Advisory

Avalanche Concern #1 Wet Snow Instability: Wet snow avalanches can occur anytime during the spring and early summer. This time of year the sun is nearing maximum strength. If night time low temperatures are above freezing, the snow warms up rapidly. If skies are cloudy overnight, you can expect wet snow by mid morning because clouds emit longwave radiation- cloudy nights are warmer than clear nights. If the snow gets a good refreeze, avalanche conditions can occur later in the day. You can use the Mammoth Mountain Ski Patrol site, http://patrol.mammothmountain.com/OtherWxStations.html or http://www.wrh.noaa.gov/mesowest/mwmap.php?map=hnx to monitor air temperatures throughout the forecast area. Boot-top deep wet snow, significant roller ball activity, or any wet snow avalanche results from small test slopes all indicate that wet snow instabilities can occur. Moving to a different aspect with less sun exposure, terrain less than 25 degrees in slope angle without steeper terrain above it, or simply heading over to the local crag all represent great alternatives to getting cart wheeled around in a wet slide. Avalanche Concern #2 Wet Slabs: This spring, wet slab avalanches represent the second spring avalanche concern. Over the cold, dry winter, the snowpack in the Rock Creek, Mammoth and Tioga Pass areas became faceted and weak. Most of us have experienced sinking through weak wet snow on low angle terrain, around rocks and buried logs or in the middle of open areas. A buried persistent weak layer still exists on some NW-N-NE aspects. Melt water may percolate through the snowpack and form melt channels through the layers and strengthen the snow or the melt water may simply weaken this layer and cause it to lose strength and fail. There is no way to know where this snow structure exists unless you probe or dig and take a look. Slopes where free water has not yet percolated to this layer will hold the best potential for these avalanches. The time period when free water percolates into the deeper layers of the snowpack and drainage channels are not well established though the heat wave caused water to drain out of the bottom of the pack, producing the spring runoff last week. Areas where the snowpack rests on top of granite slabs are good places for glide avalanche and wet slab avalanches to occur. The domes in Tuolumne Meadows have wet slab and glide avalanches every spring. The Sonora Pass area also has glide and wet slab avalanches each spring. These events are very difficult to predict and can result in large, destructive avalanches. OTHER HAZARDS Other hazards such as cornice collapse, moats, glide cracks, and open creeks with narrow weak snow bridges are common in the mountains during the spring. 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.