During the last 24 hours, the Sierra Crest north of Hwy 50 recorded the highest snowfall amounts with 20-24 inches. South of Hwy 50 remote sensors indicate 14-20 inches fell. On the east side of Lake Tahoe, around 18 inches of new snow accumulated. Most of this snow fell yesterday during periods of high winds and intense snowfall rates. After a brief lull in the storm last night, intense snowfall should begin again this morning, and the southwest winds should climb back into the 50-60 mph range today. This third and final wave of wind and snow should also bring some warmer air with it causing snow levels to rise to around 5500 ft today. This warmer air will make the snow that falls today more dense that yesterday's snow. The forecast calls for 1-2 ft of new snow by midnight with 2-3 ft possible along the Sierra Crest south of Hwy 50. A short-lived high pressure ridge will start to build over the region tonight causing the winds to decrease and the snowfall to taper off. By tomorrow skies should become partly to mostly cloudy, and the winds should become light. Cold temperatures will remain over the region.  Click here for a weather briefing from the Reno NWS.

Recent Observations:

Yesterday widespread skier triggered shooting cracks and collapses occurred on Red Lake Peak (photos and more info), Castle Peak (photos and more info), Andesite Ridge (photos and more info), and Tamarack Peak (videos, photos, more info). On Red Lake Peak, Castle Peak, and Andesite Ridge, test slopes failed easily in response to the weight of a skier. Wind slabs existed in typically sheltered areas as well as in the more exposed terrain in these places. Several remotely triggered avalanches involving the wind slabs occurred on SE-E-NE-N aspects on Castle Peak and on Andesite Ridge. The largest of these failed on the density change buried on Dec. 17th. Steep non-wind loaded test slopes in below treeline sheltered terrain also failed on this layer resulting in soft storm slab avalanches. The density change mentioned above, a facet layer on top of the rain crust (the Dec 12th facet layer), and a facet layer below the rain crust (the Dec 2nd facet layer) all failed easily in tests in yesterday. The tests showed that fractures could propagate along any or all three of those layers. Observations around the forecast area prior to this storm showed similar snowpack characteristics. Overall the data indicates a snowpack teetering on the edge of failure.

Avalanche Problem #1: Wind Slabs and Storm Slabs

Fragile wind slabs already exist across the forecast area. These wind slabs exist in below and above treeline terrain and in areas that typically remain sheltered. Additional snow and wind today will continue to add weight and depth to the wind slabs. These wind slabs rest on top of the weak layers mentioned above. Additional weaknesses will form today as warmer temperatures add a more dense layer of snow above yesterday's lighter snow creating an upside-down snowpack. Other variations in snowfall intensity, temperature, winds, and other storm characteristics could also create weaknesses within the new snow. Large, destructive, and widespread natural and human triggered avalanches involving large wind slabs will occur today as the afore mentioned weaknesses fail. The largest and most fragile wind slabs will exist on wind loaded and cross loaded NW-N-NE-E-SE aspects in both above and below treeline terrain. In more protected areas, some softer storm slabs could also exist on top of the already weak snowpack, and failure of these storm slabs will also be likely.

Avalanche Problem #2: Persistent Slabs

The snowfall from this series of storms could add enough load to cause the persistent weak layers (the Dec 12th and the Dec 2 facets mentioned above) now buried deep in the snowpack to fail. Variability in the strength and distribution of these layers means uncertainty remains as to exactly where and when these facet layers will fail, but avalanches resulting from the failure of these layers could become more and more widespread as more weight accumulates on top of the snowpack. The most well developed and weakest facets exist on the NW-N-NE aspects between 7,500 ft. and 10,000 ft. The avalanches that result form the failure of these layers will fracture across wider areas and could easily break above or far to the side of the person who triggers them. Other avalanches could also step down to these deeper weaknesses. Any avalanches that fail on these facet layers will be large and destructive.