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 is bonding 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.
60 to 130 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.
A brief respite...
FRIDAY NIGHT: Freezing level at valley bottom, light wind generally out of the north, trace of snow possible.
SATURDAY: Broken cloud cover, freezing level holding around valley bottom, light northwest wind, trace of snow possible.
SUNDAY: Overcast, freezing level rising to about 1000 m, strong west/northwest wind, 5 to 10 cm of snow possible during the day, 5 to 15 cm Sunday night.
MONDAY: Scattered cloud cover in the morning, overcast in the afternoon, freezing level rising to about 800 m, strong west/southwest wind, 1 to 5 cm of snow possible during the day, 10 to 20 cm of snow possible Monday night.
Visibility has been limited but storm slab avalanches to size 3 have been reported through the storm that started Tuesday night and ended Thursday night.
On Thursday natural avalanches to size 3 were observed in steep alpine terrain. Rain on snow allowed for loose wet avalanches to size 2 at lower elevations too.
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.
This week's storm was a doozy, when combined with the previous storm, a total of 60 to 130 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.
