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Observation: Turnagain

Location: Seattle Creek Drainage

Date:
Observer:
Route & General Observations

Rode into the Seattle Creek drainage to investigate the weak layer responsible for the widespread natural avalanche activity last week as well as the several snowmachine triggered slides in the area from over the weekend. Bad news, the March 27 buried surface hoar was the culprit weak layer (as suspected) and this means the potential for human triggered large avalanches remains and will not go away soon in areas that have not slid.

Red Flags
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Observer Comments

No signs of obvious instability.

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

Mostly cloudy skies, temperatures in the 40's at 1,000' and 30's at 2,500'. Winds were light from the Northeast. A few flurries in the afternoon but no measurable accumulation.

Snow surface

Lower elevation (1,500' and below) had a supportable 6-8" melt-freeze crust that softened and became unsupportable by 3-4pm in the afternoon. Above 1,500', a 4-10" layer of moist settled powder existed over a 4-6" melt-freeze crust (some skiers in the area classified this as fun creamy skiing, while snowmachines were not penetrating more than 4").

Snowpack

Dug two pits today. The first was a flank profile for a suspected snowmachine triggered slab avalanche in Main Bowl. The second was near the weather station in a thin spot in the slab to assess potential trigger points for the future.

Pit 1: 2,100' NW aspect. Flank of avalanche.
Slab was 3' thick on flanks and between 2-6' thick at the crown. Average hardness was 1 finger.
Weak layer: BSH 6-8mm
1,500' wide, estimated, running 1,500'.
No stability tests, weak layer had collapsed and had strengthened.

Pit 2: 2,300', N aspect. Thin spot in the slab to assess weak layer that has not collapsed and a potential trigger point.
ECTP 16, 18, 21 down 35-40cm (16") on the Mar 27 buried surface hoar
Slab was 1 Finger hardness and capped by a 2-5cm melt freeze crust with a rimed surface (wind scoured zone, hence why it was a thin spot).

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