights make for a good refreeze; cloudy, warm nights leave a poor refreeze.  Look at mountain temperature sensors (found at and note the cloud cover before you leave home.  When you’re on slope, you can dig or probe to feel how thick that frozen block of snow is. On your hike up, if you are breaking through to your waist in wet grains, that’s a bad sign. 
(2) Wet slabs are most common during rapid or unusual warm-ups.  Plan to get off of slopes early in the day, when it is an inch of nice corn skiing rather than 8” of sloppy slush skiing.  This often means starting your hike before sunrise.  If you got a late start, go towards slopes that get sun later in the day, such as west or north aspects, rather than east or south aspects.  With your descent, anticipate that slopes lower in elevation or more easterly in aspect warmed up quicker; you might have to ski some frozen crust up high to play it safe down low.  Don’t go skiing if it is raining (duh).  If weather forecasters are talking about record high temperatures, go biking or drink beer on your deck. 
Thanks from the CBAC for another great season! We couldn’t do it without your support and observations.  Have a fun and safe spring ski season. 

-Zach Guy