In our avy classes we talk about the weather being the ‘architect’ and that the snowpack ‘blueprint’ changes every season. Ideally we pray for early season snow (and lots of it!) and mild temperatures. These weather events typically result in a well bonded snowpack and therefore a great foundation to support the mid-season snowfalls.
Unless you had a seriously festive Christmas season you will remember the couple weeks of frigid temperatures in December 2008. Combine this with the average or below average snow depths in much of the mountains areas and our blueprint isn’t structurally sound at this time.
CLICK HERE to view the CAC PAB for the South Rockies for January 6, 2009
The students brave enough to participate in our field sessions at -28*C this December figured it out. While testing the snow for resistances (the density of the snow) it was easy to notice the layers of unconsolidated snow.
They could visualize how the warm storms expected over Christmas and in early January would settle quickly and create slabs near the surface. The upper layers then act as insulation and can inhibit the warmer air temperatures from penetrating into the deeper weak layers. This can lengthen the time taken for the lower layers to heal. Unfortunately this leads to the familiar formula of ‘a strong layer over a weaker layer’ which leads to slab avalanches.
Successive snowfalls bury this weak layer deeper and deeper creating a spooky snowpack. The deeper layers become harder to trigger, however the consequences of triggering one get higher and higher.
Below is part of an article that was posted on an internet site ‘WildSnow’. It was written in response to avalanche fatalities in Colorado and Utah. Based on the description, their snowpack is very similar to the snowpack in much of western Canada this season.
Posted by Lou Dawson on December 15, 2008
Black Sunday — Avalanches Kill in Colorado & Utah
When Colorado (or for that matter, Utah) goes through an early winter with minimal snow, the result is inevitable. Cold nights cause the snowpack to metamorphose. Instead of a foundation that holds subsequent storms like cinder blocks bedded in mortar, the older snow becomes a layer of ball-bearing like crystals that hold said cinder blocks as tenuously as if they’d been dropped on sand.
Such snowpacks do inevitably stabilize….
To read the entire article, CLICK HERE to be redirected to www.wildsnow.com
Remotely triggered avalanche?! What is that?
An avalanche that is triggered from very low on the slope, sometimes way out in the flats. The failure is triggered in a weak layer beneath a slab (that is supporting the rider). The person may not even know that this has happened. If the weakness is very unstable and widespread the failure may then propagate (travel quickly) through the weak layer and up the slope, searching for weak points (tension failures) to rip the overlying slab apart. These weaknesses then connect the dots and become the fracture line.
This type of avalanche activity is possible with our reported avy conditions. Please take care.
CLICK on the picture below to view a video clip of two snowmobilers that remotely triggered an avalanche in Montana on Boxing Day.
Remotely Triggered Avalanche Little Wapiti Drainage – Dec 26, 2008
The Avalanche Guys from the Gallatin National Forest Avalanche Center always have great video clips uploaded to YouTube_TM_ as well.
Henderson Ridge Avalanche – Dec 28, 2008
Northern Bridgers, MT – Dec 20, 2008
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