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Archived

Avalanche Forecast

Feb 28th, 2020–Feb 29th, 2020
Alpine
3: Considerable
The avalanche danger rating in the alpine will be considerable
Treeline
2: Moderate
The avalanche danger rating at treeline will be moderate
Below Treeline
1: Low
The avalanche danger rating below treeline will be low
Alpine
2: Moderate
The avalanche danger rating in the alpine will be moderate
Treeline
2: Moderate
The avalanche danger rating at treeline will be moderate
Below Treeline
1: Low
The avalanche danger rating below treeline will be low
Alpine
2: Moderate
The avalanche danger rating in the alpine will be moderate
Treeline
1: Low
The avalanche danger rating at treeline will be low
Below Treeline
1: Low
The avalanche danger rating below treeline will be low

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.

Avalanche Problems

Wind Slabs

10-20 cm of new snow with winds changing in direction (from south to west to northwest) may form a complex pattern of reactive wind slabs on leeward terrain features at upper elevations. These areas of concern overlap with where cornices may also be reaching their breaking point. 

Aspects: All aspects.

Elevations: Alpine, Treeline.

Likelihood: Likely

Expected Size: 1 - 2

Persistent Slabs

A weak layer of surface hoar may be found 30-60 cm deep on north-facing aspects near and above treeline. This persistent slab problem is not typical for the South Coast, and there is high uncertainty with regard to how quickly it will heal. Reduce this uncertainty by investigating these deeper layers if you are travelling near steep, open slopes at upper elevations.

Aspects: North, North East, East, West, North West.

Elevations: Alpine, Treeline.

Likelihood: Unlikely - Possible

Expected Size: 1 - 2.5