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2025 Hazard Mitigation Plan Kittitas County, Washington 4.6. HAZARD PROFILES 4,6.1. Avalanche 4.6.1.1. Description Avalanches are a rapid flow of snow down a hill or mountainside. Avalanches can occur whenever a sufficient depth of snow is deposited on slopes steeper than about 20 degrees, with the most dangerous coming from slopes between 35 and 40 degrees. Areas prone to avalanches can be identified with some accuracy because they typically follow the same paths year after year (i.e., scarring paths). However, unusualweather conditions can produce new paths or cause avalanches to extend beyond their normal paths.53 An avalanche begins when an unstable mass of snow breaks away from a slope. Subsequently, snow picks up speed as it moves downhill, producing a river of snow and a cloud of icy particles. As the snow mass moves down the slope it picks up more snow. A large, fully developed avalanche can weigh as much as a million tons and travel faster than 200 miles per hour (mph)."54 A snowpack is layers of snow, each one from a different snowfall, which becomes compacted under the weight of subsequent layers that lie on the top. lt remains on the ground until warmer temperatures cause it to melt (i.e., snowmelt) in the spring. Snowmelt is an important source of water for many areas, such as Kittitas County..ss Bonds between the layers in a snowpack can be weak when snow refreezes in between snowfalls. When snow refreezes, it causes a slick coating of ice to form on the surface and when new snow falls it may not stick and slide off. As temperatures warm up during the spring melted snow can seep through a snowpack making the lower layer slippery. Subsequently, adding weight or vibration (e.g., skier) to these conditions can create an avalanche.-56 Factors that influence the occurrence of an avalanche are weather, temperature, wind, slope steepness, terrain, vegetation, and general snowpack conditions. Avalanches are most likely to occur when fresh snoMall adds to a new layer to a snowpack and if the new snow piles during the storm, the snowpack may become overloaded setting off a slide. Earthquakes can set off avalanches as well.57 ln the high alpine regions of the Cascade Mountains, avalanches are a year-round event. However, the typical avalanche season usually begins in November and continues into early summer.58 Avalanches are categorized based on physical, objective, and observable characteristics (e.9., hard or soft, wet or dry). Sometimes an avalanche can be classified as two (2) different types. For example, a 53 Peninsula Emergency Management Coalition. (n.d.). Avalanches. Retrieved from https://www.pep- c.orql uploadsl 1 l2l 4 l0 I 1 24049041 I av alanches.odf . 5a National Geographic. (n.d.). Avalanche. Retrieved from https://education.nationalqeographic.orq/resource/avalanche/. 55 National Geographic. (n.d.). Snowpack. Retrieved from https://education. nationalqeoqraphic.orq/resource/snowpacl</, 56 National Geographic. (n.d.). Avalanche. Retrieved from https://education. nationalqeoqraphic.orq/resource/avalanche/. 57 lbid. 58 Washington State Emergency Management Division. (2023). Washington State Enhanced Hazard Mitigation Plan. Retrieved from https://mil.wa.qov/asseV651ec296d76a9l2023 WA SEHMP tinal 20231004.pdf. Chapter 4: Hazard ldentification and Risk Assessment 69