Powder hounds love the sound of avalanche bombs in the morning. To wake to the boom and rattle of windows, to feel the deep compression reverberate across the valley, to open your eyes to the alarm clock of explosives means only one thing. Powder day.
But how do we ski patrollers decide when to go out for avalanche control? Some days the rumble of explosives promises fresh powder, and other days the hillsides are quiet only to reveal deeper and lighter snow than before. So what gives? Why do we go out some days with 3″ of new and not others with 8″?
There are no hard and fast rules. This is weather we are talking about, after all. Our avalanche forecasters decide how the current weather will affect the snowpack, and make the decision to wake us all up at 4AM to come in for avalanche control. Also known as “Avi” or “AC”, we mitigate the avalanche hazard by using explosives or ski cutting to create avalanches while the slopes are closed, so that they don’t happen later, when skiers or boarders take their first turn.
The decision to “go out early” is always made before anyone actually sets out on the snow. It would be easy to decide when to go out if we had a clear rubric, if anything 5″ or more meant an automatic callout. But it doesn’t work that way. 5″ of light fluff that falls without wind doesn’t add much stress to the snowpack. However, 5″ of wet, heavy snow that comes in on a southwest wind and deposits snow in our north-facing starting zones could trigger big slides. Avalanche hazard is determined by the strength of the snowpack versus the stress of new snow. The snowpack can weaken or strengthen over time. In a maritime climate, such as ours, the snowpack tends to strengthen over time. The stress of new snow is our biggest determinant in avalanche hazard. We base our avalanche control almost entirely on new snow.
Here’s how we make that decision.
If the weather is nuking all day long, and we have not had much skier compaction, Chet, the Snow Safety Director, may decide to make the callout “automatic.”
Regardless of what happens overnight, the determination has already been made before the previous ski day ends. If the upper mountain is on “wind hold” during a big snow event, and no one has been up to ski the new snow, we will almost always have an “automatic callout”.
We set our alarms to arrive early to work at about 6AM. Then we all head up the Gondola and disperse from there to our various routes, each consisting of at least one avalanche blaster and one blaster apprentice.
Crystal Mountain has numerous avalanche paths. We are right up there with bigger areas like Squaw and Snowbird for number of detonations.
At other ski patrols, it may take years for a new patroller to gain enough hours to sit for their blasters exam. At Crystal, new patroller usually get enough apprentice hours in a single season.
Every night a ski patroller stays at the Summit House. Back in the day, the late Jack Lewis lived there, and we still refer to the night patroller as “Jack”. It can be a sweet gig with views of alpenglow and starlight or it can be windy and stormy and full of calls to cat drivers and midnight walks along the ridge to the top of Grubstake to determine the snowfall. If Jack determines we’ve had enough snow for AC, he or she will call our Snow Safety Director. If Chet agrees, Jack gets on the phone to wake us all up to come in early.
But how does Jack decide? That’s the 5 Million Dollar question. The snowpack is likea layer cake. Sometimes that cake is hard and dense and well compacted. Other times a light layer sits pretty on top of denser layers. We do not usually go out then.
Dense snow can come from the sheer weight of the snow–a foot of new snow in any form will almost always bring us out for AC. Dense snow can also be transported by wind onto lee slopes. When we have dense snow on top of weaker snow, that’s a recipe for avalanches.
A few inches of new snow with just the right wind direction can increase avalanche hazard dramatically. Wind, water amounts and temperature all play a role in avalanche control.
Avalanches happen when the stress on the snowpack outweighs the strength. Those days we wake to the sound of booms in anticipation of great skiing.
Ski patrollers are looking at the forecast, anticipating some early mornings in the coming days. It is already snowing as of 8AM Saturday and should continue for the next several days. The 72 Hour snowfall total, above, shows over 20 inches of snow in the next few days. Monday morning could be our biggest day yet. The hills will be alive with the sound of avi bombs.
This post was reblogged from kimkircher.com.