October 1, 1998
New studies could help predict Snoqualmie Pass avalanches
SUN RIVER, Ore. – Two new studies of avalanches in Snoqualmie Pass in the Washington Cascades near Seattle could bring about more accurate predictions that will safeguard travelers in quickly changing conditions.
In one study, Howard Conway, a University of Washington research associate professor in geophysics, concluded that a forecast model that uses measurements of new snow density, amount of precipitation and air temperature appears to provide a more reliable indicator of avalanche danger.
Presenting his findings to the International Snow Science Workshop in Sun River, Conway showed that it isn’t how much new snow falls but the stability of the snow that matters. His model compares the strength of the underlying layer of snow and the burden imposed by new snow.
The work essentially quantifies much of what the state Department of Transportation avalanche forecast team, headed by Craig Wilbour, has learned from years of experience in predicting avalanche hazards. Conway and Wilbour, a forecaster at Snoqualmie Pass since 1967, have conducted the study for five years, and Conway sees the formula as a starting point for predicting avalanche danger.
“It’s an aid. You can’t get away from the experience factor and replace it with a computer model,” he said. “These avalanches come down particular paths so you have to have some local knowledge of what’s going on in those paths.”
The second paper, presented by Hans-Peter Marshall, a senior in the UW geophysics program, outlines work he did with Conway. It shows that rain on snow – a common occurrence even during midwinter at Snoqualmie Pass – typically triggers avalanches within 10 minutes. After that, changes in the snowpack’s stability slow sharply.
“We’re quantifying how the snow changes when the water hits. It changes really fast,” Marshall said. “If you can imagine putting a clump of snow in water, it shrinks up very fast.”
The finding is particularly significant because the relatively low elevation of Snoqualmie Pass means that warm moist air from Puget Sound can end up on top of a layer of frigid air from Eastern Washington. That means the lower pass could be receiving snow or freezing rain, while higher elevations have just rain – and a quickly escalating danger of avalanches that might rumble all the way down to Interstate 90.
While avalanches can strike quickly, they don’t always show up when you might expect. Conway’s study illustrated the variations between storm cycles. A storm on March 1, 1997, that produced about two feet of new snow triggered a series of small avalanches that closed I-90 for 24 hours, buried or partially buried a number of vehicles and swept a skier over a cliff. No one was injured. By contrast, storms between Jan. 8 and Jan. 15, 1989, dumped more than 4 feet of snow in the pass, but no avalanches occurred until after rain began falling on Jan. 15.
Conway hopes the research will give avalanche forecasters a new tool to combine with their experience so avalanche danger can be assessed more quickly. That would lead to more timely road closures.
“Part of the reason for some of these problems is the pressure to keep the roads open,” he said. “There is a lot of traffic, especially a lot of truck traffic.”
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For more information, contact Conway at (206) 685-8085 or conway@geophys.washington.edu, or Marshall at (206) 616-5393 or hpm@geophys.washington.edu.
