Avalanche: Turnagain

Location: Seattle Ridge (Warm-up Bowl / -1 Bowl)

Date:
Observer:
Route & General Observations

We rode up Seattle Ridge to Warm-up Bowl (-1 Bowl) to investigate yesterday’s snowboard-triggered avalanche.

Avalanche Details
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Trigger SnowboarderAvalanche Type Hard Slab
Aspect West SouthwestElevation 3000ft
Slope Angle 38degCrown Depth 24in
Width 1000ftVertical Run 800ft
Near Miss / Accident Details
Number Caught/Carried? 1Number Partially Buried? 1
Number Fully Buried?0Number Injured?0
Number Fatalities?0  
Avalanche Details

Snowboarder-triggered avalanche from Wednesday, 12/23/20.

This was a very large wind slab avalanche that failed on a layer of precipitation particles (stellar dendrites) at the interface between the old snow and the 12/22 storm snow. The 12/22 storm deposited around 3' of moist sticky snow, which was blown into large wind slabs on leeward slopes by very strong winds. The winds were so strong that much of the wind loading occurred toward the mid-slope and not just the top of the slope as is often the case.

The average crown depth was 2', but there were large sections with a depth of up to 4' or deeper. The slab was a firm (4-finger to pencil hardness) layer of wind deposited snow. The avalanche propagated approximately 1000' wide and ran roughly 800 vertical feet. The wide propagation and character of the crown was similar to a persistent weak layer, which sometimes these stellars can act like before they round and bond.

HS-ARu-R3-D2.5-I

Events of the day

Confirmed details are limited at this time but they include:

A snowboarder, who was the 5th person on the slope, triggered a very large and connected fresh wind slab just under the ridgeline. The boarder was caught and carried approximately 500 vertical feet. They deployed their airbag and when the debris came to rest they were buried up to the neck. The boarder was dug out by their partner(s) and was ok. The group had accessed the area by snowmachine and the debris had buried one snowmachine 5-7' deep and another partially that were at the bottom on the slope when the avalanche occurred. No other group members were caught.

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

Skies were clear and sunny on the day of the avalanche. The following day of the avalanche investigation 12/24, visibility was poor and lifted just long enough for us to get in and get a good look at the crown. Light to moderate winds were variable in direction, from NE to NW.

Snow surface

We saw a lot of recent wind loading at higher elevations, and relatively undisturbed snow in the flats. Storm snow depth at 3000' varied from around 2' to over 4', depending on wind loading. The storm finished with moderate winds, so even areas that were heavily wind-loaded still had a few inches of soft snow on top.

Snowpack

The 2-4' of new/wind-loaded snow is sitting on a layer of rounding dendrites at the new/old snow interface. Both the slab and the bed surface layers were 1-finger to pencil hard, while the weak layer was much softer. Probably 4-finger, but it was too thin to really get a good sense.

From two pits just above the crown (3030', WSW aspect): ECTP 29, down 60 cm failed on the layer of dendrites at the interface. ECTX where weak layer was buried 80 cm deep.

Photos & Video
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