Avalanche hazard will increase into the weekend as storm snow accumulates. With ongoing slab development over weak surface hoar, conservative terrain choices are recommended.
40-60 cm of snow sits above a layer of surface hoar and/or crusts from early December. The new storm snow could easily step down to this interface, with surprising propagation.
Intermittent snow and flurries continue to stack up, 30-40 cm fresh snow overlies a layer of surface hoar. Reactive slabs have formed in the storm snow, expect to find increasingly reactive slabs as storm snow accumulates, especially in wind-loaded areas.
THURSDAY NIGHT - Isolated flurries, up to 5 cm / southwest wind, 20-40 km/h / alpine high temperature near -7
FRIDAY - Snow, 10-25 cm / southwest wind, 30-60 km/h / alpine high temperature near -6
SATURDAY - Snow, 15-20 cm / southwest wind, 25-55 km/h / alpine high temperature near -3
SUNDAY - Snow, 20-30 cm / southwest wind, 20-40 km/h / alpine high temperature near -7 / Freezing level rising to 1500 m
Small storm and wind slabs continued to be reactive underfoot/sled on Thursday. Explosives triggered avalanches to size 2 along ridgelines and with good connectivity.
On Wednesday, small storm and wind slabs showed reactivity on predictable, loaded features on the Dec 7 surface hoar/crust. Explosives and skiers triggered size 1-1.5 slab avalanches on northerly (or leeward) aspects above 2000 m. An loose, dry sluffing was easy to initiate in steep terrain. With the most reactivity, a small (size 1) avalanche was triggered remotely by a vehicle traveling along an exposed ridgeline.
On Tuesday, several small (size 1-1.5) loose dry avalanches were reactive to skier traffic. Skiers triggered a small (size 1.5) storm slab avalanche in Glacier National Park. And in the South Columbia's, storm slab avalanches were triggered by explosives and skier traffic on East, North, and West aspects above 2000 m; average depth 5-25 cm.
Ongoing flurries and snow has accumulated 20-50 cm fresh snow around the region. This new snow has buried a weak layer of surface hoar in many areas. This new snow combined with southwest winds has formed reactive slabs, especially at higher elevations.
A weak layer that was buried in early December is now down 40-60 cm. This layer is variable and has been reported to exist as either a crust or surface hoar. In any case, it is a persistent weak layer with a significant load above it now, and it may react easily to human triggers.
Another crust that was buried in early November can be found near the base of the snowpack. There have been no recent avalanches reported on this layer, however it may be possible to trigger it from shallow, rocky terrain.
