The recent winds have formed wind slabs in much of the higher elevations; especially steep terrain features prone to wind loading (e.g., leeward or cross loaded terrain features above treeline where new snow has accumulated). In more protected terrain (mid to lower elevations) soft slabs of 1-1.5 feet are possible on slopes of 35 degrees and steeper where bonding or anchoring is insufficient. The underlying early season snowpack is shallow, variable, and predominantly weak. The additional loading from the next system will further increase the risk of avalanche.
Thursday’s storm system came in as advertised with high winds and significant snowfall. The system started out initially warm and wet depositing dense snow on top of a relatively warm snow surface. By Thursday night, temperatures cooled dramatically and snow density decreased (aka Powder!) as a second wave of moisture moved through the region. Mammoth Ski Area recorded 14 – 20 inches of new snow with avalanche control producing failures primarily within the new snow. The snowpack is adjusting to this new load and bonding between layers is suspect. A report of a slide in the trees above Horseshoe Lake from an intentional ski cut further confirms that the snowpack is still adjusting to the add stress of the new snow. Areas of concern are drifted areas, loaded leeward slopes of 30 degrees and steeper. Overall conditions have greatly improved with good riding on slopes less than 30 degrees. The snow is not too deep, just fun.
The strong winds and heavy snowfall associated with this system has rapidly loaded an already weak and shallow snow pack. This combination generally requires a little time to adjust to the new load and for bonds between layers to strengthen. However, Friday night’s cold temperatures did little to aid settlement and with another strong system due in on Sunday, the stress will only increase. Areas of current concerns are wind slabs and soft slabs that have formed on leeward slopes and side loaded features in the Mid to Upper elevations. Snow pit data (12/11/15) suggests there are at least a couple of week layers to be aware of. The upper most layer of the new snow is relatively light (low density) with weak bonding to the new snow below and is easily transported by wind, forming soft slabs and tender drifts in lee areas. The second layer is a layer of preserved facets just below a melt-freeze crust that formed on the old snow surface. Stability Test results: ECT15, Q2 on 2-3 mm facets, CTM 12, 13, 15, Q2 on 2-3mm preserved facets. The two layers together could result in a failure within the new snow that steps down deeper into the snowpack or fails on the preserved facets with the potential for deep burial. The weather system due Sunday may be the tipping point needed for extensive natural avalanches. Stay tuned and stay alert.
|0600 temperature:||11 deg. F.|
|Max. temperature in the last 24 hours:||16 deg. F.|
|Average wind direction during the last 24 hours:||SSW|
|Average wind speed during the last 24 hours:||15 mph|
|Maximum wind gust in the last 24 hours:||NA mph|
|New snowfall in the last 24 hours:||3 inches|
|Total snow depth:||30 inches|
Saturday should provide a brief break for most of the area before the next significant system approaches the region early Saturday night through Sunday. This is a very dynamic system with a strong pressure gradient ahead of it (wind) and a short-lived moisture tap. A Winter Weather Advisory is in effect from 10 am to 10 pm PST Sunday. Timing: snow will begin late Sunday morning with the heaviest snowfall expected late Sunday afternoon into the evening with snow accumulations of 10 to 18 inches possible in the Sierras west of highway 395, 2 to 6 inches east. Snow levels: near 7000 feet late Sunday morning, falling to 5000 feet late Sunday afternoon. Winds will be southwesterly, 20 to 30 mph with gusts to 55 mph, gusts of 115 mph are possible at ridge top. Anticipate snow and blowing snow with periods of near white out conditions. Snow is expected to taper off by late Sunday night for all areas with the far south seeing the heaviest showers through the night.
This snowpack summary applies only to backcountry areas outside established ski area boundaries. This snowpack summary only describes general avalanche conditions and local variations always occur. This snowpack summary expires 24 hours after the posted time unless otherwise noted. The information in this snowpack summary is provided by the USDA Forest Service who is solely responsible for its content.