The victims of Saturday's killer avalanche in British Columbia were caught in a snow slide powerful enough to destroy a house, one that had been building all season and lacked only a trigger, an avalanche specialist said Sunday.
"All these things came together," Karl Klassen of the Canadian Avalanche Centre, said in an interview from Revelstoke near the site of the disaster.
Two people were confirmed dead and 30 were hurt when a slab avalanche tumbled down on dozens who had come out to watch a snowmobile event dubbed the "Big Iron Shoot-Out" on a stretch known as the Turbo Bowl at the base of Boulder Mountain.
The avalanche centre had warned the danger of a slide was high that day. Witnesses said the avalanche began after three thrill-seeking snowmobilers began racing each other up a steep slope - in a stunt called high-marking - to see who could go the highest before gravity and the steep angle forced a turnaround.
"Certainly a snowmobile can be a trigger for an avalanche," said Klassen
"The machine and the rider add weight to the snowpack, and any weak layers in the pack may then release."
The snow slide was 150 metres wide and roared down the hill in seconds, tossing people and machines like toys for close to a kilometre.
The slide, said Klassen, is considered a size three, powerful enough to damage a truck, destroy a car or a house. It's the mid-range of a five-degree scale. One is relatively harmless, a two can bury you. A four can destroy several buildings and a five can wipe out a village.
The slide had been building all season.
Usually, said Klassen, the region has a heavy dump of snow, followed by avalanches that allow the snowpack to settle and stabilize.
But in the last three years, he said, that cycle has gone out of whack.
Now, he said, it's a heavy snowfall, followed by a long dry spell, followed by another heavy dump of snow, followed by another long dry spell. Those dry spells don't allow the snow to sink and become compact, he said. Instead, the dryness creates weak, brittle layers of snow that can easily crack and send the snow on top roaring down the slope in what is known as a "slab avalanche."
He said there had been a recent snowfall in the disaster area, followed by high winds that could have pushed more snow on top of the fault line. Added to that, he said, was warm weather that heated the snowpack and made it even more unstable.
Then, he said, came that beautiful Saturday afternoon, with clear skies and little wind.
"Within our business, there is what we call the 'Blue Sky Syndrome.' People do not seem to perceive risk the same way when it's a beautiful, cloudless, windless, warm day as when the wind is blowing and the snow is falling," he said.
The snow was ripe to go.
Recently, said Klassen, he sent a wave of snow down the mountain by simply standing on a ridge on his skis.
"(Saturday's disastrous snowpack) obviously was less sensitive. It took at least one snowmobile to trigger it, but it was not sensitive enough to go on its own naturally."
When a slab avalanche falls, the results can be catastrophic. Unlike a loose snow avalanche, the slab comes down in one quick mammoth swoosh. The ground shifts underneath the unsuspecting victims in its path. They fall and are overwhelmed by wave after wave of cascading snow.
One snowmobiler caught in Saturday's avalanche described nearly surfing the wave of snow. While he thought he was driving his sled away from the slide, he eventually realized he was actually being propelled by the cascading snow before it jolted him into the air.
In general, the life expectancy rates for those caught in such a disaster start at about 50-50 and drop precipitously from there, said author Tony Daffern in his work "Avalanche Safety For Skiers and Climbers."
More than half of those who survive an avalanche do so because they dug themselves out or a part of their body or ski equipment was visible above the snow, said Daffern.
Of those completely covered, they have an 80 per cent chance of survival if found just below the surface and within minutes of the disaster. For every hour under the snow, the survival rate is cut in half or worse.
Some of the witnesses Saturday said they thought they were safe because they were about a kilometre away from the slope.
Not so, said Klassen.
"People underestimate how far avalanches can and will run. Big avalanches can run across flat terrain for long, long distances. They can even run uphill under the right circumstances and destroy trees that have been standing for 100, 200 years."
He said the key to stay safe is for those who go into the avalanche zones to be aware of the risks and know how to spot snow that is ready to tumble.
"My worry is that some of those people standing at the bottom of the slope just did not understand the risk, and they weren't really making an informed decision. They were just in the wrong place at the wrong time, and that's a very tragic circumstance."