Avalanche Forecast

Low - Timing, track, or intensity of incoming weather system is uncertain on Tuesday

Heavy snowfall continues Sunday overnight with another 30-40 cm and extreme southwest wind in the alpine. The storm is expected to ease slightly on Monday but another 15-20 cm of snowfall is forecast during the day with strong alpine wind from the west. Freezing levels are expected to be around 700 m on Monday. Light to moderate snowfall is forecast to continue Monday overnight and Tuesday. Alpine wind should remain strong from the west and freezing levels should remain around 700 m. Another period of heavy snowfall and extreme alpine wind is currently forecast for Tuesday night and Wednesday.

No recent avalanches have been reported in this region. Natural wind slab avalanches were observed earlier in the week resulting from the strong outflow winds. Widespread natural storm slab avalanches are expected to be occurring as a result of heavy snowfall and extreme winds. This is expected to continue during the storm. The new snow sits over a highly variable and weak interface which should increase the reactivity of the new storm slabs. The surface hoar layer from mid-November may still be reactive in isolated areas. As new load accumulates over the weekend, this layer is expected to increase in reactivity where is still exists. Storm slab avalanches have the potential to step down to this layer which would increase the size and destructive potential of resulting avalanches. Old rain crusts from November which have recently become faceted as well as basal faceting in shallow snowpack areas will also get tested during the storm with the rapid loading of the snowpack. Until more information is available, it is best to assume deep release avalanches are possible.

The new storm snow sits over a highly variable interface resulting from over a week of clear skies, cold temperatures, and strong outflow winds. This mid-December interface consists of scoured surfaces and hard wind slabs in wind exposed terrain, widespread faceting of the upper snowpack, and 5-10 mm surface hoar in sheltered areas. The new snow is expected to bond poorly to this interface. A layer of surface hoar which was buried mid-November was down 50-80 cm in many parts of the region prior to the storm. This layer has been dormant recently but may increase in reactivity as the storm adds new load to the snowpack. In many areas in the region, the snowpack is reported to be very thin and faceting has been reported through the entire snowpack. In these areas, old rain crusts from November are developing weak facets. Concerns in the north of the region are basal weaknesses that may exist in the shallower snowpack areas.