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Avalanche Forecast

Archived

Mar 1st, 2020–Mar 2nd, 2020

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, 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.

Regions

South Coast.

Incremental snow and wind over an atypical buried weak layer warrants careful evaluation and terrain selection on Monday.

Confidence

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

Weather Forecast

Sunday night: Cloudy, up to 5 cm of snow, moderate northwest winds, freezing level 900 m.

Monday: Cloudy, 5-10 cm of snow, moderate northwest winds, freezing level rising to 1200 m.

Tuesday: Cloudy, 5-10 cm of snow, moderate southwest winds gusting to strong, freezing level 1100 m.

Wednesday: Mix of sun and cloud, 10-15 cm of snow overnight, west winds decreasing to light, freezing level below 1000 m.

Avalanche Summary

No new avalanche observations have been reported. 

This MIN from last 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 last Monday produced small (size 1-1.5) storm slabs running on a crust.

Snowpack Summary

20-30 cm of snow has accumulated since Friday. Strong winds from the south switched to the northwest. The winds redistributed the storm snow into a complex pattern of wind slabs on lee features at upper elevations and rapidly loaded cornices. Periods of strong solar radiation over the weekend warmed the recent snow on steep, sunny slopes.

40-70 cm of recent snow may overlie a layer of surface hoar on north-facing aspects near and above tree line, and observers have reported reactivity in snowpack tests. Check out this helpful MIN that illustrates this problem. 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

  • Recent wind has varied in direction so watch for wind slabs on all aspects.
  • 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.