The winter snowpack is formed as successive storms deposit unique layers of snow, or it may change internally as other types of layers form during fair or cold weather between storms. A slab avalanche occurs when an upper layer of snow breaks away as a unit. A slab avalanche usually requires a cohesive layer of snow, a weak layer, and a bed surface, though the weak layer may just be a poor attachment of the slab to an underlying crust. Strong winds and heavy snowfall during warming weather are most likely to build a slab layer. Some common weak layers in the Northwest include low-density stellar crystals deposited during light winds or light snowfall intensities, graupel snow deposited during frontal passages, and surface hoar which forms on the snow surface during clear cold periods between frontal systems.
Some weak layers can persist within the snowpack for long periods.
Bed or sliding surfaces for overlying slabs may be provided by older crusts. Close attention to snowpack characteristics, changes in the terrain and weather, and field tests are key in assessing the local danger. Other factors being equal, cool fair weather following warm periods or rain is likely to refreeze snow and build crusts that can minimize the potential for avalanches. Back-country travelers should make terrain, weather and snowpack stability evaluations as they travel over snow covered terrain. Use forecasts as a starting point only and constantly reassess and update your mental picture of the snow structure and the weather and terrain that may be changing it.