Avalanche Forecast

Issued: Mar 24th, 2013 10:36AM

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

istorm, Avalanche Canada

Purcells.

Summary

Confidence

Good

Weather Forecast

Synopsis: Stable pattern under a ridge of high pressure expected to continue. In other words, little change.Overnight and Monday: Dry, light westerly winds, solid overnight freeze (-10 to -15 C) with daytime warming (around -5 in alpine). Monday freezing level rising to near 1600 m but above freezing temperatures likely to go higher on slopes facing the sun.Tuesday: essentially a nearly identical forecast to Monday.Wednesday: some high cloud drifting in. Tuesday night may stay warmer limiting the overnight freeze and crust recovery at treeline elevations and lower.

Avalanche Summary

Natural and explosive cornice releases are a major theme. Explosives control in several parts of the region resulted in some large cornice falls that released some slab avalanches in the tracks that were restricted to the recent storm snow.A size 3 avalanche taking out the whole basin was triggered by snowmobilers Saturday in Hell Roaring Creek, just south of the region resulting in one fatality. Details are limited at this time; it may be indicative of the persistent slab problem (surface hoar and/or crust buried about 70 - 120 cm) we continue to describe.

Snowpack Summary

Moist snow may be found on solar aspects up to about 2000 metres; surface faceting elsewhere. New melt/freeze crusts are developing due to cold clear nights and periods of strong solar radiation during the day. The new storm slab is about 30-50 cms deep, with wind transported pockets that are about 70 cms deep. Downflowing or katabatic winds have created small slabs in gulley and atypical low alpine and treeline elevations. Recent cornice growth is reported to be poorly bonded and breaks off easily with light triggers. A weak layer of surface hoar and/or a crust is buried down about 70-120 cms in the alpine and treeline elevations; this layer continues to produce "sudden" results. This surface hoar layer is producing variable results in snow profile tests, and continues to be a concern for professionals.

Problems

Cornices

Do not travel on slopes that are exposed to cornices overhead.>Give cornices a wide berth when travelling on or below ridges.>

Aspects: North, North East, East.

Elevations: Alpine.

Likelihood

Possible

Expected Size

3 - 7

Persistent Slabs

Small solar triggered slides,cornice fall, or thin-spot triggering may overload a persistent weakness buried near 100 cm deep. This could lead to surprisingly large avalanches.

Be aware of the potential for large, deep avalanches due to the presence of buried surface hoar.>Avoid areas with a thin or variable snowpack.>

Aspects: All aspects.

Elevations: Alpine, Treeline.

Likelihood

Unlikely - Possible

Expected Size

3 - 6

Storm Slabs

Periods of strong solar radiation may release the slabs where it is sitting on a buried crust or loose wet slides. Katabatic & localized moderate ridge top winds may be forming new pockets of wind slab.

Avoid sun exposed slopes when the solar radiation is strong, especially if snow is moist or wet.>Be cautious as you transition into wind affected terrain.>

Aspects: All aspects.

Elevations: Alpine, Treeline.

Likelihood

Possible

Expected Size

1 - 4

Valid until: Mar 25th, 2013 2:00PM