Reactive wind slabs may lurk around ridge features and steep, convex slopes. And keep an eye on large and looming cornices, a strong sun make make them more prone to failure.
Northwest to southwesterly winds have impacted the 10-30 cm recent storm snow resulting in wind slabs on a variety of aspects in exposed alpine and treeline terrain. Most of Saturdays reactive wind slabs were in upper treeline areas and above, generally near a large area of loose snow available for wind transport. Where wind slabs sit over a crust, they may be more sensitive to triggering.
We’re moving into a dryer period that is expected to be with us for the foreseeable future. Freezing levels look very reasonable for the middle of February and there is a lot of sun in the forecast Sunday through Wednesday.
SUNDAY NIGHT: Mostly clear. Alpine low temperature -10 C. Northwest wind 15-30 km/hr. Freezing level valley bottom.
MONDAY: Mix of sun and cloud. Alpine high temperature -7 C. Northwest wind 10-25 km/hr. Freezing level 800 m.
TUESDAY: Sunny. Alpine high temperature -4 C. East wind 10-15 km/hr. Freezing level 1100 m.
WEDNESDAY: Sunny with clouds. Alpine high temperature -1 C. South wind 15-30 km/hr. Freezing level 1500 m.
On Saturday, small size (1-1.5) wind slab avalanches were triggered by skiers and explosives in the alpine and treeline. On Friday fast moving storm slabs with crowns averaging 20 cm in depth were quite sensitive to human triggers and explosive control work to size 1.5. Control work also produced cornice failures to size 2.5.
Cornices have grown with wind and recent fresh snow. On Thursday, large cornices were reactive to explosives and skiers, and one failed naturally. Explosives triggered wind slab avalanches to size 1.5, which failed over a crust.
A very large (size 3.5) avalanche was observed on February 9th near Whistler on a steep north face at 2400 m. It is suspected to have failed on a layer of facets on a crust from late November. This very large event demonstrates the ongoing need for caution in aggressive alpine terrain.
South and westerly winds have impacted 10-30 cm recent storm snow producing pockets of windslab in the alpine and upper treeline. The new snow covered a variable surface of harder wind slabs, a sun crust on solar aspects, and settled snow in sheltered terrain. A rain crust sits 30-50 cm below the surface at elevations below 2000 m. The bond at this interface appears to be reasonably strong. Recent winds have scoured an exposed these surfaces in higher elevation exposed terrain.
While weak faceted grains and crusts near the base of the snowpack have mostly not been a problem recently, a very large avalanche was observed on this layer on Feb 9. The problem appears isolated to very aggressive alpine terrain and is likely more prevalent in inland parts of the region.
