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Avalanche Forecast

Archived

Mar 20th, 2022–Mar 21st, 2022

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

Regions

Northwest Coastal.

An easily triggered buried weak layer has surprised riders with large, deep avalanches recently. A tricky persistent slab problem is best managed through conservative terrain selection.

Confidence

Moderate - Uncertainty is due to the fact that persistent slabs are particularly difficult to forecast.

Weather Forecast

Sunday night: Snowfall 3-10 cm. Moderate to strong southwest wind. Alpine low around -5 °C. Freezing level 800 m. 

Monday: Snowfall 2-8 cm. Moderate to strong south wind. Alpine high around -1 °C. Freezing level rising to 1300 m through the day. 

Tuesday: Snowfall 25-50 cm. Strong south wind. Alpine high around 0 °C. Freezing level 1500 m. 

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

Avalanche Summary

Reports of persistent slab avalanches have been steadily coming in over the last few days from northwest of Terrace. These avalanche 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, some remotely and sympathetically, and and propagating long distances.

  • On Saturday, a natural cornice-triggered storm slab stepped down to the persistent slab problem, simultaneously triggering a sympathetic persistent slab avalanche a short distance away. 
  • On Thursday and Friday, persistent slab avalanches were being triggered by riders and vehicles.

Other avalanche activity includes small natural and skier triggered wind slabs and loose dry avalanches up to size 1.5.

Snowpack Summary

40-80cm of recent snow has seen redistribution into wind slabs lee terrain features. Low elevations may hold 10-30 cm of wet/moist snow sitting over a widespread crust on all aspects below 1200 m.

50-100 cm below the snow surface, a weak layer of feathery surface hoar crystals could be preserved in sheltered terrain at and above treeline. This weak layer has started producing large avalanches, mostly north and west of Terrace, where moderate to heavy snowfall and wind have continued through the week. In areas further inland, where the storm tapered off earlier, this layer may not be at its tipping point, but I'd still be suspicious of it through the weekend. 

A thick rain crust from mid-February is buried 100-150 cm. The snowpack below is well consolidated.

Terrain and Travel

  • Uncertainty is best managed through conservative terrain choices at this time.
  • Be alert to conditions that change with aspect and elevation.
  • Caution around convexities or sharp changes in terrain.
  • Be aware of the potential for remote triggering and large, deep avalanches due to the presence of buried surface hoar.

Problems

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.

Wind Slabs

Wind Slab avalanches are the release of a cohesive layer of snow (a slab) formed by the wind. Wind typically transports snow from the upwind sides of terrain features and deposits snow on the downwind side. Wind slabs are often smooth and rounded and sometimes sound hollow, and can range from soft to hard. Wind 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.