Avalanche Forecast

Issued: Mar 22nd, 2015 8:53AM

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

rbuhler, Avalanche Canada

Purcells.

Hazard ratings reflect conditions in the north and west parts of the region that received the most storm snow. Touchy storm or wind slabs are expected to remain reactive to human-triggering. Use a conservative approach to travel.

Summary

Confidence

Fair - Due to the number of field observations

Weather Forecast

A cold front should bring light snowfall to the region on Monday morning and convective flurries are expected in the afternoon. Models are currently showing around 5mm but amounts are expected to vary across the region and localized areas could receive larger amounts. Freezing levels are expected to reach around 1700m and alpine winds are forecast to be light-moderate from the SW switching to NW in the afternoon. Good overnight recovery is expected through the forecast period with freezing levels falling to valley bottom each night. Unsettled conditions with convective flurries are expected for Tuesday. Freezing levels should reach around 1500m in the afternoon and alpine winds should be light. A weak ridge of high pressure is expected for Wednesday bringing a mix of sun and cloud.

Avalanche Summary

On Saturday in the deeper snowfall areas, natural storm slab avalanches were reported. Explosives triggered avalanches up to size 2.5 which were up to 100cm deep. In the Dogtooth, touchy wind slabs were reported. On Monday, thin wind slabs are expected to reactive to human-triggering. Where enough new snow exists, it may be possible to trigger storm slab avalanches.

Snowpack Summary

Recent snowfall amounts vary greatly across the region. The NW part of the region received up to 60cm while the Dogtooth received around 30cm and the far south received close to zero. At higher elevations, this dense storm snow sits over the mid-March interface which had included crusts, moist snow, wind affected surfaces, and/or old wind slabs. In exposed terrain, strong SW winds have redistributed new snow into wind slabs on leeward features. On Saturday, the snow surface was reported to be wet to around 1800m and moist to around 2400m. The snow surface is now expected to undergo melt-freeze cycles as freezing levels fluctuate overnight. 20-50 cm below the surface is the mid-February facet/crust interface. This interface has not been reactive in the Purcells but has been very reactive in the neighboring North Columbia region. With additional loading, this layer could become a concern. Deeper persistent weak layers have been dormant for several weeks and are not currently a problem but still have the potential to wake-up with major warming or heavy loading.

Problems

Wind Slabs

Wind slabs found in lee terrain continue to be reactive to human triggering. This is expected to be a fairly widespread problem across the parts of the region that received new storm snow.

Avoid freshly wind loaded features.>Use ridges or ribs to avoid pockets of wind loaded snow.>

Aspects: North, North East, East, South East, South.

Elevations: Alpine.

Likelihood

Possible - Likely

Expected Size

1 - 4

Storm Slabs

This problem is likely isolated to the NW part of the region where recent snowfall amounts of 40-60cm were reported. Touchy storm slabs may be reactive to human-triggering.

The new snow will require several days to settle and stabilize.>Use conservative route selection, stick to moderate angled terrain with low consequence.>

Aspects: All aspects.

Elevations: Alpine, Treeline.

Likelihood

Possible - Likely

Expected Size

1 - 4

Valid until: Mar 23rd, 2015 2:00PM