Rain on snow avalanches near the ridgeline on Grouse Rock

Location Name: 
Grouse Rocks
Region: 
Blackwood Canyon or Ward Canyon Area
Date and time of avalanche (best estimate if unknown): 
Sun, 12/02/2012 - 04:00
Location Map: 
United States
39° 8' 18.654" N, 120° 14' 44.9988" W
See map: Google Maps
US


Red Flags: 
Recent avalanche activity
Recent loading by new snow, wind, or rain
Obvious avalanche path
Observation made by: Forecaster
Avalanche Observations
Avalanche Type: 
wet,slab,
Slope: 
38degrees
Trigger type: 
Natural
Crown Height: 
2 ft
Aspect: 
Northeast
Weak Layer: 
Storm Snow
Avalanche Width: 
150ft.
Terrain: 
Above Treeline
Elevation: 
8 300ft.
Bed Surface: 
Storm Snow
Avalanche Length: 
500ft.
Number of similar avalanches: 
4
More detailed information about the avalanche: 

This slide is indicative of several others noted along the ridgeline in this area. These slides all had several inches of snow on top of the debris and bed surface indicating that the avalanches occurred before snow level dropped on Sunday. It looked like natural cornice failure initiated these avalanches. Rain falling on the new snow likely caused these failures. These slides measured between 1 and 2 ft deep and all ran down hill between 300 and 500 ft. The bed surface was a crust layer about 1-2 ft deep. The debris from the avalanches was large, blocky and firm.

Photo 1: View of the crown from mid slope.

Photo 2: View from the base of the debris.

Snowpit or crown profile photo or graph: 
Avalanche Photos: 
Weather Observations
Air temperature: 
Precipitation: 
Air temperature trend: 
Wind Direction: 
Accumulation rate: 
More detailed information about the weather: