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Avalanches will continue to occur this spring, but it is time to close the avalanche center. Thank you to all of the groups and individuals who made generous donations to the Sierra Avalanche Center this winter. This includes our corporate sponsors, community groups, and the general public who made cash donations and purchased Ski for SAC day tickets. Another thank you goes to everyone who submitted snowpack, weather, and avalanche observations this season. Both the financial and informational support made our avalanche advisory possible during the 2007-2008 winter season. Here are a few notes on spring snowpack and avalanche conditions.

There are three major avalanche concerns for the remainder of this spring and into early summer. The first is near surface loose wet snow avalanche activity. Usually this type of instability forms in response to daytime warming. You can use the Snotel and National Weather Service sites to monitor hourly temperatures at many points throughout the forecast area. If skies were cloudy overnight and air temperatures were above freezing, plan a different activity that does not involve over snow travel. The snowpack does not refreeze well under these conditions. Under clear skies, the top few inches of the snowpack will often refreeze despite near or slightly above freezing air temperatures. This light refreeze will often allow 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 an early afternoon 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. Sinking into wet snow up to the tops of your boots, significant roller ball activity, or any wet snow avalanche results from small test slopes are all signs of wet snow instability. They indicate that it is time to move to a different aspect with less sun exposure, to move to terrain less than 25 degrees in slope angle without steeper terrain above it, or time to call it a day. 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.

The second major concern is naturally occurring deep wet slab avalanches. Higher elevation north through east aspects are the most likely locations for this to occur. In these areas, free water did not percolate to the bottom of the snowpack during the warm periods of March and April. With high late spring sun angles and warm air temperatures, free water percolation to the basal snow layer depth will eventually occur. The time period when free water percolates into the deeper layers of the snowpack and drainage channels are not well established is when deep wet slab avalanches are most likely to occur. These events are very difficult to predict and can result in large, destructive avalanches.

The third major concern involves avalanche activity that may occur during and 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. During the storm, watch 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 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.

Other hazards such as cornice collapse, moats, glide cracks, and open creeks exist. Stay well back from abrupt edges along ridgelines as human triggered cornice collapse will remain possible during the spring. Stay out from under cornice areas that are not well frozen, especially if you can see 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.

As the season begins to change, the morning air becomes crisp, and the days become shorter, check back on our home page for early season fundraising events for the 2008-2009 season. Enjoy your spring and summer and we will see you next fall.

Brandon Schwartz and Andy Anderson, Avalanche Forecasters 

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