Observations made over the past two days show a snowpack in transition from one composed of rounded, well bonded forms to facets. The widespread pre-Christmas “crust” that formed on December 20 and strengthened during the mild days before Christmas Day is crumbling as facets form above and below the layer. The rounded grains that formed a supportive layer for skiing and riding is composed of facets and rounds turning into weak cohesion-less grains.
Cold days, colder nights and a shallow snowpack are the perfect ingredients for facets to form. If the ground temperature is 0C (32F) and the snow surface is – 18C (0 degrees F), there is a huge difference in temperature over a short distance of 50 to 60 cm (19 to 23 inches). In less than a week, a snowpack that could withstand the loading of a real Sierra storm, measured in footage, is changing to a structure similar to last December and January. When storms finally reached our area at the end of January and early February, there was a widespread natural avalanche cycle.
While we don’t know what the weather will bring in the next few weeks, a faceted shallow snowpack is the setup for a persistent weak layer avalanche problem.
For now, small, soft, shallow wind drifts may pose an avalanche problem above tree line. I do not expect these small wind slabs will extend very far downslope. If you do get out and brave the subzero wind chills today, exercise normal caution and be alert for areas of wind drifted snow.