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Avalanche Forecast

Archived

Dec 24th, 2019–Dec 25th, 2019

Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches possible, human triggered probable.
Below Treeline
Natural avalanches unlikely.
Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches possible, human triggered probable.
Below Treeline
Natural avalanches unlikely.
Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches possible, human triggered probable.
Below Treeline
Natural avalanches unlikely.

Regions

South Coast Inland.

A serious persistent slab problem is dominating avalanche concerns in the north of the region. Danger in the south of the region is MODERATE and should be limited to wind slabs lingering on leeward features near ridgetop.

Confidence

Moderate -

Weather Forecast

Tuesday night: Cloudy. Light southwest winds.

Wednesday: A mix of sun and cloud, clearer in the north of the region. Isolated flurries bringing a trace of new snow in the south. Light northeast winds. Alpine high temperatures around -7, closer to -6 in the south of the region.

Thursday: Mainly cloudy with scattered flurries developing late in the day and bringing up to 5 cm of new snow by morning. Light to moderate southwest winds, becoming strong at ridgetop. Alpine high temperatures around -8

Friday: Mainly cloudy with lingering isolated flurries and a trace of new snow. Light to moderate southwest winds. Alpine high temperatures around -7, closer to -5 in the south of the region.

Avalanche Summary

A widespread avalanche cycle was observed across the region on Friday and Saturday. Avalanches were reported to be running to valley-bottom in the north of the region, failing on the weak layer described in the snowpack summary below.

Human-triggered avalanches are expected to remain likely to occur on Tuesday at higher elevations, but mainly in the northern half of the region (e.g., Duffey, Hurley, etc.). See this MIN report of an avalanche involvement on Monday for an example of the issue at hand.

Given our current snowpack structure, we recommend very conservative route-finding and decision making in higher elevation avalanche terrain in the north of the region.

Snowpack Summary

Over 120 cm of snow accumulated in the south of the region with the recent storm and over 40 cm in the north of the region. The snow fell with strong southwest wind. This snow is likely gaining strength as the days pass but snowpack conditions found beneath this layer are variable over the region and in some areas highly problematic.

In the north of the region, the above-mentioned storm snow has brought a critical load to a now deeply buried weak layer composed of sugary faceted grains and hard melt-freeze crust from mid-November. This structure is a recipe for large and destructive avalanches and a problem that may persist for weeks to months. This problematic layer is largely absent in the south of the region.

Terrain and Travel

  • Don't let the desire for deep powder pull you into high consequence terrain.
  • Persistent slabs have potential to pull back to lower angle terrain.
  • Avoid steep convexities or areas with a thin, rocky, or variable snowpack.
  • Carefully evaluate steep lines for wind slabs.

Problems

Persistent Slabs

Persistent Slab avalanches are the release of a cohesive layer of snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Deep Persistent Slab.

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.