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

Regions

South Rockies.

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, moderate northeast winds, alpine temperature -12 C.

Friday: Cloudy, 5-10 cm of snow, strong northeast winds, alpine high temperature -15 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. 

There have been recent reports of large (size 2-2.5) explosive-triggered avalanches releasing 20-30 cm deep in the storm snow. In a few cases in the neighbouring Lizard Range and Flathead region, these avalanches were triggered by cornice falls.

Snowpack Summary

Up to 15 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 25 to 45 cm of snow from the previous storm has been redistributed by wind or is well-settled. This snow sits on older wind slabs in exposed areas, and a sun crust on solar aspects (south through west facing slopes). Melt-freeze crusts extend up to about 1900 m on other aspects. 

A thick crust/facet layer currently sits 40-80 cm below the surface and can be found up to 2100 m. The middle of the snowpack is generally strong, but the base of the snowpack contains 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.
  • Be aware of the potential for large avalanches due to the presence of a deep persistent slab.

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.