New snow sitting on a layer of surface hoar may be a recipe for thin but reactive storm slabs over the next few days.
This weak layer formed in late November may present as surface hoar, a crust, facets or a combination, depending on elevation and aspect. It is buried 80-140 cm below the surface and can most likely be found in areas sheltered from the wind, but open to a view of the sky, such as near treeline.
5-20 cm of new snow fell yesterday over a layer of feathery surface hoar crystals. This can be a recipe for touchy storm slab avalanches, sometimes involving remote triggers and wide propagation. The new storm snow also hides wind slabs that have surprised a number of riders over the last few days.
Thursday Night: Flurries accumulating up to 5 cm. Alpine low -8, light west wind.
Friday: Flurries accumulating up to 5 cm overnight, alpine high -5, light west wind.
Saturday: Mainly cloudy, isolated flurries with trace accumulation, alpine high -6, light northwest wind.
Sunday: Mainly cloudy, isolated flurries with trace accumulation, alpine high -10, moderate northwest wind.
On Thursday, we received many reports of sloughing and small loose snow avalanches up to size 1. Reports of soft storm slabs propagating at ridgetops but breaking up quickly as they ran, were also limited to size 1.
Prior to the new snow, numerous natural, explosive and skier triggered avalanches size 2-2.5 on a variety of aspects have been reported recently throughout the region and in neighboring Glacier National Park. Some of these avalanches were triggered in areas of shallow snowcover or wind slab and stepped down to the persistent November layer resulting in avalanches large enough to injure or bury a person.
Thursday saw 15-20 cm of snowfall in the Monashees, with 5-10 cm elsewhere in the region. The new snow covered a widespread layer of large, feathery surface hoar crystals.
The new snow also buried soft wind slab found in the lee of features such as ridge tops, ribs and gulleys.
A weak layer formed in late November is now buried around 1 m below the surface. This is the layer of concern relating to the persistent slab avalanche problem. The weak layer may present as surface hoar, a crust, facets or a combination, depending on elevation and aspect. It can most likely be found in areas sheltered from the wind, but open to a view of the sky, such as near treeline. Below this, variety of crusts from late October are buried deeper in the snowpack.
