Seeing this deep and fairly large wet slab really threw us for a loop given the perceived danger rating and made us re-think and change our intended line for the day.
Could have been caused by a cornice fall to trigger the upper-most part of the slide (in the chute) but difficult to tell. There was a snowmobile side hill track that spanned across the middle area of the slab as well. It looked fairly fresh when we arrived but with so many sledders out it was difficult to tell old tracks from more recent. It was definitely triggered in the morning or early afternoon as the debris was fresh and didn't have the 1" of dust from last night.
We were pressed for time so didn't inspect the crowns but it looked to be about 4 feet deep and was likely a slow mover judging by the debris and length of track. Maybe 8-10 foot deep debris pile. I would assume it failed on the old rain crust.
Photo below outlines the interestingly shaped crown and debris pile. Arrows point to possible trigger points. Maybe cornice fall into upper chute or could have been the snowmobile track entering and exiting on the left and right.
I went up to the avalanche site on 3/13/17. The impressive and unexpected hard slab avalanche was an isolated event. No similar avalanches were reported or observed in the area. However, an avalanche that appears to have run on the same 2/10 ice-crust and also on an upper elevation east facing slope was triggered by a sledder on 3/3 near Hidden Lake in Gibson Basin, just north of the state line. (view report here)
Toby Weed UAC, Logan Zone Forecaster