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Peregrine News

Earth: Managing Deep Slab Avalanche Conditions

Posted by Joseph Anderson

Earth: Managing Deep Slab Avalanche Conditions

In 1910, there was a small village and railroad station high in the Cascades just west of Stevens Pass. During the last week of February, the town saw many days of consecutive snowfall with as much as 11 feet on Feb. 28th.  At 1am On March 1st, a rain event set off a half mile long and quarter mile wide wall of snow barreling down Windy Mountain, sending a parked train 150 feet down into the Tye River and killing 96 sleeping passengers.  This remains as the single biggest death toll of any avalanche event in US history.  It’s known as the Wellington Avalanche.

Over 11 days between late January and February of 1999, 20 feet of snow fell at Mt Baker ski area.  On Valentine’s day of that year, a giant slab released 14’ deep and across the entire face of Shuksan Arm Back Country area, burying and killing 2.  This Avalanche was reported as a D5, the biggest class on the scale of 1 – 5.  One of the victims was not unburied until the spring time.

Both of these Cascade tragedies are due to conditions that are not anomalies in the Cascade snowpack.  At least a few times every year, we get these Deep Slab Avalanches.  Last year the same avalanche happened on Shuksan Arm with a crown said to have been bigger than the 1999 avalanche.  Several deep slab cycles last year destroyed various bridges, deforested the landscape and created massive piles of rubble throughout the Cascade Mountains.  So when do these deep slabs occur?  And how do you stay clear of them?  Well, right now we have ripe conditions for deep slabs.  Thus far, the weather patterns of January make for a perfect recipe:
The first week of January was warm with heavy rain up to almost 6,000 ft. in the N. Cascades.  Then the storm ended cold, only dropping minimal amounts of snow and freezing the old soaked snowpack solid.  The following week consisted of cold weather with varying wind.  By the time we arrived at the MLK weekend, the Pacific hose turned on again.  Now 12 days and 10 feet of new snow later, the maritime bonding process is racing to catch up with the critical mass of new snow.  Slides like this can go at any time and occasionally, like in the case of Wellington, a big change, like rain, sets it off.

The best and only way to stay safe is to not be there when it slides.  This is the technique the residents of Wellington opted for – the town no longer exists.  On the highway, we depend heavily on the DOT to take on this responsibility.  In the Back Country you need to stay clear of big features like Shuksan Arm.  This means not only staying off the slope itself, but also not even traveling on moderate terrain below avalanche paths, even if it’s forested.

In most cases, the majority of these deep slabs do slide after a critical mass of overloading snow piles up, eventually amounting to too much too quickly.  But in some cases the persistent weak layers deep in the snow pack can remain “dormant” throughout the season and “wake up unexpectedly”.  With the extreme exceptions, I propose that the sudden “wake up” is not such a big mystery.  What sets it off is a large change in the snow pack – which can be caused by such things as heavy rain, big spikes in temperature, and increasing amounts of radiation as the encroaching season shifts from winter to spring and the sun rays continue to strengthen and the days get longer.

The skills in identifying the terrain and the snow pack in which any avalanche conditions occur will never be obtained from taking courses and reading articles alone.  The importance of hours upon days upon months and then years of witnessing and traveling in snow with constantly changing conditions is the only way to begin to understand what the snow is up to.  Even then it’s important to be paying close attention to a winter’s varying weather and it’s current effect on the stability of the snowpack you will be traveling in.