Avalanche Forecast

Issued: Mar 25th, 2015 10:12AM

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

swerner, Avalanche Canada

Purcells.

A complex upper snowpack exists, especially in the northern areas of this region. Avalanche danger ratings may be a step lower in the south.

Summary

Confidence

Fair - Timing, track, or intensity of incoming weather system is uncertain

Weather Forecast

A Pacific frontal system reaches the coast today and affects the Interior regions tonight through the forecast period. Warm air aloft will invade the region until Friday night with associated freezing levels near 2800 m. A trailing cold front will bring light precipitations and strong ridgetop winds. On Thursday, light precipitation amounts up to 5 mm expected, ridgetop winds strong from the SW and freezing levels rising to 2800 m. On Friday, solar radiation could come into play with a mix of sun, cloud and freezing levels steady at 2900 m. Unsettled conditions on Saturday will bring light precipitation amounts 5-10 mm, ridgetop winds moderate from the NW and freezing levels dropping to 2000 m.

Avalanche Summary

On Tuesday we received two separate reports of a skier triggered storm slab size 2 and a remotely triggered slab size 3 which occurred on Monday. A solar induced natural avalanche cycle occurred tuesday afternoon up to size 2, and explosive controlled size 2-3 slab avalanches were initiated from steep N-NE aspects above 2300 m. On Thursday, wind slabs are expected to be touchy and reactive to human-triggering. Where enough new snow exists, it may be possible to trigger storm slab avalanches up to size 3.

Snowpack Summary

At upper elevations recent storm snow amounts vary. The NW part of the region received up to 60 cm while the Dogtooth received around 30 cm and the far south received close to zero. This dense storm snow sits over the mid-March interface which consists of a series of crusts, wind affected surfaces, and/or old wind slabs. Snowpack tests are showing moderate- hard results with a sudden planar fracture characteristic. In exposed terrain, strong SW winds had redistributed new snow into wind slabs on leeward terrain features and slopes. On Sunday, the snow surface was reported to be wet to around 1800m and moist to around 2200m. Snow surface are moist up to 2200 m with no significant re-freeze expected. Digging deeper, 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. Deeper persistent weak layers in the snowpack that 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. Persistent weak layers may be suspect in shallower snowpack areas.

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 most of the region for wind loaded areas of the alpine.

Use ridges or ribs to avoid pockets of wind loaded snow.>Be cautious as you transition into wind affected terrain.>

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

Elevations: Alpine, Treeline.

Likelihood

Possible - Likely

Expected Size

1 - 4

Storm Slabs

This problem is likely specific to the NW part of the region where the recent snowfall accumulations were greater. Storm slabs are expected to be reactive to human-triggering in steep terrain.

Use conservative route selection, stick to moderate angled terrain with low consequence.>Avoid convexities or areas with a thin or variable snowpack.>

Aspects: All aspects.

Elevations: Alpine, Treeline.

Likelihood

Possible

Expected Size

1 - 5

Loose Wet

Rising freezing levels and an extended period of warm temperatures will likely deteriorate the upper snowpack, resulting in loose wet avalanche activity.

Watch for clues, like natural avalanche activity, sluffing off of cliffs, moist/ wet snow surfaces, and snowballing. These are indicators that the snowpack is warming up and deteriorating.>

Aspects: All aspects.

Elevations: Alpine.

Likelihood

Likely

Expected Size

1 - 2

Valid until: Mar 26th, 2015 2:00PM