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Avalanche Forecast

Archived

Mar 7th, 2020–Mar 8th, 2020

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 possible, human triggered probable.
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

Northwest Inland.

Persistent slab avalanches have been a standout feature of the avalanche cycles brought on by recent storms. Maintain conservative terrain selection with this problem in mind and avoid assuming avalanche danger will end as surface snow stabilizes.

Confidence

Moderate -

Weather Forecast

Saturday night: Clear. Light northwest winds.

Sunday : Mainly sunny, becoming cloudy overnight. Light to moderate west winds, increasing over the day and overnight. Alpine high temperatures around -13.

Monday: Cloudy with scattered flurries and a trace of new snow, increasing overnight. Moderate to strong southwest winds, peaking overnight. Alpine high temperatures around -10.

Tuesday: Cloudy with continuing flurries bringing 5-10 cm of new snow, 10-20cm with overnight accumulations. Moderate to strong southwest winds. Alpine high temperatures around -8.

Avalanche Summary

Over the past few days there have been reports of wind slab, persistent slab (failing on the February 19 surface hoar layer), and deep persistent slab (failing on basal facets) avalanches. This pattern highlights loading and surface avalanches from recent stormy weather straining multiple weak layers in the snowpack.

Reports from Thursday showed several more wind and storm slab avalanches from size 1-2 (small to large) releasing naturally as well as with skier and snow machine traffic in the Kispiox area. North through east aspects again proved most reactive.

On Wednesday a cornice failure in the same area initiated a size 3 wind slab avalanche on a northeast facing feature at 1600 m that cleared out large timber in the valley. The recent storm snow was sensitive to human triggering, even in low angle terrain, running on the March 1st surface hoar. A few shallow natural wind slabs were also observed in steep terrain.

Human triggered avalanches on the Feb 19 surface hoar layer have been reported almost daily since last Thursday.

Cornice falls have been a common trigger for slabs on the slopes below them. Last Friday a cornice fall triggered a size 3.5 deep persistent slab on a northeast facing slope at 1800 m.

Snowpack Summary

The Wednesday into Thursday night storm produced 5 to 25 cm of new snow with the deepest accumulations found in the south and accompanied by strong east/southeast wind. Previously, extreme wind from the southwest, west, and northwest has hammered open terrain, scouring some slopes and forming wind slabs on leeward slopes.

Loading from new snow and wind has made several persistent weak layers problematic over the past week. A combination of surface hoar and sun crust layers were buried on February 19 and currently sit 30-60 cm below the surface that seem to be most sensitive to human triggering at treeline elevations. 

A couple of weak layers that formed in January are buried in close proximity to one another 80-140 cm below the surface. Below that, an early season crust/facet layer lurks at the base of the snowpack.

Terrain and Travel

  • Avoid freshly wind loaded features, especially near ridge crests, roll-overs and in steep terrain.
  • Use careful route-finding and stick to moderate slope angles with low consequences.
  • The trees are not the safe-haven they normally are at this time. Terrain at treeline is primed for human triggered avalanches.
  • Use extra caution around cornices: they are large, fragile, and can trigger slabs on slopes below.

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.

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.

Deep Persistent Slabs

Deep Persistent Slab avalanches are the release of a thick cohesive layer of hard snow (a slab), when the bond breaks between the slab and an underlying persistent weak layer deep in the snowpack. The most common persistent weak layers involved in deep, persistent slabs are depth hoar or facets surrounding a deeply buried crust. Deep Persistent Slabs are typically hard to trigger, are very destructive and dangerous due to the large mass of snow involved, and can persist for months once developed. They are often triggered from areas where the snow is shallow and weak, and are particularly difficult to forecast for and manage.