Remotely triggered persistent slab avalanche on Carpenter Ridge

We snowmobiled out snow-covered Forest Service roads to the base of Carpenter Ridge. On the sled out there, we could feel the snowmobile tracks digging through the recent snow into softer old snow whenever we got off the road. We parked the sleds at the base of the ridge and skied up low angle slopes to gain Carpenter Ridge. We probed the snowpack with our poles to feel for old weak layers buried below the surface while we skinned. We could feel them when we pushed the poles about 2 ft into the snowpack. We skinned along the ridge looking for a place to dig into the snowpack. We crossed over the top of a steep wind-loaded slope to lower angle terrain on the other side. We chose to dig a pit on the 26-degree slope in an opening just below the ridge.

Brandon started digging and chose to leave his skis on for his first test, leary of punching his boot down into the Dec 11 PWL. This first ECT propagated across the column on the 23rd tap (ECTP-23) at the interface between yesterday's new snow and the old snow surface - about 12 inches (30 cm) down. The snow crystals at the old/new interface were 1 mm rounds and 2 mm graupel. After ski packing down a platform that felt like it would be boot supportable, Brandon stepped off his skis. As he then took an additional step into his pit, his boot penetrated down into the weak faceted snow below (the Dec 11 PWL) and we heard a large whumpf and felt a large collapse. About 150 yds (150 m) behind us, we saw an avalanche release on the steeper wind loaded slope behind us. We went back to check it out.

We could see shooting cracks connecting from the place where Brandon dug the pit over to the avalanche. When we got back to the avalanche, we measured the slope at 38 degrees in most places. However, this persistent slab avalanche had failed across some sections of the slope that were only 32-34 degrees. It had also fractured up into lower angle terrain on the ridge and pulled some of the snow from the ridgetop into the avalanche. The crown varied from 2 ft deep to about 5 ft deep, average 3 feet deep. It was deepest where slabs of wind drifted snow had piled up near the ridgetop. We dug into the crown at a few places and saw that the avalanche had failed on the Dec 11 PWL facet layer (see crown profile for details).

We continued up the ridge on our tour. We saw signs of wind loading and got some small cracking in wind loaded test slopes. When we got to the top of the ridge we chose to descend low angle slopes. These low-angle slopes held soft snow with good coverage.  They also had the added bonus of not being steep enough for persistent slab avalanches.