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Avalanche Forecast

Archived

Mar 12th, 2020–Mar 13th, 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

Lizard-Flathead.

Fresh wind slabs will likely form in new places as winds change direction and increase. Stick to sheltered areas and be mindful of overhead hazards on Friday.

Confidence

Moderate - Forecast snowfall amounts are uncertain.

Weather Forecast

Thursday night: Cloudy, up to 5 cm of snow, light northweast winds, alpine temperature -14 C.

Friday: Cloudy, 5-10 cm of snow, strong northeast winds, alpine high temperature -14 C.

Saturday: Cloudy, 5-15 cm of snow, strong northeast winds, alpine high temperature -20 C.

Sunday: Mix of sun and cloud, light northeast winds, alpine high temperature -10 C. 

Avalanche Summary

Strong winds are expected to form fresh wind slabs that may be reactive to human triggers. Cornices have also grown large with the recent weather, and a cornice failure could trigger a wind slab avalanche on the slope below.

Over the past few days, there have been several reports of natural, human-triggered, and explosive-triggered avalanches releasing in the recent storm snow, primarily on lee aspects above 1900 m. These avalanches were small to large (size 1.5-2), breaking 20-60 cm deep, and in a few cases, triggered by cornice falls.

Snowpack Summary

Up to 10 cm of new snow may accumulate by Friday afternoon. Winds are forecast to shift to the northeast and to increase to 40-60 km/hr, building fresh wind slabs in a reverse-loading pattern.

A total of 30 to 60 cm of snow from the previous storm has been redistributed by wind or is well-settled. This snow sits over a layer of buried wind slabs in exposed areas and a sun crust on solar aspects (south through west facing slopes). Melt-freeze crusts extend up to 1900 m on other aspects. 

A thick crust/facet layer currently sits 30-60 cm below the surface. There has been only one avalanche reported on this layer since February 17th. The middle of the snowpack is generally strong, but the base of the snowpack contains weak basal facets that are most prominent in shallow rocky start zones.

Terrain and Travel

  • Watch for newly formed and reactive wind slabs as you transition into wind affected terrain.
  • Avoid exposure to slopes that have cornices overhead.

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.