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Large Natural Avalanche Near Bunny Flat

Location Name: 
Road below Bunny Flat 6,800 feet
Region: 
Mt. Shasta
Date and time of avalanche (best estimate if unknown): 
Mon, 03/18/2019 - 3:40pm
Observation made by: 
Forecaster
Red Flags:
Rapid warming
Obvious avalanche path
Terrain Trap

Location Map

United States
41° 21' 6.8796" N, 122° 14' 17.3652" W
See map: Google Maps
US

Avalanche Observations

Details

A large (R3D3) natural wet-slab avalanche occurred on the road cut near Bunny Flat trailhead around 3:40 pm this afternoon. This avalanche was caused by water trickling through melting snow.  The water pooled and lubricated a bed surface made of clustered rounded melt forms 3 millimeters thick with a thin (<1 millimeter) ice crust.  The slab consisted of wet rounded grains F-4F hard. The bottom inch (2-3 cm) was saturated with water. Dryer rounded snow was found under the bed surface 4F-1F hard. The avalanche ran on a south southeast aspect (154°). The crown ran 150 feet along the slope and averaged 19 inches thick (50 cm). The debris pile filled up one lane of the road 8 feet deep.

Avalanche Classification Code: WS-NS-R3-D3-U

Path Description

  • Elevation range start zone: 6,930 - 6,900 feet
  • Elevation range of track: 6,900 - 6,880 feet
  • Elevation range of run out zone: 6,880 - 6,850 feet
  • Vertical fall (top of start zone to lower run out zone) : 80 feet
  • Length of path run: 150 feet
  • Angle (α) of start zone / runout: 39-40° / 5°

Path Characteristics

  • Exposure to wind/drifting snow: Leeward in northerly wind
  • Exposure to solar: Early morning sun to full sun afternoon 
  • Surface roughness: Scattered small cliffs, rocks and roots
  • General terrain shape: Concave feature
  • Estimated frequency: Frequent
  • Destructive Potential: D3 = could bury or damage a car, break a few trees, injure or kill a person

  • View of bed surface


  •  Avalanche crown - 19 inches (50 cm)


  • View of bed surface. Note: Boot pen of 3 inches (8 cm)


  • Debris pile was taller than the truck and filled up one lane of the road


 




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Characteristics

Avalanche Type: 
Wet
Trigger type: 
Natural
Slope: 
40degrees
Aspect: 
Southeast
Elevation: 
6 930ft.
Terrain: 
Below Treeline
Weak Layer: 
Old Snow
Bed Surface: 
Old Snow
Crown Height: 
2 ft
Avalanche Width: 
500ft.
Avalanche Length: 
80ft.

Weather Observations

Details

Tempature have been stedily rising over the last week. High temperatures for the last three days recorded at Sand Flat 6,750 feet are 50, 50 and 53 degrees F. Low temperatures have stayed just below freezing for the last three nights. Little wind has been recorded over the last three days. 

Statistics

Cloud Cover: 
Clear
Precipitation: 
None
Accumulation rate: 
None
Air temperature: 
Above Freezing
Air temperature trend: 
Warming
Wind Speed: 
Calm
Wind Direction: 
South