Forecasting Juneau's avalanches a full time job
Published: 03/15/2012 17:49:00
Parts of Juneau are built in known avalanche zones, and a recently released study highlighted some of the risk areas. Every day during the winter Juneau issues an avalanche forecast for those in and around those higher risk areas. Juneau's avalanche forecaster Tom Mattice watches daily weather patterns and builds models to predict when and where avalanches are most likely. But the best source of data is high up in the mountains where avalanches start.
Mattice uses the Mt. Roberts Trams to access the alpine conditions: "We come up here on the tram a lot; it's got the same aspects along the channel and it gets us to elevation real quick. And then we can go up the ridge and look around on different sides in different places to better analyze what's going on."
Using backcountry skis, crews make a short trek further up the mountain to make observations. On the way up the mountain, we passed a slide that had happened earlier in the day as we continued up to the starting zone. "Down below us here on the steep section below where the wind had loaded, we saw a natural avalanche today. We're a little bit higher on the slope, in a little more of a safe spot; we're going to dig down, we're going to look at the layering in the snow and test that weak layer we saw natural over here with a little more steepness and a little more wind-load" said Mattice.
By digging a hole in the snowpack, it's possible to see and analyze the layers in the snow caused by varying weather over time, like the rings of a tree. Weaker layers are more likely to fail and cause the upper layers to slide, causing an Avalanche.
Mattice cut out a column of snow into the wall of the hole and tapped on it with a shovel, causing a large slab to break off. "So the fracture propagated all the way across the column, but it didn't slide right off, so it's sticking there a little bit. Once again the question is: is that a nice clean sheer plane? And what are those crystals and why is that bond so poor? So now we know that if this fracture starts in the top 6 inches; it doesn't take much energy, once that avalanche gets going, to propagate down into deeper and deeper weak layers" he said.
The research teams never work alone, because of the risk of avalanches. On the way back to the tram, we witnessed the that top layer of snow fail under one of our group as he skied over the edge to the steeper part of the slope. He skied clear as a wide 6" deep slab broke off and slid down the short slope.
The first hand observations in the field help provide a more accurate forecast for those living below says Mattice. "The urban avalanches don't happen very often, but the fact that people are paying attention to the forecast and understanding what that forecast means to them, hopes that someday if we do have a big avalanche we'll have a lot less people in those affected areas."
The Juneau urban avalanche forecast is published online daily on the city's website.
By: Mikko Wilson - firstname.lastname@example.org