Whumpfs, Buried Surface Hoar, and Wind Slabs on Lincoln Ridge

Today we went back to Lincoln Ridge to see how the surface hoar buried on Jan. 29 was reacting to the new load that yesterday's fresh snow added to it. We found that the surface hoar could barely support the load above it. As soon as we moved into areas where it existed, our weight on the snow caused the surface hoar to collapse. We heard ten whumpfs during our tour and felt ten accompanying snowpack collapses. These all occurred on open or sparsely treed N-NE facing slopes below treeline. When we dug into the snow to confirm that the surface hoar was the weak layer, we could see it clearly in the pit wall buried about 18 inches below the surface. In some places, the surface hoar measured 20 to 30 mm in size; in others, it was closer to 8 to 10 mm. In addition to the repeated collapses and whumpfs, snowpit tests also yielded unstable results (ECTP). Despite all of these signs of instability, we did not see any avalanches that failed on this layer. This lack of observed activity could stem from our choice to avoid steeper terrain where the surface hoar might be present. We did talk to some snowmobilers who had found a small N-facing test slope on their ride. When they cut it, blocks of snow about 18 inches thick broke loose and slid down the slope. They also avoided larger steeper terrain today. 

As we moved into near treeline terrain where last week's E-NE winds had hammered the more exposed slopes, we saw fewer signs of buried surface hoar and more signs of wind-loading from both the SW winds during yesterday's storm and NE winds today. We came across one natural wind slab avalanche that failed yesterday on a slope below large overhanging cornices. Ski kicks on test slopes loaded by today's NE winds triggered some shooting cracks. These winds had also scoured some snow away from the N-NE slopes and left sastrugi and other wind textures on the ridges. We could also see plumes of blowing snow on English Mountain and along the Sierra Crest.