Wind slab failure on the proletariat on Tamarack Peak in the Mt. Rose backcountry

Location Name: 
Proletariat
Region: 
Mount Rose Area
Date and time of avalanche (best estimate if unknown): 
Thu, 04/22/2010 - 11:00
Location Map: 
United States
39° 19' 2.28" N, 119° 55' 2.9568" W
See map: Google Maps
US


Red Flags: 
Recent avalanche activity
Whumphing noises, shooting cracks, or collapsing
Recent loading by new snow, wind, or rain
Obvious avalanche path
Observation made by: Forecaster
Avalanche Observations
Avalanche Type: 
dry,slab,
Slope: 
37degrees
Trigger type: 
Skier
Crown Height: 
1 ft
Aspect: 
East
Weak Layer: 
Storm Snow
Avalanche Width: 
30ft.
Terrain: 
Near Treeline
Elevation: 
9 600ft.
Bed Surface: 
Storm Snow
Avalanche Length: 
200ft.
More detailed information about the avalanche: 

A thin sun crusts exists near the base of the new snow on most aspects in this area. This thin crust most likely formed on Tuesday when a brief period of sun occurred over the eastern part of the forecast area. Below this recent melt-freeze crust was a thin layer of recent snow. Below this was the older more firm and frozen melt-freeze surface. In areas below treeline that are sheltered from the wind, the new snow remains soft, and unconsolidated. In more wind affected areas a thin but more dense wind slab had formed on the snow surface with softer new snow below it. Skier-triggered shooting cracks occurred where these stiff wind slabs existed with failures occurring at the base of the wind slabs and on top of Tuesday's sun crust. In the more wind affected areas near and above treeline these wind slabs were thicker and the lighter snow below them was thinner creating an upside down snowpack similar to what has been seen in other parts of the forecast area over the last few days. Several skier triggered shooting cracks that extended up to 15 ft away from the end of skis occurred due to to failure of these wind slabs. One 30 ft wide, 1-1.5 ft deep skier-triggered avalanche occurred on a ENE facing 37 degree wind-loaded slope.

As the winds shifted to the NE, they started scouring the N-NE-E aspects leaving a mix of wind pillows, scoured surfaces, and wind slabs.

Photo 1: Skier-triggered shooting crack in near treeline wind loaded terrain.

Photo 2: Skier-triggered avalanche described in this ob.

Avalanche Photos: 
Weather Observations
Blowing Snow: 
Yes
Cloud Cover: 
100% of the sky covered by clouds
Air temperature: 
Below Freezing
Wind Speed: 
Moderate
Precipitation: 
Snow
Air temperature trend: 
Cooling
Wind Direction: 
Northeast
Accumulation rate: 
Less than 1 in. per hour
More detailed information about the weather: 

Winds shifted to the NNE and increased during the day. These winds started to transport significant snow away from the N-NE-E aspects and start to wind load and cross load NW-W-SW-S-SE aspects.