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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 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.

Regions

Northwest Coastal.

Storm slabs are likely reactive to skier and rider triggering on Saturday, especially where they sit above a weak layer down 30-70 cm. Obvious clues of instability like natural avalanche activity may not exist, so conservative terrain choices are best.

Confidence

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

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 -5 and light wind from the West.

Sunday: Mix of sun and cloud. Alpine temperatures near -15 with moderate ridgetop wind from the northwest.

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

 

Avalanche Summary

On Friday, numerous loose-dry avalanches were reported up to size 2 during the storm in steep terrain. 

On Thursday, reports indicated several small features were reactive up to size 1 on the late-January buried surface hoar interface. I suspect that once the persistent slab gains more cohesion and stiffens, this problem may be more widespread. Even a small avalanche can catch you by surprise and have enough mass to push you into a terrain trap below. 

Fresh storm slabs may be touchy on Saturday, especially where they sit above surface hoar, crust, or old surface facets. 

Snowpack Summary

Up to 30 cm of recent low-density snow fell by Friday bringing 40-80 cm of accumulative storm snow from the past week over a variety of older snow surfaces. These old surfaces, at upper elevations (upper treeline and the alpine), include surface hoar in locations sheltered from the wind, surface facets, and stiff wind affected snow. At lower elevations (lower treeline and below treeline) 10-30 cm of snow sits above isolated pockets of surface hoar and a crust that is more predominant on solar aspects. 

Additional snow and changing wind combined with cohesion may stress these potentially weak interfaces, creating the persistent slab problem. This MIN from last weekend is a good example: MIN Report.

The mid-pack seems to be well settled. Deep persistent layers appear to have mostly gone inactive with the exception of the Bear Pass area and the far reaches south of Kitimat. 

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.
  • Pay attention to cornices and give them a wide berth when traveling on or below ridges.

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.