New snow and wind will form fresh wind slabs at upper elevations. These may be especially reactive where they sit over a weak layer or a crust.
New snow and strong overnight wind will form wind slabs in lee terrain features at upper elevations. They may be especially reactive where they sit on a weak layer of surface hoar or a slippery crust.
This problem may exist in the north of the region. We have few observations and plenty of uncertainty around whether a buried weak layer remains a problem at upper elevations. Very large avalanches are suspected to have run on this layer during the atmospheric river.
Greatest snowfall amounts for Tuesday will be concentrated near Hope.
Tuesday night: New snow 5-10 cm. Strong southwest wind. Freezing level 800 m.
Wednesday: New snow 5-10 cm. Moderate southwest wind. Treeline high temperature around -8. Freezing level dropping to valley bottom.
Thursday: New snow 5-10 cm. Light westerly wind. Treeline high temperatures around -12.
Friday: Flurries up to 5 cm. Moderate southwest wind. Treeline high temperatures around -9.
Over the weekend, some loose dry sluffing and thin soft slab reactivity were seen in steep terrain and near ridge crests in the neighboring Sea to Sky region. The most recent reports we have from the inland are of explosives producing a size 2 wind slab avalanche in the Hurley last Thursday.
10-20 cm of new snow falls over wind affected surfaces and/or a crust.
The south, having endured warmer temperatures and more rainfall, is largely below threshold for avalanches at most elevations. Previous snow has seen extensive wind effect, scouring the rain crust clean in some areas, while piling up to 40 cm over it in others.
In the north, the new snow may be sitting over a layer of surface hoar crystals, older wind affected snow in the alpine, a thick crust below 1800 m or a combination of these.
A couple of buried weak layers produced large avalanches during a previous storm. Snowpack models and observations in the neighboring Sea to Sky region indicate that these layers have been reset by previous rain at treeline, but uncertainty remains around their status in the alpine. These include a layer of surface hoar down an estimated 90-120 cm and deeper crust/facet combo layer.
Average snowpack depths in the alpine are now likely closer to 150-200 cm. Below treeline, depths of 30-50 cm have been reported around 500 m, decreasing dramatically with elevation, and still below threshold for avalanches in many areas.
