Avalanche Forecast

Archived

Feb 27th, 2020–Feb 28th, 2020

Alpine

Natural avalanches unlikely, human triggered possible.

Treeline

Natural avalanches unlikely, human triggered possible.

Below Treeline

Natural avalanches unlikely, human triggered possible.

Alpine

Natural avalanches possible, human triggered probable.

Treeline

Natural avalanches unlikely, human triggered possible.

Below Treeline

Natural avalanches unlikely, human triggered possible.

Alpine

Natural avalanches unlikely, human triggered possible.

Treeline

Natural avalanches unlikely, human triggered possible.

Below Treeline

Natural avalanches unlikely.

Regions

South Coast.

A weak layer atypical of this region is buried 30-60 cm deep. It is most prevalent on north-facing aspects near treeline and may become more reactive as new snow accumulates. Avoid terrain traps, and approach deeper drifted areas and convex roll-overs with caution.

Confidence

Moderate - Uncertainty is due to the timing, track, & intensity of the incoming weather system. Uncertainty is due to whether buried persistent weak layers become active, triggering avalanches, with the arrival of the forecast weather.

Weather Forecast

Thursday night: Cloudy, up to 5 cm new snow, light to moderate southwest winds, freezing level 1300 m.

Friday: Mostly cloudy, 5-10 cm of snow above 1200 m, southwest winds increasing to strong by end of day.

Saturday: Cloudy, 15-25 cm new snow, southwest winds decreasing from strong to light by end of day, freezing level dropping below 1000 m.

Sunday: Mostly clear, light variable winds, freezing level 1000 m.

Avalanche Summary

No new avalanche observations. This MIN post from Monday reports a skier triggered size 1 wind slab in the alpine, suspected to have run on surface hoar. Explosive control work conducted Monday produced size 1-1.5 storm slabs running on a crust.

Snowpack Summary

By Friday afternoon, 5-15 cm of snow is forecast to accumulate. Strong winds from the southwest will likely drift the new snow into touchy wind slabs on lee features at upper elevations and may bring cornices to their breaking point.

30-60 cm of recent snow sits over a layer of surface hoar on north-facing aspects. Surface hoar is an exceptionally weak layer not often seen in this region. There may also be areas where the buried surface hoar sits on a sun crust, which makes an excellent bed surface for avalanches.

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. Little snow exists below 1000 m.

Terrain and Travel

Storm snow and wind is forming touchy slabs. Use caution in lee areas in the alpine and treeline.
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.