Wind slabs in Johnson Canyon

Location Name: 
Johnson Canyon
Region: 
Donner Summit Area
Observation Date & Time: 
Wednesday, December 29, 2021 - 12:12
Location: 
39.344581, -120.293181
Is this an Avalanche Observation?: 
Yes


Conditions Alerts:


Terrain Alerts: 
Slopes Steeper than 30 degrees
Obvious Slide Path

Observation made by: Educator at West Wind Collective

Tabs

Observation
Description of Snow, Weather, and Avalanche Conditions: 

Today was day 2 of of a 3 day AIARE 1, we went to Johnson Canyon to take observations, monitor conditions and see how the snowpack was setting up. Weather was as expected for most of the day with mostly cloudy skies, below freezing temps, scattered snow showers, calm SW winds below treeline with moderste gusts near treeline. The snowpack at the lower elevations had little to no wind effect, with ski penetration between boot top and mid shin. As we gained elevation, wind effect was more apparent and our first small test slope produced a shooting crack, which was our first sign of instability during the day. When the sun would break through, we could see wind transport along the higher ridge lines. Our goal today was to find fresh windslabs in non consequential terrain, we were able to do this on a near treeline ridge with a small slope that had a start zone of 35 degrees. A small cornice cut released a Size 1 windslab that a crown depth of 1' and propagated 20' wide and ran about 40' downhill. This result matched our anticipated hazards of the day and we opted to not go any higher and ski the more wind protect trees back to the cars. 

Hide Avalanche Details
Avalanche Type: 
Wind Slab
Failure Plane/Weak Layer: 
Old Snow
Hide Trigger
Trigger: 
Snowboarder
Trigger Modifier: 
Intentionally Triggered
Hide Terrain
Start Zone Slope Angle: 
35degrees
Aspect: 
Northeast
Starting Elevation: 
near-treeline
Hide Size
Destructive Size: 
D1 Relatively harmless to people.
Relative Size: 
R2 Small
Crown Height: 
1 ft
Avalanche Width (Average width): 
20ft.
Avalanche Length (Vertical Run): 
40ft.
Hide Weather Details
Elevation of Observation: 
7000 - 8000 ft.
Blowing Snow: 
Moderate
Wind Direction: 
Southwest
Wind Speed: 
Moderate
Sky Cover: 
Overcast - Cloudy - 8/8 covered
Highest Precipitation Rate: 
Very Light Snowfall (S-1 = up to .5cm /~.25 in per hour)
Precipitation: 
Snow
Air temperature: 
Below Freezing
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