Dangerous slab avalanches are a concern at all elevations. Conservative low-angle terrain choices are recommended because there is uncertainty about how well the new snow will bond to old layers.
Cornices have grown larger during the storm. When large chunks of cornice fall there is potential for very large slab avalanches as evidenced by recent activity.
50 to 100 cm of snow from the past week combined with strong southwest wind has left thick reactive slabs. This snow may be resting on well-preserved surface hoar at and below treeline. South and west facing slopes may also have a buried crust that could serve as a bed surface for large avalanches. Overall there is serious potential to trigger large slab avalanches in the wake of the storm.
THURSDAY NIGHT: Another 5-10 cm of snow as the storm tracks south of the region, strong southwest wind, freezing level drops to 400 m, alpine temperatures drop to -4 C.
FRIDAY: Broken cloud cover with clearing skies in the afternoon, light wind from the north, freezing level between 400-800 m with alpine high temperatures around -2 C.
SATURDAY: Mostly clear skies, light variable wind, freezing level stays below 500 m, alpine high temperatures around -4 C.
SUNDAY: Light flurries start in the afternoon, wind increase from light to strong from the southwest, freezing level stays below 500 m, alpine high temperatures around -4 C.
Widespread avalanche activity has likely occurred throughout a storm that started Tuesday night and is expected to end Thursday night. By Friday natural activity should wind down, but storm slabs will remain primed for human triggering.
Despite the poor visibility, several size 2-2.5 slab avalanches were reported in alpine terrain on Wednesday. These large natural avalanches mostly released on lee wind-loaded slopes, and in some cases were triggered by collapsing cornices. Storm slab avalanches were also very reactive to human triggers on Wednesday. Numerous small (size 1) slabs were triggered in the top 20-30 cm of snow and a few large (size 2) slabs were triggered remotely by riders on north and east aspects at treeline. These large slabs released on the weak surface hoar layer that sits beneath 60 cm of storm snow. There were also numerous smaller (size 1-1.5) wet loose avalanches below 1000 m where rain has fallen on snow.
On February 24 in the Ningunsaw zone a natural cornice failure resulted in a very large (size 3.5) deep persistent slab avalanche on a south/southeast facing feature.
Glide crack openings and failures continue to be reported (almost daily) too.
By Friday morning the three day storm will have delivered another 30-50 cm of snow to the region and a left storm slabs primed for human triggering. When combined with the previous storm, a total of 50 to 100 cm of snow has fallen in the past week. There is potential for "step down" avalanche activity as the fresh snow interacts with "old" storm slabs that overly a couple layers of surface hoar that may be found in sheltered terrain at and below treeline or a melt-freeze crust on sun-exposed aspects.
A thin layer of weak and sugary faceted grains that formed in January may be found about 120 to 170 cm deep, and an early-season melt-freeze crust lingers at the base of the snowpack. These layers produced a few large natural avalanches in early-February and were largely nonreactive until the Feb 24th avalanche listed above was reported.
