Central Sierra Nevada

Avalanche problems

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  Loose Wet

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  Loose wet avalanche activity will continue to occur this spring. Usually this type of instability forms in response to daytime warming. It will occur naturally following late season snowfall events. It can be human triggered when surface wet snow is present. With many previous melt-freeze cycles and rain events having already occurred this past winter and spring, free water drainage from the snowpack is well established.

  You can use data from the remote weather stations listed on the SAC weather station map to monitor hourly temperatures at many points throughout the forecast area. Mountain weather forecasts are issued twice per day by the NWS Reno. Cloudy skies overnight and air temperatures above freezing do not allow the snowpack to refreeze very well. When the snowpack does not refreeze overnight, other springtime activities that do not involve over snow travel on steep slopes represents a more prudent choice. Under clear skies, the top few inches of the snowpack will often refreeze despite near or slightly above freezing air temperatures. This superficial refreeze usually allows for a short period of good travel conditions during the early morning hours before surface wet snow instability becomes a concern.

  If a solid overnight refreeze occurs, getting out early and finishing in time to have a mid day barbecue should be your goal. Start with east aspects and follow the sun to south, then to west, and finally to north aspects. Get off of your equipment on a regular basis and check boot penetration depth. Boot-top deep wet snow, significant roller ball activity, or any loose wet avalanche results from small test slopes all indicate that wet snow instabilities can occur. When signs of loose wet instability exist it is time to exit avalanche terrain. Move to a different aspect with less sun exposure or even better, terrain less than 30 degrees in slope angle without steeper terrain above. As a matter of etiquette, do not leave deep ruts in a slope that will freeze overnight and ruin the slope for others the next day.

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  Wind Slabs

  • All elevations

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  Natural and human triggered wind slab avalanches may occur during and immediately after any late season storms. Expect a period of snowpack instability during the storm itself, then a second cycle of avalanche activity as rapid warming occurs post storm. Use clues such as blowing snow, cornice formation, and wind pillow formation to identify where potentially unstable wind slabs exist. Look for typical signs of mid winter instability such as recent avalanche activity, wind loading, collapse, audible whumpfing sounds, and/or shooting cracks. Post storm, new snow will be very sensitive to rapid warming and direct sunlight. Pay close attention to layer bonding both within the new snow and to the old snow surface beneath it. It can lose strength rapidly as the day progresses causing a significant increase in avalanche danger.

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  Persistent Slabs

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  During periods of rapid warming after new late season snowfall, storm slabs and wind slabs can transition to wet slabs. The high angle sun in late April and May allows for significantly more incoming solar radiation to affect the snowpack on NW-N-NE-E-SE aspects than what occurs during December, January, February, and March. If an active weak layer exists at the bottom of a recently formed storm slab or wind slab, rapid warming of the snow surface can increase the deformation rate of the slab adding additional stress to the weak layer below. This can cause an increase in snowpack instability and in some cases cause natural avalanches to occur during periods of rapid warming post storm under sunny skies.