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Avalanche: Turnagain

Location: Seattle Ridge

Date:
Observer:
Route & General Observations

Seattle ridge common up-track into the backside bowls. Very unstable snowpack. Saw 9 human triggered avalanches – no one caught – 4 of these we witnessed in action.

Avalanche Details
If this is an avalanche observation, click yes below and fill in the form as best as you can. If people were involved, please provide details.
Trigger SnowmachinerAvalanche Type Hard Slab
Aspect UnknownElevation 2300ft
Slope AngleunknownCrown Depth 24in
WidthunknownVertical Rununknown
Avalanche Details

Several remotely triggered slab avalanches along the backside of Seattle ridge. 7 of these were in Main Bowl (1st Bowl), 2 near the weather station and one in south facing Warmup Bowl (-1 Bowl). We witnessed 2 riders making their way along the ridge proper above Main Bowl and triggering 4 of these.

We investigated one of them in Main Bowl, buried surface hoar was the weak layer. It is very likely this was the weak layer in all human triggered avalanches seen today.
Crown heights were 20" to 60" due to wind loading. Average slabs depths were 15"-30".

There was evidence of natural slab avalanche activity in Mamma's Bowl (0 Bowl) and in north facing Warmup Bowl (-1 Bowl).

Red Flags
Red flags are simple visual clues that are a sign of potential avalanche danger. Please record any sign of red flags below.
Obvious signs of instability
Recent Avalanches?Yes
Collapsing (Whumphing)?No
Cracking (Shooting cracks)?No
Observer Comments

Recent avalanches were the only sign of instability. No cracking or collapsing was observed.

Weather & Snow Characteristics
Please provide details to help us determine the weather and snowpack during the time this observation took place.
Weather

Partly sunny then overcast in the afternoon. Light easterly winds. Temperatures in the 20'sF.

Snow surface

Soft settled powder begins around 1,500' and gets better by elevation. Crusty surface snow and conditions below 1,500'

Snowpack

Very reactive snowpack found today along Seattle Ridge.

There is 15" - 25" of new snow from last week's storm that is sitting on surface hoar formed during the short cold spell early last week (Jan 18-21). We are calling the layer the MLK BSH layer. We found no other weakness below this layer. All avalanche activity seen, both today triggered by people, as well as older slabs from the storm all appeared to be relegated to the storm snow unable to bond to the surface hoar.

Snow hardness was fist to 4F in the top 8" before hardening to pencil hard above the buried surface hoar. Below the buried surface hoar, it was the same - pencil hard. No crust was observed below the buried surface hoar. However, there was a thin rain crust (3mm) in the slab around 15cm above the MLK BSH, you can see it in the pit photo.

Pit #1: Near weather station and close to the Widow Maker slide path. West aspect, 2,300', 20 degree slope. ECTP V 22" down on BSH (meaning failure in the block when cutting it on the buried surface hoar). This is the worse possible result for a stability test and can only confirm the high instability of the snowpack. In this pit, the snow hardness was closer to 1finger above and below the MLK BSH.

Photos & Video
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