Issued 4/6/15 by CBAC
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.
Spring snowfall will usually fall on some form of crust. These surfaces do 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. Expect leeward and crossloaded features at higher elevations to hold thicker and more sensitive windslabs following a storm with moderate or strong winds. 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 small and manageable terrain until you’ve assessed how large and how sensitive 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.
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. These are usually small and predictable, but can carry significant mass after a large storm or in terrain with significant vertical relief. They can also run when old snow looses cohesion as crusts thaw, and have potential to entrain a large amount of snow during prolonged warm-ups. Wet slabs can be a larger and more dangerous problem. These are caused by liquid water percolating to and compromising the strength of buried weak layers. This year, our snowpack is about a month ahead of schedule, meaning our April avalanche problems are more typical of May problems. Most slopes facing east through south through west, as well as lower elevations that face north, have seen water run through the entire snowpack and have already been delivered the dry to wet “spring shock”. These slopes saw a wet avalanche cycle in mid-March. A number of these slopes have matured into a stable, spring snowpack, but it is difficult to identify which ones could still be harboring the threat of a wet slab. Northerly facing slopes near and above treeline have seen less, if any, meltwater, and still primed for wet slab activity when temperatures and the higher sun angle turn on the water factory. Last year, these slopes began their wet slab cycle around late May to early June. The size of wet slabs failing on deeply buried weak layers will be likely be large at these elevations. Wet slabs are most likely to occur during prolonged warm-ups and/or following multiple nights without a good refreeze. If we see a dust-on-snow event, this will expedite surface warming. Wet avalanche danger is usually lower 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. Look for evidence of recent wet avalanches on similar slopes to clue you in to dangerous conditions. Monitor mountain temperatures from these weather stations and expect cloudy nights to prevent a better refreeze.
Springtime is the season that large cornices that have been growing all season begin to weaken and fall. These can also be triggers for slab avalanches. 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.
The CBAC will continue to monitor the snowpack and post updates to our website and our facebook page if conditions warrant. The CAIC will issue statewide updates on Mondays, Wednesdays, and Fridays into the spring season.