Persistent Weak Layers and the Winter of 2007-2008
Persistent Weak Layers and the Winter of 2007-2008
Prepared by: Karl Klassen
Avalanche Forecaster, Canadian Avalanche Centre
Posted January 30, 2008
This winter, backcountry users in western Canada are faced with a particular problem—a
persistent weak layer in the snowpack of many popular mountainous areas. This
discussion is intended to help you better understand how these layers formed and the
challenges related to managing this type of risk. This article is not intended to provide
answers to all the questions you might have. Instead, it provides general guidance and
some tips to help you ask the right questions and gather information that can help you
make an informed decision.
I must stress there is no 100% guarantee of safety in the mountains. You could be caught
in an avalanche, injured, or killed even if you follow every piece of advice in this
discussion and “do everything right.” This being said, in at least half of the avalanche
fatalities to date this season, the victims were either not equipped with appropriate rescue
gear or did not heed clear warnings. Many avalanche accidents are preventable—it’s up
to you to get the training, information, and appropriate equipment to properly manage
your risk if you choose to go into the mountains.
What’s the problem?
A persistent weak layer (PWL) is so called because it does not strengthen over time, and
in some cases it even becomes weaker over time. A PWL can remain unstable for weeks
or even months and is often the cause of avalanches long after it originally forms and is
buried.
A PWL often results in what is referred to as a “deep slab instability”—a deeply buried
weak layer beneath a thick slab or slabs of snow comprised of accumulated snow from
numerous storms. Deep slab avalanches are generally large and highly destructive due to
their size and mass. A PWL that formed in the fall of 2002 and the resultant deep slab
instability was the underlying cause of most of the 29 fatalities that occurred in the winter
of 2002-2003 (the worst avalanche season in recent years), including two large
avalanches that each killed seven people in the Selkirk Mountains.
A variety of crystal forms can contribute to a PWL, but the current concern is a “facets on
crust” layer. Back in the early days of the season, rain saturated the surface of the
snowpack (this is called a “rain on snow event”). As or shortly after that wet layer froze,
facets (sugary snow grains) formed on top of the icy rain crust. Now, that layer of facets
has become a PWL.
This year’s facet on crust PWLs are known or strongly suspected as the cause behind nine
of 11 avalanche fatalities to date, spanning a time frame from December 8, 2007 to
January 16, 2008 and occurring in the Alberta Rockies, the Cariboo Mountains, and the
southern Monashee Mountains. The most active period to date was the first week of
January when numerous fatal accidents made avalanches front page news. The untold
story are the many close calls involving this year’s PWLs that didn’t make the news
starting in mid-November and, as of this writing, continuing to January 24th, occurring in
practically all the mountain ranges of BC and Alberta. You can see reports about many of
these close calls on the Canadian Avalanche Centre’s Discussion Forums at:
http://www.avalanche.ca/default.aspx?DN=586,558,3,Documents.
Experience has shown that PWLs go through cycles where periods of high avalanche
activity are interspersed with periods during which few avalanches occur. When, where,
and why these cycles occur can be difficult to predict with a high degree of accuracy and
are not fully understood. Unstable periods are often related to changes weather factors
such as increased wind, snowfalls, rain, temperature changes (especially from cold to
warm), and solar radiation. A PWL that has produced avalanches and then shows signs of
stability is generally treated with scepticism by knowledgeable practitioners, most of
whom describe such a layer as dormant rather than stable, the assumption being (as
Monty Python once said): “…it’s not dead, it’s just sleeping.”
From CAA website
