Snow Avalanche Climatology of the Western United States Mountain Ranges

Cary J. Mock and Karl W. Birkeland

The Western United States contains three climate zones that can produce avalanches; coastal, intermountain, and continental. Each zone has distinct avalanche characteristics. The coastal zone has abundant snowfall, higher snow densities, and higher temperatures and contains the pacific mountain ranges. The continental zone has by lower temperatures, lower snowfall, lower snow densities, higher snow temperature gradients, and a more persistently unstable snowpack resulting from depth hoar. The continental zone contains the Rockies. The intercontinental zone contains mountains in Utah, Montana, and Idaho and has characteristics of both the other zones. The zones generally follow an east to west gradient starting with continental in the Rocky Mountains and moving to the intermountain and then Pacific zone on the west coast. Defining these zones and describing the climate and environmental characteristics that are associated with each zone is imperative for improving forecasts of avalanche danger and preventing avalanches.

The two primary types of avalanches are loose snow slides or slab avalanches. Slab avalanches pose the greatest threat to life due to their greater size, greater distance they can travel and increased degree of unpredictability. Slab avalanches form when a cohesive slab overlays a less cohesive, weak layer. The slab breaks off from the layer beneath when stress is introduced. This stress can come from new or wind-blown snow, falling cornices, explosives, or the weight of a person on a slope.

The question this article was attempting to answer was could zones that can produce environments that avalanche be defined by characteristics unique to each zone. By answering this question it could enable better long-term predictions of avalanche risk and assist with short-term risk management and forecasting.

The researchers used interval/ratio climate data from public and private records of the Westwide Avalanche Network WWAN to calculate means of temperature, precipitation, snowfall, snow depth, and snow density for each of the three climate zones. Data from 45 stations and a few ski resorts from 1969 to 1995 created data sets that the researchers quantitatively analyzed for correlations. The unit of analysis was the same as the unit of observation because the locations the data was collected from was also what parameter for how the correlations were generated, from trends at those locations.

The researchers used a different method for data analysis as well. They used box plots to generate ranges for the snow avalanche climate classification, by comparing the variability of temperature, snowfall, snow water equivalent, snow depth, December temperature gradient, and rainfall for each of the three major avalanche climate regions. For example, this method of analysis visually displayed that coastal zones are characterized by warmer temperatures, ranging from approximately -3 to 0 degrees C on average.

This research provides more of the defining characteristics of the environments I will be investigating for my project. The method was interesting because it used patterns of temperature, precipitation, snowfall, snow depth, and snow density, snow water equivalent (SWE) and others to identify spatially where they are correlated to instead of looking at locations and finding patterns in those locations. It allowed for a greater scope and to not make conclusions about areas that you believe are important before the data has been analyzed.

Another valuable piece of information I learned from this article was that WWAN sites collect more information needed for avalanche hazard research such as snow water equivalent (SWE). However WWAN stations rarely observe backcountry avalanches, so Department of Transportation Data can be used as well.

One question this article prompted for my project was are climates shifting from continental to intermountain? There was little variability of zones demonstrating characteristics of other zones occurred during the data analysis but some areas in the southwestern (Colorado) portion of the continental zone expressed certain intermountain characteristics periodically. Could this variation be amplified by climate change and would that increase or decrease the risk of avalanches in that area?

Mock, C. J., & Birkeland, K. W. (2000). Snow Avalanche Climatology of the Western United States Mountain Ranges. Bulletin of the American Meteorological Society, 81(10), 2367-2392. doi:10.1175/1520-0477(2000)081<2367:sacotw>2.3.co;2