Avalanche Forecast

Issued: Mar 20th, 2017 5:17PM

The alpine rating is considerable, the treeline rating is considerable, and the below treeline rating is moderate. Known problems include Storm Slabs and Persistent Slabs.

mgrist, Avalanche Canada

Purcells.

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.

Summary

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.TUESDAY: Isolated flurries (local amounts 5-10cm possible) / Light to moderate southwest wind / Freezing level 1600mWEDNESDAY: Broken skies with isolated flurries (local amounts 5-10cm possible) / moderate southwest wind /Freezing level 1700mTHURSDAY: Flurries in the morning with 5cm possible / Light south wind / Freezing level 1500m

Avalanche Summary

We've had several reports recently of surprising persistent (deep) slab avalanches to Size 4, many scrubbing to ground. See the link in the headline for detailed discussion of these low probability, high consequence events.A reconnaissance flight to Silent Pass in the north part of the region reported numerous natural avalanches to Size 3-4 on a wide variety of aspects and elevations. Some crowns were several hundred meters wide and avalanches ran to valley bottom... something to keep in mind when traveling up logging roads is your overhead exposure. In the wake of Saturday's storm expect the likelihood of triggering an avalanche to remain elevated.

Snowpack Summary

We had heavy 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 20-50cm) sits on older windslabs (or soft slabs) at treeline and above. Below 1800m, the new snow sits on a melt-freeze crust from last week's warm storm, and reports so far are that the new snow is bonding well to the old crust.The persistent weakness buried late-February is now down 90-140cm, and is composed of weak facetted crystals on a thick rain crust as high as about 2000m and facets on sun crust on steep southerly aspects. This layer has produced easy-moderate results in recent snowpack tests and has proven especially reactive on steep southerly aspects.Several deeper persistent weaknesses also remain a concern, including surface hoar buried early-February and mid-January (primarily in the northern Purcells). The november crust and basal facets are still reactive in shallow, rocky start zones.

Problems

Storm Slabs

Warm storm snow on Saturday and strong southwest wind created a storm slab problem. The snowpack will need time to adjust to the new load - stick to low angled terrain (with no overhead hazard) and avoid sunny slopes if/when the sun comes out.

The new snow will require several days to settle and stabilize.If triggered the storm slabs may step down to deeper layers resulting in large avalanches.

Aspects: All aspects.

Elevations: Alpine, Treeline.

Likelihood

Possible - Likely

Expected Size

1 - 3

Persistent Slabs

Various persistent weaknesses strewn throughout the snowpack create the potential for large step-down avalanches. Heavy loads such as a smaller storm/wind slab avalanche or a cornice fall increase the likelihood of triggering these layers.

Avoid steep convexities or areas with a thin or variable snowpack.Be aware of the potential for large avalanches due to the presence of deeply buried weak layers.

Aspects: All aspects.

Elevations: All elevations.

Likelihood

Possible

Expected Size

2 - 4

Valid until: Mar 21st, 2017 2:00PM