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Avalanche Forecast

Archived

Dec 29th, 2019–Dec 30th, 2019

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

Although the likelihood of triggering avalanches is decreasing, a persistent slab problem remains a concern in the northern part of the region. Danger in the south of the region is LOW.

Confidence

Moderate - Uncertainty is due to extremely variable snowpack conditions reported through the region.

Weather Forecast

Sunday night: Mostly cloudy, isolated flurries with a trace of accumulation, light southwest winds, alpine temperatures -5 C and freezing levels near 1000 m.

Monday: Cloudy with isolated flurries with 1-3 cm of accumulation, moderate southwest winds, alpine high temperatures around -3 C, with freezing levels around 800 m.

Tuesday: Cloudy, 5-10 cm of snow, moderate to strong southwest winds, alpine high temperatures around -1 C, with freezing levels around 1000 m.

Wednesday: Cloudy, 5-10 cm of snow, moderate to strong southwest winds, alpine high temperatures near 0 C with freezing levels rising to 1500 m.

Avalanche Summary

Avalanche activity has decreased since the widespread avalanche cycle a week ago, and no recent activity has been reported. During that cycle, avalanches were reported to be running to valley-bottom in the north of the region, failing on deeply buried weak layers. See this MIN report of an avalanche involvement from last week for an example of this avalanche problem.

The possibility for large human-triggered persistent slab avalanches remains a concern at higher elevations in the northern part of the region (e.g., Duffey, Hurley, etc.), especially as recently formed wind slabs create the potential for small avalanches to step-down to these layers. These persistent weak layers continue to produce concerning snowpack test results and are expected to heal slowly. 

Snowpack Summary

Snowpack variability between the northern and southern parts of the region is significant. 

In the north (e.g. Duffey, Hurley) 5-15 cm of recent snow has been drifted into shallow, reactive wind slabs on leeward features at upper elevations. Below the surface, a weak layer from late November (down 30-70 cm), composed of sugary faceted grains and hard melt-freeze crust, is a recipe for large and destructive avalanches. It presents a persistent slab problem that may persist for weeks to months. This persistent weak layer is largely absent in the south of the region.

Terrain and Travel

  • Avoid shallow snowpack areas, rock outcroppings and steep convex terrain where triggering is most likely.
  • Uncertainty is best managed through conservative terrain choices at this time.
  • If triggered, wind slabs avalanches may step down to deeper layers resulting in larger avalanches.

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.