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Avalanche Forecast

Archived

Feb 18th, 2020–Feb 19th, 2020

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.
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.

Forecast strong winds on Wednesday should help keep snow surfaces cool as the sun beats down, but still keep an eye on steep south aspects for signs of solar warming. Watch for fresh pockets of wind loaded snow in lee features as wind slab development resumes in the alpine.

Confidence

Moderate -

Weather Forecast

Tuesday night: Clear. Light southwest wind. Freezing level valley bottom.

Wednesday: Sunny. Moderate southwest wind, building to strong. Freezing level 500 m.

Thursday: Cloudy with isolated flurries. Strong southwest wind, approaching extreme at ridgetop. Freezing level 400 m.

Friday: Flurries bringing 5-15 cm new snow. Strong southwest wind. Freezing level 800 m.

Avalanche Summary

Human triggered slabs size 1-1.5 have been reported treeline and below, possibly running on patches of buried surface hoar. Over the weekend, natural cornice falls are suspected to have triggered a number of natural wind slabs up to size 2.5 on north to east aspects in steep open terrain around treeline, one of which is described in this MIN report from Friday.

Some very large and destructive avalanches have been sporadically failing on deeply buried weak layers near the bottom of the snowpack, predominantly on north through east aspects in the alpine. On Monday, two deep persistent slab avalanches size 2-3 were triggered from thin snowpack areas by very large loads pushed by snowcats. A few examples of MIN reports of large events during the period of rapid loading by new snow and wind February 8-11 include Hudson Bay, Kathlyn Face, French Peak, Pine Creek Trail, and Babines.

Snowpack Summary

Extensive wind effect in exposed alpine terrain with scoured and pressed windward surfaces and hard wind slab in lees features. In sheltered areas, recent snow may sit over patchy surface hoar 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. 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

  • Pay attention to the wind, once it starts to blow fresh sensitive wind slabs are likely to form.
  • If triggered, wind slabs avalanches may step down to deeper layers resulting in larger avalanches.
  • 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.