Avalanche Forecast

Issued: Nov 28th, 2012 9:30AM

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.

swerner, Avalanche Canada

Northwest Coastal.

If your local riding zone receives significantly more snow and wind then what this weather forecast suggests, the danger rating will be elevated.

Summary

Confidence

Poor - Intensity of incoming weather is uncertain

Weather Forecast

A deep low pressure system off the Gulf of Alaska will be bringing a very moist and active weather pattern over the next five days. Initially, the Northern sections may see cold arctic air being pushed in by a surface ridge of high pressure, then as a consistent moderate southwest flow sets up; it brings a series of systems which then the arctic air will start to retreat slowly from the Northwest regions. Expect strong outflow winds below treeline.Thursday/Fiday: FLVLâ€™s surface, Â with a short lived AFL 100-1500 m, Thurs snow 8-15 cm, Fri snow 5-10 cm, ridgetop winds SE 30-40 km/hr, alpine temps -11 Â (inversion).Saturday: FLVLâ€™s surface (no inversion), snow 7-10 cm, ridgetop winds E 15 km/hr, alpine temps -9.0.

Avalanche Summary

On Tuesday a flight through the Bear Pass area saw numerous old (previous 3 days) large natural avalanches. These were size 2-3, 50-80 cm deep, in the immediate lee around 1600-2000 m. A recent notable event that occurred over the past 2 days off colder temps were Natural icefall induced avalanches size 2.5-3.0. On Monday around the Shames area a size 3.0 avalanche was observed on a north-northeast aspect at 1500m, which may have been a thick wind slab or possibly released on a facets near the ground.Human triggered avalanches remain possible, particularly in exposed wind loaded terrain, and natural avalanche activity will likely pickup again with more snow and wind in the forecast.

Snowpack Summary

New storm slabs and wind slabs instabilities will be developing through the forecast period. These will be loading over a couple weaknesses that exist lower in the snowpack. Down 60-80 cm lies a storm snow weakness from last week. I suspect the recent cooling trend has helped stabilized this interface. The second weakness sits near the base of the snowpack. A profile at 1200m in the Shames area showed a thin layer of facets sitting on a crust 35cm off the ground. An Extended Column Test produced easy results on this layer, but the resistant fracture didn't propagate across the entire column. Total snowpack depth is probably around 150cm in most treeline areas and deeper but more variable in the alpine. The snowpack below treeline may still be below threshold depth for avalanches in some areas. Check out the Skeena/ Babine discussion forum for more information from the area.

Problems

Storm Slabs

New storm slabs will build and hide old wind slabs. Reverse loading may occur and form new wind slabs on opposite slopes, which may catch you by surprise. Natural activity is possible with heavy loading from snow and wind, rider triggers are likely.

Avoid all avalanche terrain during periods of heavy loading from new snow, wind, or rain.>Avoid areas that have been reverse loaded by winds.>

Aspects: All aspects.

Elevations: All elevations.

Likelihood

Likely - Very Likely

Expected Size

1 - 4

Persistent Slabs

Basal facet/crust weaknesses are often prone to remote triggering and step down avalanches, and typical trigger points include thin rocky areas. They may be difficult to trigger, but deep persistent slab avalanches are often very large.

Be aware of thin areas that may propagate to deeper instabilites.>Whumpfing, shooting cracks and recent avalanches are all strong indicators of an unstable snowpack.>

Aspects: All aspects.

Elevations: Alpine, Treeline.

Likelihood

Unlikely - Possible

Expected Size

2 - 6

Valid until: Nov 29th, 2012 2:00PM