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Avalanche Forecast

Archived

Mar 2nd, 2020–Mar 3rd, 2020

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

Regions

South Coast.

Strong winds and new snow continue to build wind slabs at upper elevations. Deeper instabilities cannot yet be discounted.

Confidence

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

Weather Forecast

Monday night: Cloudy, 5-10 cm of snow, strong west winds becoming northwest, freezing level rising to 1200 m.

Tuesday: Cloudy, up to 5 cm of snow, moderate southwest winds gusting to strong, freezing level 1100 m.

Wednesday: Mix of sun and cloud, 5-15 cm of snow overnight, west winds decreasing to light, freezing level below 800 m.

Thursday: Cloudy, 20-30 cm of snow, strong southwest winds, freezing level 1000 m.

Avalanche Summary

No new avalanche observations have been reported. 

Snowpack Summary

Up to 15 cm of new snow and strong winds from the west are expected to continue to build reactive wind slabs at upper elevations. 

40-70 cm of recent snow may overlie a layer of surface hoar on north-facing aspects near and above treeline, and observers have reported reactivity in snowpack tests. See this MIN for a helpful illustration. Surface hoar is an exceptionally weak layer not often seen in this region, and it typically takes longer to heal. This persistent weak layer warrants investigation and a conservative terrain use strategy. Check out the latest forecaster blog that offers a deeper dive into these conditions. 

The remainder of the snowpack is well settled. Depth varies from around 250 to 300 cm at the peaks of the North Shore mountains (1400 m), tapering rapidly with elevation. 

Terrain and Travel

  • Watch for newly formed and reactive wind slabs as you transition into wind affected terrain.
  • Be careful with wind loaded pockets, especially near ridge crests and roll-overs.
  • Be aware of the potential for large avalanches due to the presence of buried surface hoar.
  • Carefully assess open slopes and convex rolls where buried surface hoar may be preserved.

Problems

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.

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.