Observer Name
R. Kosinski
Observation Date
Tuesday, February 2, 2021
Avalanche Date
Tuesday, February 2, 2021
Region
Skyline » Birch Creek » South Fork Birch Creek » Towhead
Location Name or Route
Towhead NW chutes
Elevation
10,000'
Aspect
North
Trigger
Skier
Avalanche Type
Soft Slab
Avalanche Problem
Persistent Weak Layer
Weak Layer
Facets
Depth
3'
Width
200'
Carried
1
Snow Profile Comments
This failed on a 2" melt/freeze crust above about 1' of facets and had large facets on top of m/f crust. The layer that avalanched contained snow from storms over the last 2 weeks.
>2ft fist hard snow
>medium to large, decomposed facets
>2in pencil hard m/f crust
>1ft large decomposed facets
Comments
At around noon, I decided to drop from Skyline Dr into a "chute" like area that had trees separating the small "chute" concavities. The top of this run had a short, steep section at the entrance. I was going through this chute to cut over to an area with lower angle terrain but a more difficult entrance I was avoiding. Due to the wind loading present in the area, I decided to ski cut at the base of this short section with a safe point in the trees. I dropped in and halfway across I gave a bounce and the entire chute shattered. The snow gave out below my skis which killed my momentum and the flanks ran wider than expected. I rode the very edge of the avalanche for about 30ft on top and right as I was about to deploy my airbag, I came to a stop. I looked down and over to see the next chute over also ran and the avalanche was going fast into the flats below with a large powder cloud following. My skis and poles came off and all but a pole were recovered.
The avalanche was a D2/R4 contained enough snow to bury and kill a person and entraining most of the snow in the path. It contained new snow that failed on a small layer of decomposed facets on top of a stout 2in melt/freeze (possibly rain?) crust. This crust was on top of about 1ft of facets and the avalanche did not appear to step down. This was triggered intentionally on a ski cut. It is possible that this area was cross loaded as well. The avalanche had a width of around 20ft at the start zone and widended to around 200ft mid path. The avalanche dropped 600ft vertical and ran approximately 1300ft. The average crown depth was 2ft but was 3ft at the crown on the convexity. Starting elevation 10000', N aspect, 34 degree slope angle. The debris deposited on a bench about 5-7 feet deep and the toe of the debris was about 150ft further and was around 5 feet deep.
Looking back on this accident, I think this is a case of a lot of small things adding up. I thought the slope angle was a couple degrees lower and the aspect was more NW. Having seen the wind drifts in the area, I was thinking more about windslab problems than with PWL problems. There was a lack of familiarity with the weather in this area this winter. Based on what I was seeing elsewhere, this problem is localized to similar aspect/elevation which I had not encountered till this point on my tour. This run also appeared to have been crossloaded since the beginning of the season causing the avalanche to be bigger than I anticipated. In retrospect, performing the ski cut likely saved my life; if the slope had broken above me I would have gone on quite the ride. Some things that I think helped keep me on top were having identified a safe zone to go towards, not locking my toes/pole straps off, fighting hard to get off the slide, and remaining calm throughout and after the ordeal.
Pic 1: Overview
Pic 2: Crown
Pic 3: Looking down from where I stopped
Pic 4: Failure layer (facets)
Coordinates