Northwest Coastal

Heavy snowfall and extreme winds on Sunday are expected to drive the danger to HIGH in parts of the region. Travel in avalanche terrain is not recommended.

Moderate - Timing, track, or intensity of incoming weather system is uncertain

A series of storm systems will bring heavy precipitation to the region over the next few days. In the wettest southern parts of the region, 30-40 cm of snowfall is forecast Saturday overnight with another 30-40 cm by Sunday evening. Alpine winds are forecast to be strong to extreme from the southwest and freezing levels are expected to reach close to 1000 m by Sunday afternoon. Heavy snowfall is forecast to continue Sunday overnight and Monday with models showing the potential for another 40-60 cm by the end of Monday in the southwest of the region. Alpine winds should remain strong to extreme and freezing levels will remain close to 1000 m. A break in the storm is currently forecast for Monday night with another storm pulse arriving on Tuesday.

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. Heavy snowfall and extreme winds are expected to quickly form widespread storm slabs on Sunday. 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 is being 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 especially on smooth alpine features like glaciers.