Resolution_2025 Kittitas County Hazard Mitigation Plan

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2025 Hazard Mitigation Plan Kittitas County, Washington Chapter 4: Hazard Identification and Risk Assessment 150 Table 4-93. Types of Thunderstorms Type Description Single-Cell Thunderstorm Thunderstorms created by just one (1) convection cell in the atmosphere. Most of these are small, lasting only about an hour, and are also called ordinary thunderstorms. These storms often form during summer and include towering cumulonimbus clouds that can grow 7.5 miles high in the atmosphere. Rain and lightning are common. Sometimes hail falls. Multi-Cell Thunderstorms Thunderstorms that are made from many convection cells that move as a single unit. Often the convection cells are arranged as a cluster, with each cell at a different stage of the thunderstorm cycle. Multi-cell storms along a cold or warm front, where warm air is pushed high into the atmosphere above cold air, often form a line, called a squall line. The squall line can be up to 600 miles long. Strong wind gusts can often be found just ahead of the storm. Supercell Thunderstorms Thunderstorms with deep, rotating updraft winds (i.e., supercells) are very large and last for hours releasing significant amounts of rain and sometimes even baseball -sized hail. They include fast moving convection – air zooming upward at as much as 175 mph. Rotation in supercells sometimes forms violent tornadoes, the largest and most damaging type, because the storms are so long-lived. Several tornadoes can be produced from one (1) supercell thunderstorm. Supercell clouds grow up to 11 miles high in the atmosphere. Although the least common types of thunderstorms, supercell thunderstorms are the most destructive. Heavy Rainfall There is no single definition for heavy (or extreme) rainfall. However, meteorologists consider instances where the amount of precipitation (rain or snow) experienced in a location substantially exceeds normal amounts. The amount of precipitation needed to qualify as heavy rainfall varies with location and season. Rainfall events share characteristics such as high moisture and an atmospheric disturbance (e.g., winter storm, atmospheric river) and when these conditions persist over an area, the more rainfall an area will receive.182F 182 Heavy rainfall is most frequently measured by tracking the frequency of events, analyzing the mean return period, and measuring the amount of precipitation in a certain period (most typically inches of rain within a 24-hour period)183F 183. Refer to the flood section of this Plan for further details on flooding as a result of heavy rainfall. Hail Hail is a form of precipitation that consists of solid ice that forms inside thunderstorm updrafts. During a thunderstorm, hail forms when raindrops are carried upward by the thunderstorm’s updrafts into extremely cold air and freeze. As the hailstones continue to be carried upward, they continue to collide with liquid water drops that freeze into the surface of the hailstone. Once the thunderstorm’s updraft can no longer hold the weight of the hailstone, which occurs when the updraft weakens or the hailstone is too heavy, then the hail falls. When a hailstone encounters different liquid water content conditions and temperatures within the thunderstorms, it causes hailstone to have layers of clear and cloudy ice. The speed at which hail falls primarily depends on four (4) characteristics – hail size, friction between the hailstone and surrounding air, local wind conditions (horizontal and vertical), and degree of melting of the 182 National Oceanic and Atmospheric Administration. (2018). Ask the Scientist: Extreme Rainfall, Why it Happens and How We Predict It. Retrieved from https://www.noaa.gov/stories/ask-scientist-extreme-rainfall-why-it- happens-and-how-we-predict-it. 183 United States Environmental Protection Agency. (2023). Climate Change Indicators: Heavy Precipitation. Retrieved from https://www.epa.gov/climate-indicators/climate-change-indicators-heavy-precipitation.