Bollettino valanghe

Central Sierra Nevada

Sierra Avalanche Center
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General Spring Avalanche Statement

The 2019-2020 operating season has come to a close. Thank you to everyone who supported the avalanche center this season, making everything possible. Avalanche activity will continue to occur this spring. Expect wet snow instabilities and cornice failures on warm days. When spring storms impact the region, they bring the possibility of fresh slabs of wind drifted snow and storm slabs. Keep reading for some information and resources to aid in trip planning and decision making.

Problemi valanghivi

  • 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.

  • 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.

  • 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.

Avalanche Discussion

The observations page will remain available for sharing snowpack and avalanche observations submitted to the avalanche center.

Other hazards besides avalanches exist for over snow travelers in the backcountry. A few of these are cornice fall, moats, glide cracks, exposed/barely-covered obstacles, and open creeks.

Warm spring weather and melting snow also weaken the large cornices that have built up along many ridgelines. Expect continued cornice failure throughout the spring. Cornices often break farther back from their edges than expected or at unexpected times. Stay far from abrupt edges along ridgelines where cornices exist and stay out from under melting cornices, especially if you can see or hear water dripping from the cornice.

Areas of weak snow around rocks, vegetation, and along the base of cliff bands exist. Move carefully around these features as the thin bridges of snow could collapse under body weight, allowing you to fall into a melted hole next to the feature.

Exercise caution when traveling near or attempting to cross creeks as wet snow along the banks can collapse under the weight of a person. Take these additional hazards into account during route planning.

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