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Avalanche Forecast

Archived

Mar 21st, 2017–Mar 22nd, 2017

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 possible, human triggered probable.
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.

Regions

North Columbia.

We've weathered the storm but we're not out of the woods yet. There's a great blog post on critical factors to watch out for this spring. Click here for details.

Confidence

Moderate - Due to the number of field observations

Weather Forecast

We're looking at classic unsettled spring weather for the forecast period: cloudy with isolated (sometimes intense) flurries.WEDNESDAY: Broken skies with isolated flurries (local amounts 5-10cm possible) / moderate south wind /Freezing level 1600mTHURSDAY: Flurries in the morning with 5cm possible / Light east wind / Freezing level 1300mFRIDAY: Isolated flurries (local amounts 5-10cm possible) / Light to moderate southwest wind / Freezing level 1500m

Avalanche Summary

We had a widespread natural avalanche cycle (to Size 4.5) at all elevations and aspects during the recent storm, including multiple adjacent avalanche paths running simultaneously with 1 Km wide propagation.Many of these avalanches ran in the storm snow with some also stepping down to persistent weaknesses (see snowpack summary below) deeper in the snowpack. Of particular concern are the ongoing large destructive avalanches (some scrubbing to ground) that have been occurring daily: one or two deep spooky avalanches in each bulletin region of the Columbia Mountains. See the link in the headline for detailed discussion of these low probability, high consequence events.

Snowpack Summary

We had heavy wet snow (or rain up to 2000m in places) and strong to extreme southerly winds during the weekend's storm. Temperatures also warmed up significantly. The end result: Widespread storm slabs and wind slabs at treeline and above, with significant cornice growth as well.The most recent storm snow (totals of 30-50cm) sits on older windslabs (or soft slabs) at treeline and above. Below 1800m, the new snow (with a breakable surface crust) sits on a melt-freeze crust from rain events last weekend. Reports so far are that the new snow is bonding well to the old crust, but that the storm snow instabilities (down 25 and 45cm) are showing some reactivity to rider traffic.Approximately 100-140 cm below the surface you may find the late-February persistent weakness / crust interface. This layer has woken up from time to time as smaller avalanches still have to potential to 'step down' and trigger this layer.The deep mid-December facet layer (and November raincrust) still linger at the bottom of the snowpack and are the suspected culprit (running to ground/glacier) in Glacier National Park avalanches. See here for one spooky picture. See here for another.

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.