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2025 Hazard Mitigation Plan <br />Kittitas County, Washington <br /> <br /> <br />Chapter 4: Hazard Identification and Risk Assessment 69 <br />4.6. HAZARD PROFILES <br />4.6.1. Avalanche <br />4.6.1.1. Description <br />Avalanches are a rapid flow of snow down a hill or mountainside. Avalanches can occur whenever a <br />sufficient depth of snow is deposited on slopes steeper than about 20 degrees, with the most dangerous <br />coming from slopes between 35 and 40 degrees. Areas prone to avalanches can be identified with some <br />accuracy because they typically follow the same paths year after year (i.e., scarring paths). However, <br />unusual weather conditions can produce new paths or cause avalanches to extend beyond their normal <br />paths.52F <br />53 <br /> <br />An avalanche begins when an unstable mass of snow breaks away from a slope. Subsequently, snow <br />picks up speed as it moves downhill, producing a river of snow and a cloud of icy particles. As the snow <br />mass moves down the slope it picks up more snow. A large, fully developed avalanche can weigh as <br />much as a million tons and travel faster than 200 miles per hour (mph).53F <br />54 <br /> <br />A snowpack is layers of snow, each one from a different snowfall, which becomes compacted under the <br />weight of subsequent layers that lie on the top. It remains on the ground until warmer temperatures cause <br />it to melt (i.e., snowmelt) in the spring. Snowmelt is an important source of water for many areas, such <br />as Kittitas County.54F <br />55 Bonds between the layers in a snowpack can be weak when snow refreezes in <br />between snowfalls. When snow refreezes, it causes a slick coating of ice to form on the surface and <br />when new snow falls it may not stick and slide off. As temperatures warm up during the spring melted <br />snow can seep through a snowpack making the lower layer slippery. Subsequently, adding weight or <br />vibration (e.g., skier) to these conditions can create an avalanche.55F <br />56 <br /> <br />Factors that influence the occurrence of an avalanche are weather, temperature, wind, slope steepness, <br />terrain, vegetation, and general snowpack conditions. Avalanches are most likely to occur when fresh <br />snowfall adds to a new layer to a snowpack and if the new snow piles during the storm, the snowpack <br />may become overloaded setting off a slide. Earthquakes can set off avalanches as well.56F <br />57 <br /> <br />In the high alpine regions of the Cascade Mountains, avalanches are a year-round event. However, the <br />typical avalanche season usually begins in November and continues into early summer.57F <br />58 <br /> <br />Avalanches are categorized based on physical, objective, and observable characteristics (e.g., hard or <br />soft, wet or dry). Sometimes an avalanche can be classified as two (2) different types. For example, a <br /> <br />53 Peninsula Emergency Management Coalition. (n.d.). Avalanches. Retrieved from https://www.pep- <br />c.org/uploads/1/2/4/0/124049041/avalanches.pdf. <br />54 National Geographic. (n.d.). Avalanche. Retrieved from <br />https://education.nationalgeographic.org/resource/avalanche/. <br />55 National Geographic. (n.d.). Snowpack. Retrieved from <br />https://education.nationalgeographic.org/resource/snowpack/. <br />56 National Geographic. (n.d.). Avalanche. Retrieved from <br />https://education.nationalgeographic.org/resource/avalanche/. <br />57 Ibid. <br />58 Washington State Emergency Management Division. (2023). Washington State Enhanced Hazard Mitigation <br />Plan. Retrieved from https://mil.wa.gov/asset/651ec296d76a9/2023_WA_SEHMP_final_20231004.pdf .