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Avalanche Forecast

Archived

Feb 5th, 2021–Feb 6th, 2021

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

Regions

Northwest Inland.

Fresh storm slabs may be reactive to skier and rider triggering on Saturday. A persistent weak layer of surface hoar, facets and a crust is buried 30-60 cm down. This avalanche problem is less obvious and harder to predict, so conservative terrain choices are best. 

Confidence

Low - Forecast snowfall amounts are uncertain. Uncertainty is due to whether buried persistent weak layers become active, triggering avalanches, with the arrival of the forecast weather.

Weather Forecast

An arctic front gradually pushes its way South bringing cold air and some light embedded flurries. A cold northwest wind will persist through the forecast period.

Saturday: Cloudy with flurries 5-10 cm. Alpine temperatures near -12 and light wind from the southeast.

Sunday: Mix of sun and cloud. Alpine temperatures near -20 with light ridgetop wind from the northwest.

Monday: A mix of sun and cloud. Alpine temperatures near -25 and mostly light wind from the northwest. 

Avalanche Summary

On Thursday a MIN report from the Seaton South Ridge triggered a size 1 wind slab on a north aspect at treeline. The report said that the bed surface had a few cm's of soft snow possibly surface hoar above a somewhat icy crust. The slab was small but ran fast on the bed surface. Awesome report! Thanks for sharing this information. I suspect that once this new low-density storm snow forms more of a slab, conditions may increase in reactivity and hazard. Skier and rider triggering is likely. 

 

Snowpack Summary

Up to 10-25 cm of new snow fell by Friday bringing 25-50 cm of accumulative low-density storm snow from the past week over a variety of old snow surfaces. These old surfaces include surface hoar in locations sheltered from the wind at all elevations, surface facets, and stiff wind affected snow. On solar aspects, a buried sun crust can be found and a thick crust exists near the surface below 1000 m. Additional snow and changing wind combined with cohesion may stress these potentially weak interfaces, creating the persistent slab problem.

The lower snowpack has two decomposing crust layers. The upper crust is 70-140 cm deep and continues to show occasional hard sudden results in snow pits. The deeper crust at the bottom of the snowpack is more prevalent in shallow snowpack ranges. These deep persistent layers seem to be dormant under the current conditions, but shallow rocky slopes should still be carefully assessed and approached with caution. 

Terrain and Travel

  • Potential for wide propagation exists, fresh slabs may rest on surface hoar, facets and/or crust.
  • Watch for signs of instability like whumpfing, hollow sounds, shooting cracks or recent avalanches.
  • Watch your sluff: it may run faster and further than you expect.
  • Watch for changing conditions today, storm slabs may become increasingly reactive.

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.

Persistent Slabs

Persistent Slab avalanches are the release of a cohesive layer of snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Deep Persistent Slab.