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Avalanche Forecast

Archived

Jan 26th, 2012–Jan 27th, 2012

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

Regions

Purcells.

Confidence

Fair - Timing, track, or intensity of incoming weather is uncertain on Saturday

Weather Forecast

A surface ridge moves into the region Friday keeping the area dry while northwesterly flow aloft keeps scattered clouds in place. Ridgetop winds will be at strong values out of the W near ridgetop Friday & temps will remain cool; Expect a daytime High of -5 and an overnight Low of -12 @ 1500 m. Things begin to change a bit Saturday as the ridge breaks down allowing moist flow in from the pacific. The first storm associated with the new pattern enters the region Saturday night, sticking around through Sunday. Freezing levels are expected to rise to 1300 m Saturday night and storm totals from 20 - 40 cm are forecast at this time.

Avalanche Summary

A couple of large natural avalanches in the alpine (to size 3) were reported on NE and SE- E aspects in the Dogtooth range Wednesday. Smaller avalanches of the loose snow & soft slab variety (to size 1.5) were also reported running in the new/old snow interface. Elsewhere in the region a natural avalanche that started in steep un-skiable terrain was observed. All the reported avalanches were running fast and entraining snow as they ran down the slope.

Snowpack Summary

The Purcell’s are one of the more complex snowpacks in the province right now.Tuesday night/Weds the region received 35 – 60 cm of new snow accompanied by strong winds out of the SW. Just a few days ago the region received 20 – 40 cm with S & SE winds. A test profile on Tuesday revealed a sudden planer shear down 40 cm failing on surface hoar with a hard trigger. That was before the storm, that layer now has 70+ cm of snow on it and is likely more sensitive. Resistant shears were noted in last weekend’s storm snow yesterday too. The bottom line is that there are a number of different weaknesses/sliding layers in the snowpack which are now under additional strain.The mid-December surface hoar/facet persistent weakness, now down around 70cm on the eastern side of the range and as deep as 230cm on the western side, is still causing operators concern, especially in shallow snowpack areas. Recent snowpack tests show moderate to hard, sudden planar results on this layer. The consequences of a failure on this layer would be large. Facets and depth hoar exist at the base of the snowpack. Cornices are large and weak in some areas.

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.