Avalanche: Turnagain

Location: Seattle Ridge - backside bowls, multiple avalanches

Route & General Observations

Rode the common route up to Seattle Ridge, cleared off the weather station and headed south to see how the snowpack was doing at the higher elevations. Upon entering Triangle Bowl (-3 Bowl) there were large crowns on all sides, N, W, and South. Several avalanches had been triggered by riders earlier in the day, no one caught to our knowledge.

Also received a couple photos from the air of two additional remotely triggered (from ridge) avalanches in the bowl directly south of Triangle/-3 bowl. All from today.

Avalanche Details
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Trigger SnowmachinerRemote Trigger Yes
Avalanche Type Hard SlabAspect Unknown
Elevation 3000ftSlope Angleunknown
Crown Depth3ftWidthunknown
Vertical Rununknown  
Avalanche Details

At least three remotely triggered large hard slab avalanches in Triangle Bowl (-3 Bowl). The highest slide was remotely triggered by riders about 100' away, next to, slope. The other two slides a bit lower on the opposite side of the bowl are thought to have been sympathetically triggered by the first avalanche, since the avalanches were already there when the group descended afterward (meaning triggered by a collapse in the snowpack caused by original avalanche).

Elevations of crowns: 3,000 - 2,500'
Aspects: N, NW, and SW
Weak layers: Facets near the ground and/or near the base of the snowpack; possibly involving the Thanksgiving crust and/or the mid-Dec facets. My guess is a bit on a combination.
Crown depths: 1-8' deep (estimated)
Widths: Between 500' and 1,500' wide (estimated) One slab wrapped around from a SW face to West that went out of view
Lengths: Between 300' and 1,000' - essentially they ran until the slope lessened or stopped

We were able to investigate one crown. It was on a North aspect at 3,000' on a ~40deg slope.
Total snowpack varied from a couple feet to 10' or so. The weak layer was likely facets under the Thanksgiving crust. The bed surface was 1-2' of sugary faceted snow, easy to punch a boot through to rocks underneath.

Red Flags
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Observer Comments

The main Red Flag today was the extent of recent avalanches. There were 5 slabs recently triggered on the front side of Seattle Ridge (posted in another report) and up to 5, maybe more, on the backside.
No cracking or collapsing.

Weather & Snow Characteristics
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Sunny! Some valley fog. Temps in the 20'sF. No wind.

Snow surface

1-3" of soft snow on crust from the parking lot to 1,400' or so.
Soft settled snow above 1,500', becoming drier with some wind effect in the higher exposed ridges.


We looked as closely to these slides as we could from below. All appeared to be similar to the one we investigated. Essentially the Xmas, New Years, and yesterday's storm snow all have settled into a slab anywhere from 1-8' feet deep depending on the amount of wind distribution. Average depth however seemed to be in the 3-4' range. This is all sitting on the December facets, Thanksgiving crust, and October Facets at the very bottom.

Photos & Video
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