By Jason D. Martin | Images courtesy of the Northwest Avalanche Center
It snowed on Wednesday. On Thursday, it rained. Today it was sunny. Tomorrow, it’s supposed to snow again … So, should I go backcountry skiing tomorrow? Will it be dangerous? Will there be avalanches?
It’s almost always possible to ski in the backcountry with some margin of safety. But where? It’s common for some slopes at some elevations to be more dangerous than others. Some aspects are more impacted by sun or wind, while others are more sheltered. The Northwest Avalanche Center (NWAC) tracks all this and provides backcountry users with the baseline information they need to make a decision about whether and where they should ski.
NWAC provides backcountry users with daily avalanche forecasts at nwac.us. Forecasts are based on a combination of archived information, snow data, field observations and weather forecasts. To make use of NWAC’s forecasts, you have to know how to read them.
The NWAC report starts with three tabs titled avalanche forecast, mountain weather, and observations and weather data. We’ll focus on the avalanche forecast. An avalanche forecast encompasses four separate components: the type of avalanche, where the avalanche danger is, the likelihood of an avalanche, and how big an avalanche is likely to be. The NWAC forecast starts with a summary of all four parts titled, “The Bottom Line.” This provides a well-rounded understanding of the avalanche hazards and should be the first thing you read.
Following the summary, there’s a chart that shows the likelihood of an avalanche in three major locations: above tree line, near tree line and below tree line. Each area gets a likelihood rating of low, moderate, considerable, high or extreme. Most avalanche fatalities do not happen on high or extreme days. Why? Most backcountry skiers stay home. Instead, fatalities happen on days forecast as moderate or considerable.
The next chart breaks down the avalanche concerns, including types of avalanches, the likelihood of an avalanche at a given aspect and elevation, and the size. There are two major categories of avalanche, loose-snow avalanches and slab avalanches. Within those there are nine subcategories that we might define as avalanche problems. The NWAC forecast will generally identify one, two, or three different problems for a given day.
The problems are defined on NWAC’s website, but here’s a quick breakdown to aid in reading the daily forecast:
Loose Dry: Release of dry, unconsolidated snow. These avalanches typically occur within layers of soft snow near the surface of the snowpack. Loose-dry avalanches start at a specific point and entrain snow as they move downhill, forming a fan-shaped avalanche.
Storm Slab: Release of a soft, cohesive layer of new snow that breaks within the storm snow or on the old snow surface. Storm-slab problems typically last between a few hours and a few days.
Wind Slab: Release of a cohesive layer of snow formed by the wind. Wind typically transports snow from the upwind side of terrain and deposits snow on the downwind side. Wind slabs are often smooth and rounded and sometimes sound hollow and can range from soft to hard.
Persistent Slab: Release of a cohesive layer of snow in the middle to upper-snowpack when the bond to an underlying persistent weak layer breaks. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky.
Deep Persistent Slab: Release of thick cohesive layer of hard snow when a bond breaks between the slab and an underlying persistent weak layer, deep in the snowpack or near the ground. Deep persistent slabs are typically hard to trigger but are very destructive and dangerous due to the amount of snow involved, and can persist for months. They are often triggered from areas where the snow is shallow and weak, and are particularly difficult to forecast and manage.
Loose Wet: Release of wet, unconsolidated snow or slush. These avalanches typically occur within layers of wet snow near the surface of the snowpack, but they may quickly gouge into the lower layers of snow. Like loose-dry avalanches, they start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. They generally move slowly, but can contain enough mass to cause significant damage to trees, cars or buildings.
Wet Slab: Release of a cohesive layer of snow that is generally moist or wet when the flow of liquid water weakens the bond between the slab and the surface below (snow or ground). They often occur during prolonged warming periods or rain-on-snow events.
Cornice Fail: Release of an overhanging mass of snow that forms as the wind moves snow over a ridge and deposits snow on the downwind side. Cornices can break off suddenly and pull back onto the ridge top and catch people by surprise, even on flat ground above the slope. Even small cornices can have enough mass to be destructive and deadly.
Glide: Release of the entire snowpack as a result of gliding over the ground. Glide avalanches typically occur in very specific paths, where the slope is steep enough and the ground surface is relatively smooth. These are often preceded by cracks running the full depth of the snowpack (glide cracks). The time between the appearance of a crack and an avalanche can vary between seconds and months. Glide avalanches are unlikely to be triggered by a person and are nearly impossible to forecast.
In the next column, there’s a graph called an “aspect and elevation rose.” This graphic looks a little like a spider web, with 24 distinct compartments. The outermost segments indicate the area below tree line, the mid-level segments indicate the areas near tree line, and the central segments indicate areas above tree line. Each series of segments lines up with a direction on the compass rose. If a section within the aspect and elevation rose is shaded, it means that there is a possibility of the indicated type of avalanche at the shaded elevation and aspect.
The final two columns are for likelihood and size, which are self-explanatory.
The other two tabs on the Forecast Page – Mountain Weather, and Observations and Weather Data – provide significant details on what has happened with the weather, what is happening and what is about to happen. This provides a great deal more detail to work with in order to make the best possible plans.
In addition to professional avalanche training, the avalanche forecast should be an essential part of every backcountry travelers toolkit, and no one should leave home for a day in the backcountry without carefully reviewing it. However, that avalanche forecast is only part of the backcountry toolkit. The other part – perhaps more important – is avalanche education. Most backcountry skiers and boarders take an American Institute for Avalanche Research and Education Level I course and receive avalanche rescue training before venturing into the backcountry.
Jason D. Martin is a mountain guide, general manager at American Alpine Institute and a widely published outdoor writer. He lives in Bellingham with his wife and two young children.