There's a Canadian field guide written for travel in avalanche terrain that doesn't mess around when it describes what's known as deep slab instability – when an unstable layer of snow is buried deep in the snowpack.
The guide said that avalanches caused by the problem «are highly unpredictable and destructive; essentially not survivable.»
Unfortunately for snowmobilers and backcountry skiers and snowboarders who like to play in southwestern Montana's mountains, deep slab instability is the main concern right now. A band of sugary crystals called depth hoar, built up during cold weather in December, rests near the ground.
«It's a difficult problem,» said Mark Staples of the Gallatin National Forest Avalanche Center. «It's the most difficult problem, even at ski areas and highways where they can use explosives (to trigger avalanches). A lot of it lies in it being low probability and high consequence.»
«It's common for persistent deep slab avalanches to become dormant for extended periods of time then ‘wake up' again, weeks or sometimes months later when the weather changes or when warm spring weather arrives,» the Canadian field guide noted.
With more snow predicted over the next few days, the weather continues to add weight to what is already a hefty mountain snowpack – 8 to 12 feet deep, which is about 110 to 140 percent of average.
«It's all about the total load,» Staples said. «This is why we look at the SnoTel (snowfall report) sites so closely. Whether it's wet or dry snow, it's all about the weight.»
But in the spring, as the field guide noted, the snowpack can also be destabilized by warm weather. A couple of years ago, Staples said, warm weather caused snowmelt that released a weak layer causing «big, destructive, wet slides» in the Bridger Mountains northeast of Bozeman.
On the flip side, melting snow can also wet that weak layer and, if frozen, provide stability similar to rebar, the steel bars used to reinforce concrete, Staples said. That is a very random occurrence, however.
«How water moves through the snowpack is even more complicated, it's really unpredictable,» he said. «Just the slightest change in crystal shape or size can stop the water from moving.»
Right now, the snowpack is «most unstable during and immediately after a storm,» wrote the avalanche center's Doug Chabot in the Wednesday morning report. He rated the avalanche danger at moderate with it rising to considerable if the rain and wet snow continued.
Staples is extremely cautious about venturing into the mountains when the snow conditions are this fickle. It was about this point in the season a couple of years ago when he got swept downhill in an avalanche while skiing in the Gallatin Mountains, south of Bozeman. Luckily, he escaped unscathed.
«So I'm personally pretty sensitive to this setup,» he said.
«The key thing is to look for those times when it's been a few weeks without snow,» Staples added. «Some seasons are better than others. Last season was a good one and, personally, I was a little more aggressive. And this season I've notched everything back.»
