Our high pressure ridge of doom has finally broken down, and we are seeing our first snowfall of substance since before Christmas. While it is a modest storm, with only up to 7” or so of snow expected above 9000’, the SW winds will surely form much deeper deposits on leeward terrain at mid to upper elevations that will be today’s greatest avalanche concern. Below 9000’ the snow coverage has barely been enough to cover the ground, especially in areas south of Mammoth. Therefore the rain falling today below that elevation, which normally would be a concern for dangerous wet avalanches, is not. The dry clear weather that has persisted for weeks, combined with a shallow snowpack, has led to the formation of weak faceted snow in many areas throughout the forecast area, both at the surface in protected northerly facing areas, and deeper down in the snowpack around crusts and more dense slabs. While snow pit tests over the last couple of weeks have been showing failures and propagation in these buried weak layers, slope failures have not been seen. The addition of new snow weight, and especially the stress of a smaller avalanche, could awaken these layers and a resulting avalanche could be quite large and destructive. This 7” storm likely will not create widespread failures, but the much larger storm we are expecting in a couple of days very well could. Thursday morning's tinny precipitation event, which left up to an inch of new snow at higher elevations, was quite warm with drizzle up to 9,500'. That moisture combined with dense clouds and warm air, left melt freeze and rain crusts up to 11,000' on all aspects. Small facet formation was observed ontop of that crust in many areas, as well as some small surface hoar. While the surface hoar did not likely survive, the slick crusts, with some facets ontop, will likley act as a good sliding layer for new snow, whether it be in the form of wind slab or storm slab.
