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Avalanche Forecast

Archived

Mar 13th, 2020–Mar 14th, 2020

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

Regions

South Rockies.

Intense snowfall and strong winds are building widespread, touchy slabs. 

Stick to sheltered, simple terrain and avoid exposure to overhead hazards.

Confidence

Low - Forecast snowfall amounts are uncertain.

Weather Forecast

Friday night: Cloudy, 5-10 cm of snow, areas in the eastern half of the region favored by the up-slope weather pattern could see up to 20 cm, moderate east winds, alpine temperature -28 C.

Saturday: Cloudy, 5-10 cm of snow, areas in the eastern half of the region favored by the up-slope weather pattern could see up to 15 cm, moderate northeast winds decreasing to light, alpine high temperature -20 C.

Sunday: Mix of sun and cloud, light northeast winds, alpine high temperature -14 C. 

Monday: Mostly clear, light variable winds, alpine high temperature -10 C, freezing level 1000 m. 

Avalanche Summary

Strong easterly winds and intense snowfall are expected to form widespread, touchy slabs. Cornices have also grown large with the recent weather, and a cornice failure could trigger a large slab avalanche on the slope below. 

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

Snowpack Summary

Some areas of the South Rockies may see intense and significant snowfall accumulations. Anywhere from 10-40 cm may accumulate by Saturday afternoon, with higher totals favoring up-slope areas in the east of the region. Strong easterly winds will accelerate the formation of touchy slabs, and there is ample snow for transport with this reverse-loading pattern. These conditions may also bring cornices to their breaking point.

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

  • Use increased caution at all elevations. Storm snow is forming touchy slabs.
  • Be careful to keep storm day fever from luring you out into bigger terrain features.
  • 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

Storm Slabs

Storm Slab avalanches are the release of a cohesive layer (a slab) of new snow that breaks within new snow or on the old snow surface. Storm-slabs typically last between a few hours and few days (following snowfall). Storm-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.