Avalanche Forecast

Archived

Nov 27th, 2011–Nov 28th, 2011

Alpine

Natural and human triggered avalanches likely.

Treeline

Natural avalanches possible, human triggered probable.

Below Treeline

Natural avalanches unlikely, human triggered possible.

Alpine

Natural avalanches possible, human triggered probable.

Treeline

Natural avalanches possible, human triggered probable.

Below Treeline

Natural avalanches unlikely, human triggered possible.

Alpine

Natural avalanches possible, human triggered probable.

Treeline

Natural avalanches unlikely, human triggered possible.

Below Treeline

Natural avalanches unlikely, human triggered possible.

Regions

Purcells.

Confidence

Poor - Due to limited field observations

Weather Forecast

The trailing cold front should have moved through the Interior ranges by late Sunday. On Monday a weak ridge of high pressure should bring drier conditions, maybe even some sunshine. It would be nice to get out and see what the last storm brought to the mountains. The weak ridge looks like it will be over-run on Tuesday by a Pacific disturbance that should bring light to moderate precipitation and light northwest winds to all of the interior ranges. Unsettled conditions are forecast to continue on Wednesday, ending sometime Wednesday night or Thursday morning.

Avalanche Summary

Explosive control in the Dogtooth resulted in smaller more stubborn releases to ground up to size 1.5 in the alpine on a north aspect. I suspect that there has been a natural cycle on Sunday when the freezing level was at 1800 metres and the southwest winds were howling.

Snowpack Summary

There is a lot of variation in the snowpack in this region. There is more snow in the Dogtooth area near Golden than there is further south near Invermere and Kimberley. There is probably quite a bit of snow to the west, in areas like the Bugaboos and Jumbo. We don't have any information yet from operators in the west of this region. In the early season we get most of our information from the Dogtooth. We know that there is a weak facetted layer at the ground that has been failing in the last week and resulting in very large avalanches. There has been 60-80 cms recently with strong winds and high freezing levels, creating a cohesive slab above the weak basal layer. In the higher alpine terrain there may be a rain crust from October. The storm and wind-slab instabilities should settle and bond over the next day or two. Any deeper weak layers may continue to be a problem. The danger ratings are steering towards moderate to reflect the storm layers improving. If we see evidence of very large naturals when it clears on Monday, we may change our course!

Problems

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.

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.