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Avalanche Forecast

Archived

Mar 11th, 2022–Mar 12th, 2022

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.
Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches possible, human triggered probable.
Below Treeline
Natural avalanches unlikely, human triggered possible.

Regions

Northwest Coastal.

Keep terrain selection conservative while storm snow gains strength. 

Slabs will be more reactive where they sit over a crust on south facing slopes and in sheltered terrain where buried surface hoar lingers.

Confidence

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

Weather Forecast

Snowfall tapers off Saturday afternoon. Sunday brings a brief break in the snow and wind with another active front forecast to hit late evening.

FRIDAY NIGHT: Cloudy, 10-15 cm of snowfall possible. Southwest winds ease to strong, alpine high -2 °C, freezing levels around 500 m. 

SATURDAY: Mainly cloudy, flurries continue with 5 cm possible, moderate southwest winds. Alpine high -3 °C, freezing levels rise to 1000 m. 

SUNDAY: A mix of sun and cloud. Isolated flurries possible. Light to moderate southwest winds. Freezing level around 1000m. Alpine high -3. 

MONDAY: Light snowfall begins overnight, around 15 cm by morning and another 15 cm possible over the day. Freezing levels around 1000 m. Alpine high -3. Strong southwest winds. 

Avalanche Summary

Friday morning, large explosive charges were able to trigger multiple size 1.5 storm slabs, up to 20cm deep. We expect storm slabs to be reactive to human triggers, with natural activity possible where accumulations are greatest. 

Over the last 10 days, a few size 1-2 persistent slabs have been triggered on the buried weak layers described in the snowpack summary section. Avalanches were mainly triggered on North through East aspects, between 1200 and 1800 m. Recent activity suggests they are becoming less reactive with the last reported avalanche on Sunday the 6th.

Snowpack Summary

Storm snow accumulates over a crust on sun affected slopes and on hard wind hammered surfaces in most terrain features. A layer of surface hoar may sit below the storm snow in isolated terrain features.

Low elevations may see a mix of snowfall and rain falling on a widespread crust on all aspects below 1000 m.

Several weak layers sit in the upper/mid snowpack that have been recently reactive. A layer of surface hoar buried early March is down 15-30 cm, preserved in wind sheltered terrain features at treeline. Another layer of weak surface hoar from late February is buried 35-60 cm deep, and is most prominently at treeline elevations. A thick crust from mid-February is buried 70-110 cm. The snowpack below is well consolidated.

Terrain and Travel

  • As the storm slab problem gets trickier, the easy solution is to choose more conservative terrain.
  • Storm slabs in motion may step down to deeper layers resulting in large avalanches.
  • Be careful as you transition into wind affected terrain.
  • Carefully assess open slopes and convex rolls where buried surface hoar may be preserved.

Problems

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.

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.