Central Sierra Nevada

Early season conditions update #4.

The snowpack is slowly building. At this time we are still without sufficient snow cover to warrant daily avalanche advisories. We are making snowpack observations and posting them to the observations page.

As the snowpack has slowly built since late October, a number of key trends have been observed. Prior to today's snowfall, areas of significant snow cover were limited to NW-N-NE aspects. The sun exposed aspects were melted out to bare ground. Above treeline NW-N-NE aspects were heavily wind scoured by gale force ENE winds that occurred on Nov 21-23. The only exceptions to this scouring were a few noted areas on N aspects below large cliff bands (ie. Round Top Peak complex on Carson Pass and Fourth of July Bowl in Blackwood Canyon).

The snowpack that exists in the ENE wind protected areas at treeline and below treeline on NW-N-NE aspects is 12 to 16 inches deep. It is composed mainly of layers of facets (sugary snow) and melt-freeze crusts.  In some locations in the 7,500' to 8,000' elevation range, a basal ice mass formed from rain on snow during the Nov 20 precipitation event. Overall the current snowpack holds faceted weak layers that are unlikely to support significant new snow loading in their current state. For the most part, a strong slab is not present on top of this widespread weak snowpack. The lack of this key avalanche ingredient allows for the presence of a shallow and weak but mostly stable snowpack around the majority of the forecast area at this time.

Today on Tamarack Peak (Mount Rose area) the first signs of an unstable snowpack were observed in snowpit tests. In this area, a stiff wind slab on top of a layer of near surface/near crust facets provided the necessary slab over weak layer combination. While the current known distribution of this snowpack structure is very limited, finding this instability in a narrow gully or chute could lead to to dire consequences from impact with rocks while caught in a small avalanche.