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Avalanche Forecast

Archived

Feb 23rd, 2020–Feb 24th, 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, human triggered possible.
Alpine
Natural avalanches unlikely, human triggered possible.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely, human triggered possible.

Regions

Northwest Inland.

10 to 60 cm of storm snow rests on a layer of buried surface hoar. While the surface hoar is present at all elevations, it's been particularly problematic at lower elevations at and below treeline. Stick to simple, mellow, well-supported terrain and avoid convexities. 

Confidence

Moderate - Uncertainty is due to the fact that persistent slabs are particularly difficult to forecast.

Weather Forecast

The forecast period is marked by small amounts of snow and rising mid-week temperatures.

SUNDAY NIGHT: Freezing level at valley bottom, light variable wind, 1 to 3 cm of snow possible

MONDAY: Clear skies at dawn building to scattered cloud cover by sundown, freezing level at valley bottom, light variable wind, no significant precipitation expected.

TUESDAY: Broken cloud cover, freezing at valley bottom, moderate to strong southwest wind, trace of snow possible.

WEDNESDAY: Broken cloud cover, freezing level rising to about 1500 m, strong southwest wind, trace of precipitation possible.

Avalanche Summary

On Saturday storm slabs resting on surface hoar failed naturally and were sensitive to skier triggering producing slabs up to 60 cm in depth. These slabs seem to be more sensitive at and below treeline which is the reverse of how we normally think about avalanche hazard.

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.
  • Storm slabs in motion may step down to deeply buried weak layers resulting in very large avalanches.
  • The trees are not the safe-haven they normally are at this time. Terrain at treeline is primed for human triggered 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.