What’s Behind an Avalanche Warning?

Parts of the Cascade Mountains in Washington state were under avalanche warnings Tuesday as our latest winter storm moved into the Pacific Northwest.

The warnings were due to heavy snow and high wind.

The snow will be piling up in the Cascades over the next few days. Mt Baker has received over 4 feet (not a typo) since late last week. More is on the way. This is a UW model forecast for snowfall ending early Thursday. Yellow shading is in the vicinity of 3 feet or more. #wawx pic.twitter.com/i9w83iqHaE

— NWS Seattle (@NWSSeattle) January 23, 2018

Avalanche

Simply put, an avalanche is a rapid flow of snow sliding down a slope.
Avalanches typically grow as they tear down slopes entraining more snow and at times trees, rocks, and other debris.

They can be triggered by anything from gravity, to wind, to humans or animals, or even a change in temperature– just to name a few.
The one constant between each of these events is that avalanches are never random, and no matter the type there are always precursors to a slide.

In each slide, initiation occurs when the load exceeds the strength of the snow.
The load is simply the weight of the snow, but the strength depends on everything from type of snow to the different layers of snow and even the steepness of the hill. And that just scratches the surface.

Avalanche! The Danger of an Unstable Snowpack

Wind Load

The avalanche warnings for the Northwest were for heavy snow and high winds.

This is known as a wind load. As strong winds blow new snow falling from the sky, it can do a few different things to cause an unstable snowpack.

• Wind can cause snow to pile up in certain areas almost 10 times faster than it otherwise would just falling from the sky.
• As snow blows in the wind the flakes erode and change shape, making for a much more dense and therefore heavier snowpack.
• Strong wind can deposit snow on the leeside of a mountain, piling it up to the point of instability. Often in the form of a large cornice.
• Wind can also erode snow from the windward side of the mountain, eating away at the essential portion of the snowpack for stability.

Avalanches and You

The good news is that more than 90% of all avalanches involving a person, are caused by the person or people involved in the avalanche.

This means that if you avoid the backcountry, you are safe. It is highly unlikely that a rogue avalanche will come down and sweep away your romantic mountain-side lodge.

But if you plan on heading into the backcountry. Be safe. Know the dangers. And always check your local avalanche forecast center.

For WeatherNation — Meteorologist Jeremy LaGoo