CBAC Spring Season Advisory Posted 4/15/14

Spring season is a great time of the year to get into the mountains and generally offers better stability and more manageable avalanche problems for backcountry travelers.  However, avalanches conditions can still be dangerous during spring, and there are several kinds of problems you should continue to monitor and assess in you springtime adventures.

-Storm instabilities

Spring snowfall will usually fall on some form of crust or dusty crust.  These surfaces don't immediately bond well.  Anytime a slab of new or windblown snow forms, expect touchy conditions during and shortly after the storm.  New snow becomes especially sensitive as the sun comes out immediately after the storm and quickly consolidates into more of a storm slab (ex photo).   Expect leeward and crossloaded features at higher elevations to hold thicker and more sensitive windslabs  following a storm with moderate or strong winds (ex photo).  These kinds of instabilities are generally short-lived during the springtime, but can last for several days after the storm on shaded aspects or higher elevations. The best strategy is to monitor how much snow accumulates during a storm and ease into manageable terrain until you’ve assessed how reactive new slabs are.  Be wary of windloaded slopes and avoid heavily windloaded features following a significant storm.  The Schofield Pass SNOTEL , which is northwest of town, and the Butte SNOTEL , which is on Mt. Crested Butte, are local remote snow sensors that update hourly.  Storm or wind slabs are most problematic over consequential terrain with cliffs, gullies, long vertical, or rocks and trees.  

-Wet avalanches

When the sun (or rain) comes out after a spring storm and moistens the new snow, loose wet avalanches become frequent on any slope steeper than 35 degrees, and these point releases typically fan out and entrain all of the new snow down to previous crust layers (ex photos).  These are usually small and predictable, but can carry sizeable mass after a large storm or in terrain with significant vertical relief.  They can also gouge deeper and grow larger during prolonged warm-ups.   Wet slabs are a larger and more dangerous problem. These are caused by liquid water percolating to and compromising the strength of buried weak layers.  As of April 12th, we have observed several wet slabs failing on weak layers near the ground on low elevations wrapping from south aspects to more northerly aspects more recently.  As we progress towards summer, we can expect to see the problem move higher in elevation on the southern half of the compass, and eventually reaching the higher northerly elevations. The size of wet slabs failing on deeply buried weak layers will be likely be large to very large at these elevations.  Wet slabs are most likely to occur during prolonged warm-ups and/or following multiple nights without a good refreeze. Wet avalanche danger is usually low in the morning and rises through the day. The best strategy is to exit avalanche terrain early during warm, sunny days or avoid it during rainy days.  Monitor how well the snow surface refroze overnight, and time your descent so that you are riding in a couple inches of supportive corn skiing, rather than punchy, trap-door snow or ankle deep slushy snow.  Monitor mountain temperatures from these weather stations and expect cloudy nights to prevent a better refreeze. Anticipate that near-surface dust layers will expedite surface warming. 

-Cornice Falls

Springtime is the season that large cornices that have been growing all winter begin to weaken and fall.  These can also be triggers for stubborn deep slabs.  Cornices tend to break wider than expected.  Give cornices a wide berth if you are traveling along a corniced ridgeline, and limit your amount of exposure climbing underneath cornices, especially during the peak of warming.  

We will post updates on our Facebook page  as conditions warrant.  You can also use this as a forum to post or view snowpack or avalanche observations.   The CAIC will continue a statewide snowpack discussion in May.