Avalanche Forecast

Issued: Feb 20th, 2013 8:31AM

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

pmarshall, Avalanche Canada

Northwest Inland.

Summary

Confidence

Fair - Due to limited field observations

Weather Forecast

Synopsis: A series of frontal systems is expected to affect the region over the next few days bringing steady precipitation and seasonal temperatures. Overnight and Thursday: Periods of snow – 10-20 cm. The freezing level is around 600 m. Winds are moderate to strong from the west-southwest. Friday: Moderate to locally heavy snowfall – 15-20 cm. The freezing level remains around 600 m and winds are strong from the west.Saturday: Cloudy with light snow. The freezing level is around 600 m and winds ease to moderate from the southwest.

Avalanche Summary

Natural avalanches activity has tapered off but some observers continue to report older (+48 hrs) natural slab avalanches up to size 3 from steep northeast facing terrain. These events may involve the basal weak layers.

Snowpack Summary

The previous storm snow seems to have settled and strengthened over the past couple days. Wind slabs have formed in exposed terrain in lee of ridges, in gullies, and behind terrain features. These wind slabs have not been reactive to skier triggers, even in steep unsupported terrain. There are a variety of old interfaces now down 40 - 100cm which include facets, crusts and isolated pockets of surface hoar. There is very limited information about the nature of this interface. I would stress the importance of digging down to find and test weak layers before committing to larger and steeper slopes.A strong mid-pack currently overlies a weak base layer of facets/depth hoar. It is worth noting that the snowpack in general is quite shallow compared to averages; triggering the basal weakness may still be possible from thin spots, rocky outcrops or under the weight of larger triggers such as cornice fall.

Problems

Wind Slabs

Shifting wind have formed wind slabs in exposed terrain on a variety of aspects. New wind slabs will likely form on north through east facing terrain near ridge crests.

Highmark or enter your line well below ridge crests to avoid wind loaded pillows.>Be cautious as you transition into wind affected terrain.>

Aspects: All aspects.

Elevations: Alpine, Treeline.

Likelihood

Likely

Expected Size

1 - 4

Persistent Slabs

Buried beneath the new snow sits a surface hoar/crust/facet weakness down 40 - 100 cm. This may be sensitive to rider triggering in steeper sheltered terrain or over convex rolls, especially at treeline elevations.

Be aware of the potential for full depth avalanches due to deeply buried weak layers.>Carefully evaluate and use caution around thin snowpack areas.>

Aspects: All aspects.

Elevations: All elevations.

Likelihood

Unlikely - Possible

Expected Size

2 - 7

Cornices

The recent snow and wind have been building cornices on lee features. These cornices are likely weak and may fail at anytime. Cornice failure has the potential to trigger large avalanches.

Give cornices a wide berth when travelling on or below ridges.>

Aspects: North, North East, East.

Elevations: Alpine.

Likelihood

Possible - Likely

Expected Size

1 - 5

Valid until: Feb 21st, 2013 2:00PM