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Sun Bowl Avalanche

Submitted by abeverly on Tue, 02/28/2017 - 9:57pm
Location Name: 
Sun Bowl
Region: 
Mt. Shasta
Location Map: 
United States
41° 22' 11.5428" N, 122° 12' 55.5768" W
US
Observation made by: 
Professional Observer
Date and time of avalanche (best estimate if unknown): 
Tue, 02/28/2017 - 8:00am
Red Flags: 
Recent avalanche activityRecent loading by new snow, wind, or rain
Avalanche Observations
Avalanche Type: 
DrySlab
Slope: 
38degrees
Trigger type: 
Natural
Crown Height: 
2 ft
Aspect: 
Southeast
Weak Layer: 
Old Snow
Avalanche Width: 
900ft.
Terrain: 
Above Treeline
Elevation: 
8 680ft.
Bed Surface: 
Old Snow
Avalanche Length: 
800ft.
Number of similar avalanches: 
3
More detailed information about the avalanche: 

Most of sun bowl slid. It appears that there may have been up to 4 different slides. Not clear if they all occurred at the same time. The three smaller ones were likely new windslabs sliding at the new/old snow interface. The more destructive slide occurred within the old snow where an old windslab slid on a rain crust. Some more details:

  • Type: HS (hard slab)
  • Trigger: N/NC (Natural, possibly triggered by cornice fall)
  • Destructive Force: D2-D3
  • Size: R3, medium relative to path
  • Level of Bed Surface: O (The avalanche released within old snow)
  • Slab: Knife hardness, .5 mm rounded grains
  • Slab Dimensions: 20-24 inches thick
  • Length of run: 900'
  • Depth of deposition: 8' in some areas

In a test pit dug nearby, failures were triggered below the new wind slab and below an old deep slab on a rain crust.

  • CT3 SP @ 22 cm, CT12 SP @ 73cm.
  • ECTN4 SP @ 22cm, ECTN28 SP @ 73cm
  • ECTN5 SP @ 73 cm, ECTX

Though the CT tests yielded unstable results for the new wind slab and old wind slab at 22 cm and 73 cm, the ECT tests were difficult to trigger/propagate at the 73 cm depth.

Avalanche Photos: 
Snowpit or crown profile photo or graph: 
Weather Observations
Blowing Snow: 
No
Cloud Cover: 
25% of the sky covered by clouds
Air temperature: 
Below Freezing
Wind Speed: 
Calm
Precipitation: 
None
Air temperature trend: 
Warming
Accumulation rate: 
None
More detailed information about the weather: 

A breakable melt freeze crust has formed below 8500' and was encountered on most aspects down to the Bunny Flat parking lot. Some north facing aspects may not have seen this effect yet.