Rare Natural Activity on Powderhouse

Location Name: 
Powderhouse
Region: 
Luther Pass Area (including Job and Freel)
Date and time of avalanche (best estimate if unknown): 
Sun, 03/20/2011 - 10:00
Location Map: 
United States
38° 47' 7.6884" N, 119° 58' 36.9336" W
See map: Google Maps
US


Red Flags: 
Whumphing noises, shooting cracks, or collapsing
Recent loading by new snow, wind, or rain
Observation made by: Professional Observer
Avalanche Observations
Avalanche Type: 
dry,slab,
Slope: 
40degrees
Trigger type: 
Natural
Crown Height: 
1 ft
Aspect: 
North
Weak Layer: 
Storm Snow
Avalanche Width: 
50ft.
Terrain: 
Below Treeline
Elevation: 
8 900ft.
Bed Surface: 
Other - explain below
Avalanche Length: 
50ft.
Number of similar avalanches: 
2
More detailed information about the avalanche: 

Note: given lat/long estimated.

Pic 1: Although 30 to 40cm of 24hr snow remained unconsolidated, there were occasional and subtle clues to wind transport in traditionally protected areas.

Pics 2 and 3: Two natural crowns on steep convexities. Pic 3 crown ~300ft uphill from pic 2. Neither avalanche was large enough to cause harm. Debris was soft and broken, and came to rest within a very short distance, where terrain features became less steep. Bed surface was a thin layer of denser snow from yesterday's warm temps. Weak layer 1mm stellars. Soft slab comprised of wind broken PPs with many PPco.

Upon descending, I triggered 1 more slide similar to those described above.

Several easy Q2 and Q3 hand shears at same weak layer as observed naturals. Ski cutting above skin track tended to fail on softer snow beneath the denser layer that served as bed surface in observed naturals. Similarly, very localized collapses were felt beneath denser layer. Shooting cracks limited to 1m propagation.

On the steeper, more traditional avalanche terrain just E of given lat/long, moderately buried loose debris was observed, implying several loose snow or very soft slab avalanches early in the previous 24hr cycle.

Although winds increased to moderate at ridgetop, snow surface remained unconsolidated.

 

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: 
Static
Wind Direction: 
Southwest
Accumulation rate: 
Less than 1 in. per hour
More detailed information about the weather: 

Weather obs provided for ridgetop conditions. Snowfall intensity appeared to be increasing in the early afternoon.