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Avalanche Forecast

Archived

Feb 22nd, 2020–Feb 23rd, 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 unlikely, human triggered possible.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely.
Alpine
Natural avalanches unlikely, human triggered possible.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely.

Regions

Northwest Inland.

Wind-loaded pockets of recent storm snow will likely remain reactive to human triggering especially where it sits over surface hoar or sun crust.

Confidence

High -

Weather Forecast

Saturday night: A trace of new snow. Light to moderate west wind, increasing in the afternoon. Alpine low -11 C. Freezing level valley bottom.

Sunday: 5-10 cm new snow. Southeast wind easing to light. Alpine high -8 C. Freezing level 500 m.

Monday: Mix of sun and cloud. Light to moderate west wind. Alpine high -7 C. Freezing level 300 m.

Tuesday: Flurries. Moderate to strong west wind. Alpine high -4 C. Freezing level 300 m.

Avalanche Summary

A few natural wind slab avalanches up to size 1.5 were observed on north to east aspects on Friday.

Some very large and destructive avalanches have been sporadically running on deeply buried weak layers near the bottom of the snowpack, predominantly on north through east aspects in the alpine. Last Tuesday, a natural size 2.5 was observed. It is suspected to have started as a wind slab in a thin rocky area before stepping down. Last Monday, two deep persistent slab avalanches size 2-3 were triggered from thin snowpack areas by very large loads pushed by snowcats. 

Snowpack Summary

10-15 cm of recent snow has been redistributed by strong westerly winds in exposed alpine and treeline areas. In sheltered areas, the new snow rests on a layer of surface hoar, and on a sun crust on solar aspects.

An older, patchy layer of small surface hoar crystals and a sun crust both from early February may still exist under previous storm snow, 30-60 cm deep.

A couple of weak layers that formed in January are buried in close proximity to one another 80-140 cm below the surface and continue to give hard results in snowpack tests. Below, an early season crust lurks at the base of the snowpack. Some large avalanches have been triggered on these layers, usually during periods of rapid loading by new snow or wind and by large loads or from thin, rocky areas.

Terrain and Travel

  • Watch for newly formed and reactive wind slabs as you transition into wind affected terrain.
  • Be aware of the potential for larger than expected storm slabs due to the presence of buried surface hoar.
  • Approach steep, open and/or sparsely treed slopes cautiously.
  • Storm slabs in motion may step down to deeply buried weak layers resulting in very large avalanches.

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.