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CHAPTER 7. AVALANCHE <br />7-4 <br /> <br />Figure 7-1. Areas Vulnerable to Avalanche <br />7.2.3 Frequency <br />Avalanches occur regularly every year in mountain areas. Many weather and terrain factors determine <br />avalanche danger. At lower elevations of the Cascades, the avalanche season begins in November and <br />continues until the last remnants of snow have melted in early summer. In the high alpine regions, the <br />hazard continues year-round. Hundreds of thousands of avalanches are thought to occur each year in the <br />Cascades. While the probability of avalanches occurring each year is very high (100%), there is a very low <br />impact on the planning area due to the remote locations of events and mitigation efforts along developed <br />areas. Avalanches along two key mountain highway passes are limited due to ongoing mitigation to control <br />slides during winter months. <br />7.2.4 Severity <br />A number of weather and terrain factors determine avalanche severity and danger: <br />• Weather: <br />– Storms—A large percentage of all snow avalanches occur during and shortly after storms. <br />– Rate of snowfall—Snow falling at a rate of 1 inch or more per hour rapidly increases <br />avalanche danger. <br />– Temperature—Storms starting with low temperatures and dry snow, followed by rising <br />temperatures and wetter snow, are more likely to cause avalanches than storms that start <br />warm and then cool with snowfall.