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Avalanche Forecast

Archived

Feb 19th, 2020–Feb 20th, 2020

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.
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.

Flurries and strong winds over the next few days will build fresh wind slabs in the alpine and exposed treeline areas. Watch for wind loading around ridgecrests and steep rollovers.

Confidence

Moderate - Uncertainty is due to the track & intensity of the incoming weather system.

Weather Forecast

Wednesday night: Flurries bringing a trace to 10 cm new snow. Moderate to strong southwest wind. Freezing level valley bottom.

Thursday: Mix of sun and cloud with chance of isolated flurries late in the day. Moderate southwest wind, increasing to strong. Freezing level 600 m.

Friday: Flurries bringing 5-10 cm new snow. Strong southwest wind. Freezing level 800 m.

Saturday: Flurries bringing 5-10 cm new snow. Moderate southwest wind. Freezing level 500 m.

Avalanche Summary

Human triggered slabs size 1-1.5 have been reported treeline and below, possibly running on patches of buried surface hoar. Last 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 running on deeply buried weak layers near the bottom of the snowpack, predominantly on north through east aspects in the alpine. On 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. On Monday, two deep persistent slab avalanches size 2-3 were triggered from thin snowpack areas by very large loads pushed by snowcats. 

Snowpack Summary

Extensive wind effect in exposed alpine terrain with scoured and pressed windward surfaces and hard wind slab in lee features. In sheltered areas, recent snow may sit over patchy surface hoar or crusts on solar aspects and below 1200 m.

A couple of weak layers that formed in January now sit in close proximity to one another 60-120 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

  • Be carefull around freshly wind loaded features.
  • 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.