Avalanche Forecast

Archived

Jan 16th, 2020–Jan 17th, 2020

Alpine

Natural avalanches possible, human triggered probable.

Treeline

Natural avalanches unlikely, human triggered possible.

Below Treeline

Natural avalanches unlikely, human triggered possible.

Alpine

Natural avalanches possible, human triggered probable.

Treeline

Natural avalanches possible, human triggered probable.

Below Treeline

Natural avalanches unlikely, human triggered possible.

Alpine

Natural avalanches possible, human triggered probable.

Treeline

Natural avalanches possible, human triggered probable.

Below Treeline

Natural avalanches unlikely, human triggered possible.

Regions

Northwest Coastal.

Low elevation trees are your best chance at avoiding windslabs and finding soft snow that hasn't been hammered by outflows. The short trip from the car is an added bonus on a chilly day.

Confidence

Moderate - Uncertainty is due to the track & intensity of the incoming weather system. Uncertainty is due to the fact that deep persistent slabs are particularly difficult to forecast.

Weather Forecast

Thursday night: Mostly cloudy with clear periods. Isolated flurries bringing a trace of new snow. Moderate wind from the south. Alpine temperatures around -21 C.

Friday: Mix of sun and cloud. Light south to southwest winds. Alpine temperatures around -18 C.

Saturday: Mostly cloudy with scattered flurries bringing 5-10 cm of new snow to most areas. Strong south to southeast winds, approaching extreme at ridgetop. Alpine high around -16 C.

Sunday: Mostly cloudy with scattered flurries bringing 10-15 cm of new snow to most areas. Moderate to strong south to southwest winds. Alpine high around -11 C.

Avalanche Summary

On Tuesday and Wednesday, avalanche activity was limited to size 1-1.5 windslabs, both natural and explosive triggered. A widespread natural windslab cycle around size 2 -3 was observed Sunday through Monday morning amid reverse loading by the onset of strong outflow winds. Cornice and serac failures have also observed recently, as they typically become brittle in the cold.

Reports of deep persistent slab avalanches have been trickling in over the past week. They are associated with a November crust layer near the base of the snowpack, producing very large avalanches with crown depths of around 2 m. Observations are typically in alpine terrain, on lee or cross-loaded slopes. In one case, the slab was triggered remotely by a vehicle from shallow, rocky terrain, propagating to deeper areas. Among the earliest reports was this MIN from the storm last week.

Snowpack Summary

Extensive wind effect observed at all elevations. North to east aspects in the alpine have been wind scoured, with isolated hard windslab in lees, and notable cornice growth. This wind affected surface snow may overly a layer of surface hoar in many areas, particularly at treeline. In sheltered areas, unconsolidated snow may be found well preserved by the cold temperatures.

A deep crust from mid November lurks at the base of the snowpack. A couple of recent large avalanches have run on this layer high in the alpine.

Terrain and Travel

Be careful with wind slabs, especially in steep, unsupported and/or convex terrain features.
An atypical weather pattern has created unusual avalanche problems in this area.
Be aware of the potential for surprisingly large avalanches due to deeply buried weak layers.
Avoid thin areas like rock outcroppings where you're most likely to trigger avalanches failing on deep weak layers.

Problems

Wind Slabs

Wind Slab avalanches are the release of a cohesive layer of snow (a slab) formed by the wind. Wind typically transports snow from the upwind sides of terrain features and deposits snow on the downwind side. Wind slabs are often smooth and rounded and sometimes sound hollow, and can range from soft to hard. Wind slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs.

Deep Persistent Slabs

Deep Persistent Slab avalanches are the release of a thick cohesive layer of hard snow (a slab), when the bond breaks between the slab and an underlying persistent weak layer deep in the snowpack. The most common persistent weak layers involved in deep, persistent slabs are depth hoar or facets surrounding a deeply buried crust. Deep Persistent Slabs are typically hard to trigger, are very destructive and dangerous due to the large mass of snow involved, and can persist for months once developed. They are often triggered from areas where the snow is shallow and weak, and are particularly difficult to forecast for and manage.