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Avalanche Forecast

Archived

Dec 10th, 2021–Dec 11th, 2021

Alpine
Natural and human triggered avalanches likely.
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 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.

Regions

Northwest Coastal.

Continue to make conservative terrain choice while the storm snow stabilizes. There is still some concern for buried weak layers, which means that avalanches have the potential to pull back into lower angled terrain than you might expect.

Confidence

Moderate - Uncertainty is due to the limited number of field observations. Uncertainty is due to the fact that persistent slabs are particularly difficult to forecast.

Weather Forecast

A fast moving pacific system brought heavy snow and wind to the region on Friday. A quieter weekend is in store, but we will continue to see convective flurries from the passing front.

Friday Overnight: The frontal system will pass over this evening, with easing wind and snowfall. Mainly cloudy skies overnight with strong to moderate southwest winds at ridgetop. Another 10-15 cm can be expected with freezing levels reaching valley bottom by morning. Alpine temperatures around -5 C. 

Saturday: A mainly cloudy day with some continued snowfall as the low tracks south. 5-15 cm of accumulation can be expected accompanied by moderate to strong westerly winds at ridgetop. Freezing levels near valley bottom with alpine temperatures around -10 C. 

Sunday: Partially cloudy with convective flurries, possibility for locally heavy accumulation ranging from 2-15 cm. Moderate south winds at ridgetop with alpine temperatures around -10 C. 

Monday: A mix of sun and cloud with light southerly winds and isolated flurries, trace accumulation. Alpine temperatures around -15C.

Avalanche Summary

We suspect a natural avalanche cycle occurred during the storm on Friday.

On Thursday, highways NW of terrace had several explosive triggered loose dry and storm slab avalanches up to size 1.5. They also observed several natural loose dry and storm slab avalanches up to size 1.

Reports from recreation areas remain limited! If you do head into the backcountry, please share you observations to the Mountain Information Network!

Snowpack Summary

Across the region 40-80 cm of new snow has fallen in the past 2 days. In wind exposed areas, we can expect strong southwesterly winds to have created deep deposits of wind slab in lee features. In sheltered areas, this new snow will acts as a storm slab or loose dry problem. 

There is still some concern for buried weak layers, will the possibility for buried surface hoar and faceting above the December 1st crust, now buried 40-120 cm deep. In the southern part of the region the December 1st crust has been reported up to 2000m, tapering to 500m in the northern reaches. The Terrace area has reported the crust to around 1200m.

Snowpack depths range from 150 cm- 300 cm at treeline and 200 cm- 400 cm in the alpine.

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.

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.