Tallac-Human triggered slab avalanche in corkscrew bowl

Location Name: 
Tallac Corkscrew bowl, skiers left entry
Region: 
Desolation Wilderness Area (including Emerald Bay)
Date and time of avalanche (best estimate if unknown): 
Thu, 01/14/2016 - 13:00
Location Map: 
United States
38° 54' 41.5152" N, 120° 6' 2.8836" W
US


Red Flags: 
Recent avalanche activity
Recent loading by new snow, wind, or rain

Observation made by: Public
Avalanche Observations
Avalanche Type: 
Dry
Slab
Slope: 
35degrees
Trigger type: 
Snowboarder
Crown Height: 
1 ft
Aspect: 
East
Weak Layer: 
Other - explain below
Avalanche Width: 
35ft.
Terrain: 
Above Treeline
Elevation: 
9 100ft.
Bed Surface: 
Other - explain below
Avalanche Length: 
200ft.
Number of people caught: 
1
Number of partial burials: 
1
More detailed information about the avalanche: 

Disclosure: I am not a professional or a forecaster

This was a very eye opening experience on a number of levels.  Here are a few thoughts and pitfalls that our team discussed when we debriefed following the event.  This slide was not giant but could very well have buried a human, was very heavy with large debris, and ran for quite a ways  through some trees thereby increasing the chance of full burial and blunt trauma.  So real deal stuff.

First, this is widely considered a safe zone by Tahoe locals.  The entry off the ridge certainly has steep entries, and cornices and wind slabs, but there are several "safe entries" into this bowl and this was one of them. Although it was about 35 degrees for a short time and fairly open.  So, good reminder.

We knew the set-up, the reports have demonstrated the weak layers, and we gathered our own pit data prior to reaching the ridge which clearly showed reactive wind slab on a hoar layer about 30 cm deep (CT3 for me).  We initially decided to drop in below the ridge because of this data, and for reasons that aren't clear, we went ahead up to the ridge, as it was very close and not much steeper.  

We skied our first lap on skiers right of the bowl with no evidence of weakness, whumfing, or cracks or slabs off the ridge right there.  

We opted to ski a second lap off the ridge skiers left, which has a fairly low angle entry available in spite of the majority of it being quite steep, we thought we could safely enter the bowl here,

There was already a set of tracks right through the line we chose to ski.  CLASSIC false security pitfall.

The first two skiers dropped in with no problems, although the second skier saw a small shooting crack at the entry point. The slide was triggered by the third who picked a line in between the two other tracks.  Very unusual for this third person to be the trigger.  I am unable to comment on any visible potential trigger points as I didn't visualize this area. I was below and did not drop in at that point. 

It was apparent to me that the person who triggered the slide was above, and this is a scary situation because it takes a long time to skin up.  

We immediately began a search as two of our party were not immediately accounted for. One skier went down the avalanche path, and I immediately started going up it.  Within a few minutes, we all had voice and visual conformation of each other.  The snowboarder partially buried above just needed a little help getting out of his bindings and digging out one of his lower extremities.

Take home:
-Stay away form the ridge lines on days like this especially with known buried hoar
-Use the data you have to make a plan.  In this case we realized we should avoid the ridge, and assumed our primary risk was wind slabs off the ridge, but went up anyway.  We initially decided to drop in low below the ridge and subconsciously changed out minds without even realizing it.  This is a cognitive error.  Make a decision, verbalize it, discuss it, and then do not change unless swayed by additional data.
-Be very very careful when are there buried hoar layers and wind.  Zones traditionally considered safe may not represent safe zones in these conditions.

Snowpit or crown profile photo or graph: 
This is the crown profile
The crown from below, this shows about half the total crown I believe
This is the debris field, the arrow marks where the snowboarder had ended up in the tree and was buried up to his waist.  His head and neck were above the snow the whole time, and had no injuries.
This pit below the final ridge on the skin track, East facing, 33degrees 9000ft shows the weak layer which was very reactive CT3 for me and CT 13 for the one on the right.  Not sure if this is a new layer or if this is an older, seems like a new layer and the new storm snow/wind slab.
Avalanche Photos: 
Weather Observations
Blowing Snow: 
Yes
Cloud Cover: 
50% of the sky covered by clouds
Air temperature: 
Below Freezing
Wind Speed: 
Moderate
Precipitation: 
None
Air temperature trend: 
Static
Wind Direction: 
Northeast
Accumulation rate: 
None
More detailed information about the weather: 

Wind was picking up before the storm with heavy gusts on the ridge, but was calm earlier in the day