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Avalanche Forecast

Archived

Nov 21st, 2011–Nov 22nd, 2011

Alpine
Widespread avalanches certain.
Treeline
Widespread avalanches certain.
Below Treeline
Natural avalanches possible, human triggered probable.
Alpine
Natural and human triggered avalanches likely.
Treeline
Natural and human triggered avalanches likely.
Below Treeline
Natural avalanches unlikely, human triggered possible.
Alpine
Natural and human triggered avalanches likely.
Treeline
Natural and human triggered avalanches likely.
Below Treeline
Natural avalanches unlikely, human triggered possible.

Regions

Sea To Sky.

This bulletin is based on limited data. Local variation in conditions and danger levels are likely to exist. To produce more accurate forecasts, we need information. Please send an email to [email protected].

Confidence

Fair - Due to limited field observationsfor the entire period

Weather Forecast

Very heavy precipitation is expected overnight Monday into Tuesday, with over 100mm for coastal areas, before easing Tuesday Afternoon. Winds are expected to be strong to extreme southwesterlies during the height of the storm and freezing levels generally around 1500m could briefly rise as high as 1700m.Wednesday: Another 20-40cm of snow expected with freezing levels dropping down to 1000m and strong southwesterly winds.Thursday: Continued moderate to heavy precipitation with associated strong southwesterly winds, especially for coastal areas.

Avalanche Summary

I suspect widespread natural avalanche activity will occur overnight Monday into Tuesday during the height of the storm. Large avalanches from alpine and treeline elevations could run well below treeline.

Snowpack Summary

Newly formed surface hoar is now buried in many locations, particularly sheltered open treeline slopes. A rain crust with associated facets is generally down 50-80cm also has surface hoar crystals sitting on top and/or slightly below it. A deeper crust with associated facets and/or depth hoar is approximately 50cm off the ground. Recent snowpack tests on a northeast facing treeline slope gave moderate sudden results on both of these weaknesses.

Problems

Persistent Slabs

Persistent Slab avalanches are the release of a cohesive layer of snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Deep Persistent Slab.

Wind Slabs

Wind Slab avalanches are the release of a cohesive layer of snow (a slab) formed by the wind. Wind typically transports snow from the upwind sides of terrain features and deposits snow on the downwind side. Wind slabs are often smooth and rounded and sometimes sound hollow, and can range from soft to hard. Wind slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs.

Storm Slabs

Storm Slab avalanches are the release of a cohesive layer (a slab) of new snow that breaks within new snow or on the old snow surface. Storm-slabs typically last between a few hours and few days (following snowfall). Storm-slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs.