Steady snowfall through Monday will continue to form fresh and reactive storm slabs primed for human triggering on steep slopes across the region. Triggering a storm slab or wind slab on the surface is probably the most effective way to cause a large and destructive step down avalanche.
Continuing, convective snowfall and strong southwesterly winds are building storm slabs and wind slabs that will be reactive to natural and human triggers. Expect wind loaded spots to be particularly touchy.
If triggered, slab avalanches may step down to deeper weak layers and initiate large and destructive avalanches.
A layer of large, weak facets sits near the base of the snowpack. This layer is most prominent in upper treeline and lower alpine elevations. Riders are most likely to trigger an avalanche on this layer in steep, shallow terrain.
Recent observations have shown a trend away from very large persistent slab and deep persistent slab avalanches and toward surface instabilities such as wind slabs. We expect this trend to continue, however the basal snowpack remains questionable and should still figure into terrain selection around steep features with shallow or variable snowpack areas where these layers may be more easily triggered by a person or machine as well as large wind slab or cornice releases.
As recently as last week, very large natural avalanches were observed failing on deep snowpack layers. Two were size 3 persistent slab avalanches on steep north-facing aspects with 750 m wide crowns. A size 3.5-4 deep persistent slab avalanche was also observed. It is suspected this avalanche failed in depth hoar at the base of the snowpack.
Another 15-30 cm of new snow should accumulate in the region by end of day Sunday, adding to roughly 80 to 130 cm of recent storm snow from steady stormy weather over the past week that has accumulated most recently under the influence of strong to extreme southwest wind.
Aside from shorter term storm slab and especially wind slab problems associated with successive snowfall and wind events, the recent storm snow is reportedly forming a good bond with previously wind-affected surfaces as well as the late January melt-freeze crust found on sun-exposed slopes and on all aspects below about 1600 m.
Several strengthening crust/facet/surface hoar layers can still be found in the upper and middle portions of the snowpack. The most concerning persistent weak layer, however, is at the base of the snowpack and composed of large and weak facets formed in November. This layer is strengthening slowly as well, remaining most likely to be triggered in thin, rocky alpine and upper treeline terrain.
Sunday night
Cloudy with continuing snowfall bringing 10-20 cm of new snow. Strong to extreme southwest winds easing in the morning.
Monday
Cloudy with scattered flurries bringing another 5-10 cm of new snow. Light to moderate southwest winds easing and shifting west. Treeline high temperatures around -7.
Tuesday
A mix of sun and cloud. Light northwest winds shfting southwest. Treeline high temperatures around -10
Wednesday
Increasing cloud with isolated flurries. Moderate southwest winds. Treeline high temperatures around -8.
More details can be found in the Mountain Weather Forecast.
