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Avalanche Forecast

Archived

Feb 28th, 2020–Feb 29th, 2020

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

Regions

South Coast.

Winds are strong and shifting direction as snow accumulates, creating a tricky pattern of wind slabs at upper elevations. Use caution around drifted slopes and convex roll-overs, especially where these slabs may overly a buried weak layer.

Confidence

Moderate - Uncertainty is due to the track & intensity of the incoming weather system. Uncertainty is due to how buried persistent weak layers will react with the forecast incoming weather.

Weather Forecast

Friday night: Cloudy, 10-15 cm of snow, strong southwest winds switching to northwest, freezing level dropping to 800 m.

Saturday: Mix of sun and cloud, northwest winds decreasing from moderate to light, freezing level staying below 800 m.

Sunday: Mix of sun and cloud, light variable winds, freezing level staying below 800 m.

Monday: Cloudy, up to 5 cm of snow, moderate northwest winds, freezing level 1000 m.

Avalanche Summary

No new avalanche observations have been reported. 

This MIN post from Monday reports a small (size 1) human-triggered wind slab in the alpine, which was thought to have run on surface hoar. Explosive control work conducted Monday produced small (size 1-1.5) storm slabs running on a crust.

Snowpack Summary

By Saturday afternoon, 10-20 cm of snow is forecast to accumulate as the freezing level drops below 800 m. Strong winds from the south are expected to switch to the northwest and decrease. This will likely create a complex pattern of reactive wind slabs on lee features at upper elevations.

30-60 cm of recent snow may overlie a layer of surface hoar on north-facing aspects near and above tree line. 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. This weekend offers a prime opportunity on the coast to dig in the snow to assess these conditions (and to share your observations via the MIN!).

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

  • Storm snow and wind is forming touchy slabs. Use caution in lee areas in the alpine and treeline.
  • Recent wind has varied in direction so watch for wind slabs on all aspects.
  • 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.