|Southwest B.C. Avalanche Comment #3620|
Back To Discussion List Written: 2010.12.05 by: Frank W. Baumann
We are managing avalanche hazard for several industrial operations in southwest B.C., and are quite concerned right now about one Persistent Weak Layer that formed at the contact between early melt-freeze snow that is glued to the ground in most areas, but forms a very weak bond with overlying snow. It is about 60 cm up from the ground (HS = 162 cm) in the Whistler alpine area (Oboe Basin; 1650 metres elevation) and formed on November 15.
The CAA does warn of this PWL in their bulletins- but you need to read the written summary to get information on this- the overall hazard itself is described as only being "Moderate".
The problem with Persistent Weak Layers are that they are like a trap, ready to react when a skier or snowmobiler goes over them on a steep enough slope. So the hazard, as described in the bulletin, may seem to be low- but this may give a bad false sense of security. PWL's are one, if not the greatest, cause of fatal avalanche accidents in B.C.
My suggestion: always check for both the avalanche hazard (Low, Moderate, Considerable, High, Extreme) AND also any information on Persistent Weak Layers- and then make a travel decision based on both factors. And remember AVOIDANCE of hazard by paying careful attention to terrain is the key to safety.
The November 15 Persistent Weak Layer is one to be very careful about right now- it is best described on the Whistler/Blackcomb Ski Patrol's avalanche advisory at: http://www.whistlerblackcomb.com/weather/advisory/index.htm It has produced several big avalanches- one natural in the Flute Peak area last week was particularly interesting.
You can read more about Persistent Weak Layers in this Bivouac reference article.