Avalanche Problems

The Nine Avalanche Problem Types in Most North American Avalanche Forecasts

Storm Slabs form during storms. Depending on the storm characteristics and snowfall amounts, the problem can vary from a thin, relatively harmless soft slab to a much thicker, harder, and more dangerous slab. Storm Slab problems typically last between a few hours and a few days. They are commonly distributed widely across terrain that received similar snowfall amounts. To assess potential size, monitor storm totals, and expect deeper accumulations at higher elevations and in leeward terrain. During a storm, larger, deeper, and sometimes more reactive storm slabs will form where wind-drifted snow piles up on the leeward side of terrain features. Storm slabs in these areas will share characteristics with wind slabs, with the main difference being that these storm slabs formed during a storm with new snow and wind. During these storms, unstable slabs exist across more terrain in wind-affected and sheltered areas. Wind slabs form when the unstable slabs are limited to only wind-loaded areas.

Look for feedback, such as shooting cracks, on small, steep test slopes.  Shallow Storm Slabs are most dangerous on larger terrain features or slopes with terrain traps, such as trees, gullies, and cliffs.  In some cases, Storm Slabs can be remotely triggered. You can reduce your risk from Storm Slabs by waiting a day or two after a storm before venturing into steep terrain. 

Storm slabs that form in wind-loaded areas may transition to Wind Slabs as the storm snow in sheltered areas stabilizes. Similarly, Storm Slabs that form over a persistent weak layer (e.g., surface hoar, depth hoar, or near-surface facets) may also develop into Persistent Slabs if the instabilities last longer, and their behavior will become more challenging to manage. In some cases, Storm Slabs may be termed Persistent Slabs in the first place.

Wind Slabs form when wind transports snow from the upwind sides of terrain features and deposits it into thicker drifts on the downwind side. This forms slabs in somewhat predictable and specific locations, such as below the leeward side of ridges or in cross-loaded gullies. Wind Slabs can range from soft to hard, thin to thick, and are often smooth, rounded, and sometimes sound hollow. Blowing snow and cracking or collapsing in drifted snow represent clear warning signs of the problem. 

Wind Slabs can be avoided by sticking to sheltered or wind-scoured areas.

Storm slabs that form during storms with wind and snow may form in similar locations as wind slabs, but the storm slabs also form in sheltered areas, making them more widespread during a storm. Wind Slabs that form over a persistent weak layer (e.g. surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs, in which case, their instabilities will last longer.

Persistent Slabs form when persistent weak layers break under the load above them. Persistent weak layers include surface hoar, depth hoar, near-surface facets, and faceted crusts. These layers can vary greatly over short distances and continue to produce avalanches weeks or even months after burial.  Persistent Slabs are characterized by difficult to manage and often surprising behavior.  They can be triggered long after a storm has passed or release under modest loading events. They may be triggered remotely—from flatter terrain below, above, and to the sides of steep slopes. The slabs often propagate widely and in unpredictable ways. Shooting cracks and collapses are clear warning signs of the problem. However, feedback is often irregular because of spatial differences in weak layer and slab thickness. Thus, tracks on a slope are not an indicator of stability. 

Persistent Slabs require a wide margin for error. The most reliable way to manage Persistent Slabs is to make conservative terrain choices. This often means avoiding travel on or below steep terrain where the problem exists, which may be aspect or elevation dependant. Persistent Slabs can develop into a Deep Persistent Slab as additional snow and wind events build a thicker slab.