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Avalanche Forecast

Archived

Mar 21st, 2022–Mar 22nd, 2022

Alpine
Natural and human triggered avalanches likely.
Treeline
Natural and human triggered avalanches likely.
Below Treeline
Natural and human triggered avalanches likely.
Alpine
Natural and human triggered avalanches likely.
Treeline
Natural and human triggered avalanches likely.
Below Treeline
Natural avalanches possible, human triggered probable.
Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches possible, human triggered probable.
Below Treeline
Natural avalanches possible, human triggered probable.

Regions

Northwest Coastal.

Heavy snowfall and extreme winds are forming deep and reactive storm slabs. Large natural avalanches are very likely. Avoid all avalanche terrain.

Confidence

High - We are confident a natural avalanche cycle will begin shortly after the arrival of the incoming weather.

Weather Forecast

Monday night: Snowfall 15-20 cm. Strong south wind. Alpine low temperature around -3 °C. Freezing level rising to 1500 m. 

Tuesday: Snowfall 20-40 cm. Strong to extreme south wind. Alpine high around -1 °C. Freezing level 1500 m. 

Wednesday: Snowfall 10-20 cm. Moderate southwest wind. Alpine high around -6 °C. Freezing level 1000 m. 

Thursday: Overnight snowfall around 5 cm then clearing. Light to moderate south wind. Alpine high around -4 °C. Freezing level 1200 m.

Avalanche Summary

Reports of persistent slab avalanches have been steadily coming in over the last few days from northwest of Terrace. These avalanches have been large to very large (size 2-3), failing on a weak layer buried 60-100 cm deep mostly on northeast facing slopes. These avalanches are easily triggered by riders and vehicles, some remotely or sympathetically, and propagating long distances.

Looking forward, we are expecting a large, widespread storm slab avalanche cycle Tuesday. There is a good chance of step-downs to the persistent layer, which will result in very large and destructive avalanches that could run full path.

Snowpack Summary

30-60 cm of new snow by the end of the day Tuesday is forecast to fall amid strong to extreme wind. Below 1500 m, wet snow and rain in expected to soak the previously moist snowpack.

A weak layer of surface hoar buried 70-120 cm deep has been reactive recently and will likely produce large avalanches during the current storm. A thick rain crust from mid-February 100-150 cm deep caps a well consolidated lower snowpack.

Terrain and Travel

  • Avoid all avalanche terrain during periods of heavy loading from new snow, wind, or rain.
  • If triggered, storm slabs in-motion may step down to deeper layers and result in very large avalanches.
  • Avoid the runout zones of avalanche paths. Avalanches could run full path.
  • Only the most simple non-avalanche terrain free of overhead hazard is appropriate at this time.

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.

Loose Wet

Loose Wet avalanches are the release of wet unconsolidated snow or slush. These avalanches typically occur within layers of wet snow near the surface of the snowpack, but they may quickly gouge into lower snowpack layers. Like Loose Dry Avalanches, they start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-wet avalanches include point-release avalanches or sluffs. Loose Wet avalanches can trigger slab avalanches that break into deeper snow layers.

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.