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Avalanche Forecast

Archived

Feb 24th, 2020–Feb 25th, 2020

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

Regions

Northwest Inland.

This is going to be a tricky forecasting week, the region's not getting a ton of wind or snow, but it could start to stack up to enough by mid-week to touch off another natural cycle in our tenuous snowpack. Human triggered avalanches remain possible at all elevations.

Confidence

Low - Uncertainty is due to the complexity of the snowpack’s structure. Uncertainty is due to the speed, direction, or duration of the wind and its effect on the snowpack. Uncertainties in both the snowpack structure and the weather forecast limit our confidence.

Weather Forecast

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

MONDAY NIGHT: Freezing level around valley bottom, moderate to strong southwest wind, 1 to 5 cm of snow expected.

TUESDAY: Overcast, freezing level around 1000 m, strong southwest wind, 1 to 5 cm of snow expected.

WEDNESDAY: Overcast, freezing level rising to about 1300 m, strong southwest wind, 1 to 5 cm of snow during the day with another 5 to 10 cm Wednesday night.

THURSDAY: Overcast, freezing level holding around 1300 m, strong southwest wind, 5 to 10 cm of snow.

Avalanche Summary

On Sunday human triggered avalanches to size 2 were failing on the February 19th surface hoar, crowns were 10 to 60 cm in depth.

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.

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

In most of the region 10-15 cm of recent snow has been redistributed by strong wind, the alpine is starting to look pretty wind hammered as evidenced by this MIN submission. Snow totals are deeper in the far north of the region where there is 30 to 60 cm of recent storm snow. All the new snow rests on surface hoar in sheltered locations and a sun crust on solar aspects. The surface hoar may be very sensitive to human triggering at lower elevations which is a bit different setup than we're used to.

There are few different persistent weak layers (PWL's) we're watching in the mid and lower snowpack. A patchy layer of small surface hoar crystals and a sun crust both from early February are now 30-80 cm deep. A couple of weak layers that formed in January are buried in proximity to one another 80-140 cm below the surface and continue to give hard results in snowpack tests. Below that, 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.