Avalanche Forecast Explanations

Take a moment to get familiar with the various warning colors, icons, avalanche problems and other graphics associated with our avalanche forecasts.

Danger Scale

The danger scale is the simplest form of forecasting we provide. Each color is associated with a specific degree of danger. When you see this color on the Avalanche Forecast Map, that is the highest danger or warning level for that area. These same colors are also used to define the warning levels of specific elevations, specific faces of the mountains in that area, and so on. Colors are also paired with numbers and icons to help individuals better understand the warning.

Note: The Forecast Map is only providing the highest warning level in that area, which may be associated with a very specific face or elevation of the mountains. When looking at the Detailed Forecast for that area you might determine that particular elevations and sides of the mountains are more safe than others. As always, your backcountry travel experience, avalanche training, understanding of forecasts and common sense should dictate where you choose to travel.

The Danger Scale includes these colors, numbers and icons:

0-noneNone

No forecast has been provided at this time or there is not enough information to establish an avalanche danger level.

1-low1. Low

Generally safe avalanche conditions. Watch for unstable snow on isolated terrain features.

2-moderate2. Moderate

Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.

3-considerable3. Considerable

Dangerous avalanche conditions. Careful snowpack evaluation, cautious route-finding and conservative decision-making essential.

4-high4. High

Very dangerous avalanche conditions. Travel in avalanche terrain not recommended.

5-extreme5. Extreme

Avoid all avalanche terrain!

Avalanche Problems

Mountain aspects, various types of snow, the snowpack formed throughout the season, moisture and weather conditions, and your personal activities all contribute to the frequency and severity of avalanches.

Forecasters use the graphics below to define what they expect as the most likely type and/or cause of an avalanche in their area. The problem graphics are meant to help you better understand what conditions and situations to avoid when in the backcountry.

Loose Snow

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 point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-dry avalanches include point-release avalanches or sluffs. Loose-dry avalanches can trigger slab avalanches that break into deeper snow layers.

Loose Dry avalanches are usually relatively harmless to people. They can be hazardous if you are caught and carried into or over a terrain trap (e.g. gully, rocks, dense timber, cliff, crevasse) or down a long slope. Avoid traveling in or above terrain traps when Loose Dry avalanches are likely.

Storm Snow

Release of a soft layer of new snow that breaks within the storm snow or on the old snow surface. Storm problems typically last between a few hours and few days. Cohesive storm snow that accumulates over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs.

You can reduce your risk from Storm Snow by waiting a day or two after a storm before venturing into steep terrain. Storm Snow is most dangerous on slopes with terrain traps, such as timber, gullies, over cliffs, or terrain features that make it difficult for a rider to escape off the side.

 

Falling Cornice

Cornice Fall is the release of an overhanging mass of snow that forms as the wind moves snow over a sharp terrain feature, such as a ridge, and deposits snow on the downwind (leeward) side. Cornices range in size from small wind lips of soft snow to large overhangs of hard snow that are 30 feet (10 meters) or taller. They can break off the terrain suddenly and pull back onto the ridge top and catch people by surprise even on the flat ground above the slope. Even small cornices can have enough mass to be destructive and deadly. Cornice Fall can entrain loose surface snow or trigger slab avalanches.

Cornices can never be trusted and avoiding them is necessary for safe backcountry travel. Stay well back from ridge line areas with cornices. They often overhang the ridge edge can be triggered remotely. Avoid areas underneath cornices. Even small Cornice Fall can trigger a larger avalanche and large Cornice Fall can easily crush a human. Periods of significant temperature warm-up are times to be particularly aware.

Wet Avalanches

There are two types of wet avalanche: Loose-Wet and Wet Slab. Wet Slabs are a release of a cohesive layer of snow (a slab) 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 events and/or rain-on-snow events. Wet Slabs can be very destructive.

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 lower snowpack layers. 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. Other names for loose-wet avalanches include point-release avalanches or sluffs. Loose Wet avalanches can trigger slab avalanches that break into deeper snow layers.

Travel when the snow surface is colder and stronger. Plan your trips to avoid crossing on or under very steep slopes in the afternoon. Move to colder, shadier slopes once the snow surface turns slushy. Avoid steep, sunlit slopes above terrain traps, cliffs areas and long sustained steep pitches.

Persistent Slab

Release of a cohesive layer of soft to hard snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Deep Persistent Slab.

The best ways to manage the risk from Persistent Slabs is to make conservative terrain choices. They can be triggered by light loads and weeks after the last storm. The slabs often propagate in surprising and unpredictable ways. This makes this problem difficult to predict and manage and requires a wide safety buffer to handle the uncertainty.

Wind Slab

Release of a cohesive layer of snow (a slab) formed by the wind. Wind typically transports snow from the upwind sides of terrain features 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. Wind slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs.

Wind Slabs form in specific areas, and are confined to lee and cross-loaded terrain features. They can be avoided by sticking to sheltered or wind-scoured areas.

Deep Slab

Release of a thick cohesive layer of hard snow (a slab), when the bond breaks between the slab and an underlying persistent weak layer, deep in the snowpack or near the ground. The most common persistent weak layers involved in deep, persistent slabs are depth hoar or facets surrounding a deeply buried crust. Deep Persistent Slabs are typically hard to trigger, are very destructive and dangerous due to the large mass of snow involved, and can persist for months once developed. They are often triggered from areas where the snow is shallow and weak, and are particularly difficult to forecast for and manage. They commonly develop when Persistent Slabs become more deeply buried over time.

Deep Persistent Slabs are destructive and deadly events that can take months to stabilize. You can trigger them from well down in the avalanche path, and after dozens of tracks have crossed the slope.

Glide

Release of the entire snow cover as a result of gliding over the ground. Glide avalanches can be composed of wet, moist, or almost entirely dry snow. They typically occur in very specific paths, where the slope is steep enough and the ground surface is relatively smooth. The are often proceeded by full depth cracks (glide cracks), though 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, are nearly impossible to forecast, and thus pose a hazard that is extremely difficult to manage.

Predicting the release of Glide Avalanches is very challenging. Because Glide Avalanches only occur on very specific slopes, safe travel relies on identifying and avoiding those slopes. Glide cracks are a significant indicator, as are recent Glide Avalanches.

Avalanche "Avy" Rose

Avalanche Rose Example

The Avy Rose is a quick way to visualize the avalanche danger on different aspects and elevations. The inner band represents higher elevations, with lower elevations on the outer band.  The Rose corresponds to the same danger ratings for each elevation band as in the main Danger Rating graphic above, but also breaks the elevation bands into each aspect to show more detail. This makes it easy to see patterns of wind loading, solar heating, or other aspect-dependent dangers. Like all parts of an avalanche forecast, the Rose is a tool to help convey our best understanding of current dangers. It is still up to you to decide whether any particular slope is safe on a case-by-case basis.