Natural wind slab avalanches and storm slab instability on Andesite Peak.

Natural avalanches were fairly widespread at 11 am, occurring on most near treeline and above treeline wind loaded ENE to E aspect slopes steeper than 35-38 degrees. Crown height of 8 to 12 inches.

Departing from the trailhead, our first signs of the instability ahead were very easy shears (failed on isolation) at the old/new interface in hand pits in open below treeline terrain and snow surface cracking of the surface slab while breaking trail (photo).  Looking at the snow crystals at the failure plane with a magnifier revealed 2mm surface hoar at the old/new snow interface. This would have formed under clear skies around 7-8 pm last night.

Upon reaching treeline and the first wind loaded feature encountered, we were able to intentionally remote trigger a wind slab avalanche with an 8" crown (photos). The avalanche was triggered by approaching the top corner of the slab and creating a shooting crack that became an avalanche once it propagated out into terrain 35-40 degrees in slope angle. The avalanche was 25' wide.

Very careful route finding while ascending through 30 degree terrain allowed us to reach above treeline and see numerous 1 foot crowns from natural wind slab avalanches that had all failed on slopes steeper that 35-38 degrees (photos).  Several of these avalanches had wide propagation with cracking continuing beyond the flank into sub 35 degree terrain. The widest propagation observed was around 200'.

We ski cut a few other 30-35 degree test slopes and triggered some cracking but no other avalanches. Were were able to enjoyably descend 30-32 degree terrain on our way back out of the area. Upon exiting the area at 1:30 pm, new snow accumulation from this morning was 10 inches.