SALT LAKE CITY - The deadly, roaring avalanches that have killed 19 people in western North America so far this winter may have broken away in an instant, but they were months in the making, according to avalanche experts.
Snow that fell in October and November created a slick icy slab that's now providing an unstable base for heavy snows that fell across the West in December.
Occasional rain and changing weather conditions provided additional layers in the snow that's made it easier for huge walls of snow to cleave off and plummet down the mountain. Some avalanches are so powerful they plow over trees and boulders and leave little chance of survival for those unable to get out of its path.
"There's something very tricky and very weird about the snow conditions this year that obviously caught some of the best snow-safety teams in the U.S. off guard," Doug Abromeit, director of the U.S. Forest Service's National Avalanche Center in Ketchum, Idaho, said this week.
All 19 have died since Dec. 14.
The worst was Sunday when 11 snowmobilers were swept away in back-to-back avalanches in British Columbia's backcountry. Eight were killed.
In Utah, three snowmobilers and a skier have been killed.
Colorado has also had four deaths: a skier, a snowboarder and two on snowmobiles.
Others have died in Wyoming and Washington state.
Dozens of other people have had close calls - 28 in Colorado alone, as of Wednesday.
Some of the slides are large - one near Logan, Utah, that killed two snowmobilers was estimated to be 2,000 feet wide - but many aren't.
"It's more the frequency that's unusual as opposed to the size," said John Snook, an avalanche forecaster with the Colorado Avalanche Information Center in Boulder.
Avalanche experts have been flummoxed at times by what they've seen this winter and have issued warning after warn00ing about the dangers of venturing onto steep, snow-covered mountainsides.
Bruce Tremper, director of the Utah Avalanche Center compared the layered avalanche conditions to a pile of feathers overlain by a pane of glass which, in turn, is covered in slippery potato chips.
"Then you try to park an Oldsmobile on top of it. It's just not working," he said.
Some of the problems were predictable. The snow that fell in October and November changed as temperatures fluctuated, switching from a cohesive layer to one that's more crystallized and weak.
Rain fell in some parts of high-elevation Utah on Thanksgiving, adding another slippery coat. And in December, huge snow dumps - 300 percent of average in some parts of Colorado - added heavy loads to those unstable underlying layers.
Plus, there was extreme high wind in Colorado, further impacting the snowpack.
Conditions are dangerous but it's also difficult to predict exactly when and where avalanches will occur.
"Pretty much all of the avalanche professionals I've talked to have said, 'Man, I don't know if I've seen this before,"' Tremper said.
Avalanche officials remain on alert, and danger remains high or extreme in most mountainous regions throughout the West.
Extreme conditions are expected in the Cascade Range from Washington state's Mount Rainier to central Oregon because of weak snow, high winds and new snow on the way.
Avalanche experts have been urging people to steer clear of dangerous areas. Those who do go into the backcountry should consult with avalanche centers first and be prepared with proper avalanche equipment, including beacons and shovels.
No one's sure how long the danger will last. It's possible that slick slab from the fall snowstorms will stick around until spring. Then again, maybe not.
"It all depends on what the weather does," Abromeit said.