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Avalanche Forecast

Archived

Feb 17th, 2020–Feb 18th, 2020

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

Regions

Northwest Inland.

Avalanche danger may have improved but buried weak layers make for a tricky snowpack situation. Conservative terrain choices are recommended with caution around wind loaded features, steep rollovers in open trees and steep sun exposed slopes.

Confidence

Moderate -

Weather Forecast

Monday night: Mainly cloudy. Light northwest wind. Alpine low -10 C. Freezing level valley bottom.

Tuesday: Mix of sun and cloud. Light northwest to southwest winds. Freezing level 700 m.

Wednesday: Mix of sun and cloud. Moderate southwest winds, building to strong at ridgetop. Freezing level 400 m.

Thursday: Increasing cloud. Strong southwest winds, approaching extreme at ridgetop. Freezing level 400 m.

Avalanche Summary

Reports from the weekend consisted of several human triggered storm slabs size 1-1.5 at treeline and below, as well as a few natural (possibly cornice-triggered) slabs up to size 2.5 on north to east aspects in steep open bowls at treeline. Reports from Friday included an observation of a large cornice-triggered wind slab in Morning Bowl - see the report here. 

During the first half of the month there were many very large and destructive avalanches failing on deeply buried weak layers near the bottom of the snowpack. The bulk of this activity occurred above treeline on north through east facing aspects, but any large alpine slopes that have not yet avalanched should be treated as suspect. A few examples of MIN reports of large events February 8-11 include Hudson Bay, Kathlyn Face, French Peak, Pine Creek Trail, and Babines.

Snowpack Summary

10-40 cm of snow from the past week has seen some redistribution at alpine and treeline elevations, with wind slabs forming in lee features. At treeline and below, it may sit over patches of surface hoar in wind sheltered areas or crusts on solar aspects and below 1200 m.

A weak layer of facets that formed during the January cold snap is now about 60-120 cm below the surface while an early season crust lurks at the base of the snowpack. A few large avalanches were triggered on these layers in the first half of the month.

Terrain and Travel

  • Avalanche hazard may have improved, but be mindful that deep instabilities are still present.
  • If triggered, wind slabs avalanches may step down to deeper layers resulting in larger avalanches.
  • Stay off recently wind loaded slopes until they have had a chance to stabilize.
  • Avoid exposure to steep, sun exposed slopes, especially when the solar radiation is strong.
  • Approach steep open slopes at and below treeline cautiously, buried surface hoar may exist.

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.