Sonora Pass Avalanche

Location Name: 
Sonora Pass Avalanche
Region: 
BWRA
Date and time of avalanche (best estimate if unknown): 
Mon, 03/19/2018 - 12:52
Location Map: 


Red Flags: 
Recent avalanche activity
Whumphing noises, shooting cracks, or collapsing
Rapid warming
Obvious avalanche path
Terrain Trap

Observation made by: Avalanche Specialist
Avalanche Observations
Avalanche Type: 
Dry
Slab
Slope: 
42degrees
Trigger type: 
Natural
Crown Height: 
7 ft
Aspect: 
Northeast
Weak Layer: 
Old Snow
Avalanche Width: 
120ft.
Terrain: 
Near Treeline
Elevation: 
9 800ft.
Bed Surface: 
Old Snow
Avalanche Length: 
100ft.
Number of similar avalanches: 
3
Number of people caught: 
0
Number of partial burials: 
0
Number of full burials: 
0
Did the avalanche hit a building, vehicle, or structure?: 
No
More detailed information about the avalanche: 

Rapid warming set off a few Loose wet & Cornice fall avalanches. As we rounded the corner towards Sonora Pass we noticed a very recent slab avalanche on a NE steep rollover. There were multiple snowmobile tracks in the area & none were involved with this R2D3 avalanche. This terrain feature is adjacent to a waterfall & is commonly climbed by sledders & skiers alike. The convexity collects a lot of wind driven snow & a cornice had formed. From my observation it seems as the cornice was melted enough to fall onto a tender wind slab which released. This slab stepped into an old lingering Deep Slab problem which has remained dormant for some time, possibly associated with a graupel layer. My coworker & I discussed not traveling on the slope to investigate due to increasing instability & lots of hangfire that was cracking. Not having witnessed nor gaining any public reports of this slide leaves a lot of questions; such as was this remotely triggered by snowmobiles on the adjacent slope. This is the first instance of a serious Deep Slab Avalanche problem in the BWRA.

Our patrol continued over the pass to look at a large slope that regularly avalanches huge & into Hwy 108 on the Stanislaus NF. Our intuition proved correct & the face in question ran large & also stepped into old Deep Slab instabilities mid storm. The pictures show Wind slab avalanches that failed high on the slope stepping 3' into the deep slab issues mid slope, with likely sympathetic releases from wind slabs connected to the larger slope. With rain in the forecast expect this Deep Slab avalanche problem to continue & get much worse.

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

 Intense sun & warm temperatures in the 40s changed the texture & density of the surface snow. Powder turned into mash potatoes in the afternoon. The haze & scattered clouds increased as we left the BWRA.