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Avalanche Forecast

Archived

Mar 14th, 2013–Mar 15th, 2013

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

Regions

Purcells.

Precipitation amounts vary a lot throughout the region and according to altitude. Be aware of conditions that are specific to your area and make observations continuously as you travel.

Confidence

Fair - Forecast snowfall amounts are uncertain on Friday

Weather Forecast

Thursday night and Friday: Freezing levels will start to drop (1500 m.) bringing the snow line lower. Moderate amounts are expected until Friday and will taper off becoming light during the day. Moderate to strong winds should switch from the SW to the W.Saturday: Unsettled conditions without any frontal systems bringing very light precipitation, strong W winds, mainly cloudy skies and freezing levels near 1000 m.Sunday: Similar pattern as Saturday with cooler temperatures and lighter winds.

Avalanche Summary

I suspect that a natural avalanche cycle is ongoing in the region. Slab avalanches up to size 3 have been reported out of alpine features especially on N-NW-NE aspects. Loose avalanches were also reported up to size 1.5 in the recent snow. 

Snowpack Summary

The warm storm has left from 20 to 45 cm of snow above 2000 m..  At higher elevations, snow has been transported by strong SW and W winds creating windslabs that are very touchy. Below the freezing line, the snowpack is rain soaked. Another pulse of precipitation will add weight onto the storm slab and the already moist/wet snowpack.  The new snow has not been bonding well to the underlying surfaces creating lots of recent avalanche activity, easy snowpack test results and reactivity to skier traffic. Some of the significant underlying surfaces mentioned before include a surface hoar layer in shaded-sheltered areas, a suncrust on S aspects and a melt-freeze crust below 1700 m. The surface hoar persistent weak layer buried down 100 cm is also still a concern to professionals and would generate very large avalanches if triggered.

Problems

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.