THIS AVALANCHE ADVISORY EXPIRED ON February 17, 2017 @ 6:59 am
Avalanche Advisory published on February 16, 2017 @ 6:59 am
Issued by Andy Anderson - Tahoe National Forest

Human-triggered wind slab avalanches will be possible today on wind-loaded slopes and some human-triggered storm slab avalanches may be possible on sheltered slopes. MODERATE avalanche danger exists at all elevations. If more snow falls than forecasted, the avalanche danger could rise to CONSIDERABLE. Evaluate the snowpack and terrain carefully to identify where problematic storm slabs and wind slabs may exist and plan travel routes to avoid areas with these avalanche problems.

2. Moderate

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Above Treeline
Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.

2. Moderate

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Near Treeline
Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.

2. Moderate

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Below Treeline
Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.
    Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.
Avalanche Problem 1: Wind Slab
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Gale force southwest winds and new snow will form wind slabs on leeward aspects today. These wind slabs will build quickly and human-triggering of these wind slabs will be possible today and may be likely in some of the most heavily wind-loaded areas. Depending on the amount of snow this fast moving storm deposits, some wind slabs could reach 2 to 3 ft in depth in heavily wind-loaded areas. If the lower end of predicted snowfall occurs, the wind slabs will remain smaller. If the upper end of forecasted snow or more snow than forecasted falls, wind slab avalanches will grow larger and more problematic. Wind-loaded N-NE-E aspects and cross-loaded NW and SE aspects in near and above treeline areas above 7500 ft. will hold the largest and most fragile wind slabs, but some smaller wind slabs could also exist in wind-loaded below treeline areas due to the strength of the wind.

Identify where wind slabs may exist using clues like blowing snow, cornices, wind pillows, ripples across a slope, and other wind created textures and use this information to plan a route that avoids the wind slabs.

Avalanche Problem 2: Storm Slab
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Changing snow levels and snowfall rates that may reach or exceed 2 inches per hour could allow human-triggered storm slab avalanches to become possible on steep open slopes in sheltered areas this morning. The actual amount of snow accumulation we receive will determine the potential size of this avalanche problem. Snowfall amounts on the lower end of the forecasted range will mean smaller storm slabs, while snowfall amounts on the higher end of the forecasted range will mean larger more problematic storm slabs

Human-triggered cracking, recent avalanche activity, feeling softer layers below harder layers as you probe can all indicate storm slab instabilities. Make careful observations while traveling in the backcountry to identify where storm slabs may exist. 

recent observations

Observations from Rubicon Peak and Incline Lake Peak yesterday showed a snowpack with a thick, strong rain crust buried about 25 to 30 inches below the surface. The layers of snow around this crust appear to have bonded well to the crust. Above the crust, the snowpack showed good bonding within and between the layers of snow all the way to the surface. Variable snow surfaces existed ranging from soft unconsolidated snow on sheltered N-NE aspects to firm scoured surfaces and sastrugi on exposed N-NE-E aspects to breakable refrozen sun crusts on open E-SE-S aspects to wet sticky surface snow below 7800 ft. on N aspects

Weather and CURRENT CONDITIONS
weather summary

Southwest winds increased overnight with gusts in the 130 mph + range over the ridgetops and consistent strong to gale force winds at all elevations. Precipitation started early this morning with snow levels around 7500 ft. Precipitation should increase and become heavy at times this morning with snowfall rates that reach or exceed 2 inches per hour at some times. The forecast calls for snow levels to quickly fall to around 6000 ft. during this storm with 4 to 10 inches of snow accumulating above 6500 ft. by this afternoon. This fast moving storm should begin to depart the area this afternoon and snowfall and winds should decrease as it does. Tonight brings a lull in the weather before a second weaker storm system moves into the area. The forecast calls for more southeast and east wind with this second system and 1 to 5 inches of snow during the day tomorrow above 7000 ft. 

Weather observations from along the Sierra Crest between 8200 ft. and 8800 ft.
0600 temperature: 30 to 39 deg. F.
Max. temperature in the last 24 hours: 42 to 47 deg. F.
Average wind direction during the last 24 hours: Southwest
Average wind speed during the last 24 hours: 60 mph
Maximum wind gust in the last 24 hours: 134 mph
New snowfall in the last 24 hours: trace to 1 inches
Total snow depth: 117 to 139 inches
Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 7000 ft. to 8000 ft.
Thursday Thursday Night Friday
Weather: Cloudy with snow and rain in the morning with snow levels around 7500 ft. Snow levels lowering to around 6000 ft. by mid-day. Mostly cloudy becoming partly cloudy. Slight chance of snow showers in the evening. Cloudy with snow likely in the morning and snow becoming more widespread in the afternoon
Temperatures: 38 to 43 deg. F. 24 to 29 deg. F. 30 to 35 deg. F.
Wind Direction: Southwest Southwest South shifting to east in the afternoon
Wind Speed: 30 to 45 mph with gusts to 85 mph decerasing to 25 to 35 mph with gusts to 75 mph in the afternoon 15 to 25 mph with gusts to 55 mph decreasing to 30 mph after midnight 10 to 15 mph with gusts to 30 mph increasing to gusts to 50 mph in the afternoon
Expected snowfall: 3 to 7 in. 0 in. 1 to 4 in.
For 8000 ft. to 9000 ft.
Thursday Thursday Night Friday
Weather: Cloudy with snow in the morning and snow showers in the afternoon Mostly cloudy becoming partly cloudy. Slight chance of snow showers in the evening. Cloudy with snow likely in the morning and snow becoming more widespread in the afternoon
Temperatures: 34 to 39 deg. F. 22 to 27 deg. F. 27 to 32 deg. F.
Wind Direction: Southwest Southwest shifting to the south after midnight Southeast
Wind Speed: 45 to 65 mph with gusts to 120 mph decreasing to 30 to 50 mph with gusts to 110 mph in the afternoon 20 to 35 mph with gusts to 75 mph decreasing to 15 to 25 mph with gusts to 45 mph after midnight 20 to 30 mph with gusts to 50 mph increasing to gusts to 85 mph in the afternoon
Expected snowfall: 3 to 8 in. 0 in. 2 to 5 in.
Disclaimer

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