Avalanche Forecast

Issued: Dec 1st, 2016 4:11PM

The alpine rating is high, the treeline rating is high, and the below treeline rating is below threshold. Known problems include Storm Slabs and Persistent Slabs.

shorton, Avalanche Canada

Northwest Coastal.

Heavy snowfall, extreme winds, and a buried surface hoar layer are a recipe for very dangerous avalanche conditions.

Summary

Confidence

Moderate - Due to the number of field observations

Weather Forecast

FRIDAY: 10-15 cm Thursday night and another 15-30 cm during day on Friday, 60-90 km/h winds from the southwest, freezing level up to 1000 m.SATURDAY: 10-15 cm overnight and 5-10 cm during the day, 30-60 km/h winds from the southwest, freezing levels dropping to 500 m.SUNDAY: cloudy with isolated flurries, light winds, cooling with treeline temperatures around -10.

Avalanche Summary

On Wednesday, several size 1.5-2 storm slabs were triggered by explosives on steep north aspects between 1400-1500 m in the Stewart area. A natural size 1 avalanche was triggered by a rockfall in the area running on a 20-50 cm deep surface hoar layer. A MIN report from the Terrace area describes evidence of a natural avalanche cycle from the past few days with up to size 2.5 storm slabs in large alpine features. The incoming storm will build bigger storm slabs and has the potential to reawaken buried surface hoar layers.

Snowpack Summary

Heavy snowfall and extreme southwesterly winds will form touchy storm slabs on Friday. Storm snow totals could reach 30-50 cm by Friday afternoon, with the potential for much more on lee features. The new snow will load a buried surface hoar layer reported 60-80 cm below the surface in many parts of the region. On Wednesday, a ski tourer reported numerous whumpfs caused by the surface hoar layer in the Terrace area. Also, snowpack tests indicated the layer can be triggered with moderate loads and has the potential to propagate over long distances. A thick rain crust exists 20-30 cm below the surface hoar layer, with isolated reports of weak facets forming above the crust. Treeline snow depths are around 120-140 cm in the Terrace and Stewart areas, but substantially less further north. Snow in these thinner areas, such as Ningunsaw, may be facetting and developing weak basal layers.

Problems

Storm Slabs

30-50 cm of new snow with extreme winds will form very touchy storm slabs.

Minimize exposure to avalanche terrain during periods of heavy loading from new snow, wind, or rain.If triggered the storm slabs may step down to deeper layers resulting in large avalanches.

Aspects: All aspects.

Elevations: Alpine, Treeline.

Likelihood

Very Likely - Certain

Expected Size

1 - 3

Persistent Slabs

Persistent weak layers with surface hoar, facets, and crust are buried 60-80 cm deep throughout the region, and will likely become reactive under the load of the new snow.

Avoid open slopes and convex rolls where buried surface hoar may be preserved.Minimize overhead exposure during periods of heavy loading from new snow, wind.

Aspects: All aspects.

Elevations: Alpine, Treeline.

Likelihood

Likely

Expected Size

2 - 3

Valid until: Dec 2nd, 2016 2:00PM