Avalanche Forecast

Archived

Nov 26th, 2011–Nov 27th, 2011

Alpine

Natural and human triggered avalanches likely.

Treeline

Natural and human triggered avalanches likely.

Below Treeline

Natural avalanches possible, human triggered probable.

Alpine

Natural avalanches possible, human triggered probable.

Treeline

Natural avalanches possible, human triggered probable.

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.

Regions

Sea To Sky.

The tail end of a pineapple express will cross the South Coast on Sunday keeping the avalanche danger at HIGH. Travel in avalanche terrain is not recommended.

Confidence

Fair - Due to limited field observations

Weather Forecast

A strong pacific frontal system will continue to bring heavy precipitation and strong winds to the region through Sunday morning. Clearing and cooling are expected by mid afternoon as a ridge of high pressure moves into the area. The freezing level should drop from 1800-2000m in the morning to 700m late in the day. Monday should be mainly dry with a freezing level around 600-800m. There is a chance of light snowfall on Tuesday with a freezing level around 800m.

Avalanche Summary

Recent control work produced several explosive triggered slab avalanches up to Size 3. Most of these avalanches involved the recent storm snow, but some stepped down to a facet/crust layer near the base of the snowpack. One Size 2.5 avalanche occurred in an avalanche path that had recently released. It reloaded and failed again on the facet/crust interface. Avalanche activity is expected to continue on Sunday with the potential for very large and destructive avalanches.

Snowpack Summary

Recent strong and shifting winds have created a variable snow surface and pockets of deep and dense wind slabs in exposed areas. Fluctuating freezing levels and heavy precipitation will likely produce various storm snow weaknesses. There is a persistent weakness of facets associated with a crust around 40-70cm above the ground. This weakness exists at treeline and alpine elevations and has been sensitive to human triggers. Several recent avalanches on this interface have shown very wide propagation and there have been cases of triggering from a distance. In some areas there is also a factet/crust layer just above the ground. This layer may still be a concern in areas with smooth underlying ground.

Problems

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.

Storm Slabs

Storm Slab avalanches are the release of a cohesive layer (a slab) of new snow that breaks within new snow or on the old snow surface. Storm-slabs typically last between a few hours and few days (following snowfall). Storm-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.

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.