Central Sierra Nevada

Problèmes avalancheux

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  Loose Wet

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  Avalanches of loose wet snow will continue to occur this spring on all aspects and at all elevations. The amount of instability and how soon the instability forms will vary from day to day. When recent new snow experiences rapid warming, it will almost always be problematic.

  Once the snowpack has been through several melt-freeze cycles, loose wet instability will depend on the degree of overnight snow surface refreeze and the rate of daytime warming. Cloud cover overnight can greatly inhibit snow surface refreeze. The NWS-Reno Backcountry Weather Forecast can help identify periods of expected cloud cover and warm temperatures. GEOS Satellite imagery shows how much cloud cover occurred overnight. Check the weather station table for a list of early morning air temperatures at various locations around the forecast area. If overnight cloud cover existed with above-freezing air temperatures, little to no overnight refreeze will have occurred, and traveling in avalanche terrain is not recommended. Choose a different day.

  Near or slightly above freezing air temperatures under clear skies allow the top few inches of the snowpack to refreeze on open slopes. These superficial refreezes usually allow for short periods of supportable snow in the morning. These refreezes are often weaker in areas where tree cover exists. A comparison of surface crust thickness on open slopes vs slopes under thick forest canopy will give a good indication of the overall strength of snow surface refreeze. The further and longer air temperatures drop below freezing overnight, the better the snow surface refreeze will be both on open slopes and under forest canopy.

  Once melt makes the surface crust marginally supportable to unsupportable, loose wet avalanches will become possible. Signs of potentially unstable snow also include rollerballs and pinwheels. Avoid travel in or below avalanche terrain on slopes with deep or unsupportable wet snow conditions and on slopes where rollerballs/pinwheels are occurring.

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  Wind Slabs

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  Fresh slabs of wind drifted snow may form on leeward slopes during any late-season snowstorms and natural and human-triggered wind slab avalanches may occur. Expect a period of snowpack instability during and immediately following the storm, then a second cycle of avalanche activity when rapid warming occurs post-storm.

  During the storm, use clues such as blowing snow, cornice formation, and wind pillow formation to identify where potentially unstable slabs of wind drifted snow may exist. Look for typical signs of mid-winter instability such as recent avalanche activity, wind loading, collapse, audible whumpfing sounds, and/or shooting cracks. Identify and avoid avalanche terrain in areas where recently formed wind slabs exist.

  Post-storm, new snow will be very sensitive to rapid warming and incoming solar radiation. Newly formed wind slabs could be easy to trigger post storm during rapid warming.

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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 sun angle in 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 natural avalanches can occur during periods of rapid warming post storm under sunny skies and light winds.