Feb 6 investigation of Feb 2 deep slab avalanche

We toured into the head of the Pole Creek drainage today (Feb 6) to get a look at the failure layer of the deep slab avalanche that occurred here naturally on Feb 2. This is the third time in our institutional memory that a deep slab avalanche has occurred on this NE aspect slope. The avalanche debris ran full path down two gullies where old broken trees are indicators of past avalanches. The bed surface slope angle ranged from 35 to 37 degrees. Crown depth was an average of 5' with a max depth of 6'. There was one spot low on the southern flank where a secondary crown had pulled out. We classified this avalanche as D2.5-R4-SS-O-NC.

We were able to safely access the far climber's right end of the crown and perform a crown profile. This avalanche failed on faceted snow sandwiched in between two rain crusts that made up the crust/facet sandwich of the Jan 22 rain crust layer. The lower crust was the bed surface. A series of snowpit tests targeting this failure layer gave no indication of ongoing instability.

This same weak layer has been identified as the failure layer in two other recent skier/snowboarder triggered deep slab avalanches on Waterhouse Peak and Mt. Watson. This Pole Creek avalanche occurred naturally, likely triggered by a large cornice collapse. The cornice on top of the cliffs above the crown was eroded by recent NE winds, but it appeared that a large section was missing from the center and still more built out on either side.

Other general observations from the area today were that surface wet snow was forming on all aspects below 7,200' and on all but very heavily shaded northerly aspects above 7,200'. At 12:50 pm we descended some sun cooked wet snow adjacent to the northern flank and did not trigger any roller balls. Evidence of recent wind NE wind scouring was prominent above treeline. Ridgetop winds were light to moderate in speed out of the W while we were at the crown with no blowing/drifting snow occurring. Signs of continued upper snowpack settlement existed around trees. The upper snowpack in the crown profile appeared strong and well bonded.