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Avalanche Forecast

Archived

Mar 28th, 2017–Mar 29th, 2017

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

Regions

Northwest Coastal.

A sleeping dragon lies deep in the snowpack: The persistent slab problem hasn't changed and isn't improving. Conservative terrain use is essential & avoid overhead hazards.

Confidence

Moderate - Due to the number of field observations

Weather Forecast

We'll have lingering flurries in the wake of the snow from Monday-Tuesday. The next significant weather arrives Friday. WEDNESDAY: Lingering flurries (5-10 cm possible) light southwest wind, freezing levels 1100m, alpine temperature around -4 C. THURSDAY: A short-lived clearing during the day. Light-moderate south wind, freezing level around 1100 m with alpine temperature around -3 C. FRIDAY: Snow in the afternoon (10-15cm), moderate southwest wind, freezing levels 1200m.

Avalanche Summary

On Monday, we had reports of loose wet avalanches below treeline, in response to warming temperatures. A natural glide avalanche was observed on a south aspect near Stewart. On Sunday, a machine-triggered Size 3 (with crown approximately 2m thick) was reported in the South Douglas area just north of Terrace on a northwest aspect near 1400m. The slab is suspected to have run on the late February interface and involves an incredible story of survival. Click here for details.On Friday, a spooky Size 3.5 persistent slab with impressive propagation was remote-triggered north of Terrace at 1500m elevation. See here for more details in this excellent post.A significant concern remains the potential for persistent slab avalanches triggered by people. See the forecaster blog for advice on managing avalanches that are not likely, but if they are triggered, are probably unsurvivable. See (here), and the accompanying travel advice for valley bottom (here).

Snowpack Summary

We've seen light snowfall and steady light to moderate winds over the past several days. Weekend totals brought 10-20cm of new snow and moderate southwest / southeast winds. Consequently, storm slabs (up to 40cm thick) developed at treeline and above. Cornices have also grown larger with the steady winds and regular, small amounts of snow. Seasonal spring temperatures mean the snow has been moist below ~1200m.Snow may still be poorly bonded to a 40 cm deep crust and surface hoar interface from mid-March; however, the late-February interface has been the more troublesome persistent layer. It's composed of facets, crust, and surface hoar roughly 70-140 cm deep and is still reactive throughout the region, especially in shallower areas around Stewart and Bear Pass. See the Avalanche Summary above for surprising examples of this persistent problem re-awakening.

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.