Sierra Avalanche Center: Today the avalanche danger is: HIGH

January 10, 2017 – AN AVALANCHE WARNING IS IN EFFECT through at least 7am Wednesday, January 11th. Significant new snow loading has natural and human triggered avalanches likely now into Wednesday. The avalanche danger has increasing to HIGH. Travel in or near avalanche terrain or in avalanche runout zones is not recommended.

AVALANCHE PROBLEM 1: WIND SLAB

Feet of new snow combined with gale force SW winds has and will continue to create very large wind slabs in near treeline and above treeline terrain mainly on NW-N-NE-E-SE-S aspects. Wind slabs may form in more isolated areas below treeline and on SW-W aspects today.

Travel on or below slopes steeper than 30 degrees is not recommended today. Very large cornices are expected to form along ridgelines, presenting an additional hazard.

AVALANCHE PROBLEM 2: STORM SLAB

Rapid new snow accumulation in wind protected areas near and below treeline has and will continue to create storm slabs on all aspects. Rising snow level could further contribute to storm slab formation with a potential for upside down storm snow structure. Any areas where rain on snow occurs today will allow these same storm slabareas to experience wet slab or wet loose avalanche problems.

Travel on or below slopes 30 degrees and steeper is not recommended. Trees will need to be too tight to move through on skis or a snowmobile to protect from direct human triggering of this avalanche problem. Keep in mind that avalanches from above can move through mature forest.

AVALANCHE PROBLEM 3: DEEP SLAB

By current definition, deep slab avalanches are anything that fails more than 3 feet deep in the snowpack regardless of weak layer type. Several possibilities exist for deep slab avalanches to occur today. Drifting snow associated with wind slabs will easily become large enough today to qualify as deep slabs. A new rain crust buried Jan 9 will provide an efficient bed surface and potential new weak layer at the base of the current storm snow. Failure much deeper in the snowpack down to the depth of the Dec 15 rain crust remains possible, especially above 9,000′. Deep slabs could form on any aspect at any elevation.

Human involvement in any deep slab avalanches will be difficult to survive given the expected very large to historic avalanche size. Travel on or below slopes 30 degrees and steeper is not recommended. Avoid historic avalanche run out zones characterized by small to medium sized trees with steeper slopes above.

WEATHER

A powerful storm system is impacting the region. High intensity snowfall occurred last night and is forecast to continue through tonight. Snow levels may briefly rise to around 6,500′ today, but high intensity snowfall is expected to keep snow levels lower than associated air temperature would suggest. All available ridgetop wind sensors are off line, but reported blizzard conditions and forecast gale force SW winds give a decent picture of what is occurring. Lower intensity snowfall is expected for Wednesday with a decrease in SW ridgetop winds from gale force to strong. Another storm system is forecast to impact the region Wednesday night.

Recent observations

Observations were made and received yesterday from Hidden Peak (West Shore Tahoe area) and in Negro Canyon (Donner Summit area). On Hidden Peak at 8,250′ rain wetted non-refrozen snow existed in the top 4 feet of the snowpack with signs of significantly increased moisture down to the level of the Dec 15 rain crust located 6 feet deep in the snowpack at that particular wind protected location. In Negro Canyon, strong snow temperature gradients were measured at the newly formed and buried Jan 9 rain crust. This indicates a potential for a new weak layer of near crust facets to form on top of the Jan 9 rain crust at the base of the most recent storm snow. No new avalanche activity was reported to have occurred during the day yesterday.

This avalanche advisory is provided through a partnership between the Tahoe National Forest and the Sierra Avalanche Center. This advisory covers the Central Sierra Nevada Mountains between Yuba Pass on the north and Ebbetts Pass on the south. Click here for a map of the forecast area. This advisory applies only to backcountry areas outside established ski area boundaries. This advisory describes general avalanche conditions and local variations always occur. This advisory expires 24 hours after the posted time unless otherwise noted. The information in this advisory is provided by the USDA Forest Service who is solely responsible for its content.

For a recorded version of the Avalanche Advisory call (530) 587-3558 x258