R2025-143 - Laserfiche WebLink

Home Browse Search

2025 Hazard Mitigation Plan Kittitas County, Washington ffilffil 4.6.7. Severe Weather 4.6.7.1. Description For the purposes of this Plan, severe weather includes thunderstorms (i.e., heavy rainfall), hail, strong winds, tornadoes, and extreme temperatures (i.e., extreme heat and cold). Thunderstorms A thunderstorm is a rain shower with thunder and lightning. However, a thunderstorm is classified as severe when it contains one (1)or more of the following - hail of one (1) inch or greater, winds gusting in excess of 58 mph, andior a tornado. For a thunderstorm to form, it needs three (3) basic ingredients - moisture, unstable rising air, and a lifting mechanism.-17e Thunderstorms form in three (3) stages - the developing stage, the mature stage, and the dissipating stage. The life cycle of a thunderstorm is outlined in Table 4-92...180 Table 4-92. Stages of a Thunderstorm 1 Developing Cumulus clouds form when warm moist air rises into cold air, and as it rises the moisture in the air condenses into water droplets. As long as warm air from below continues to rise, the clouds will continue to grow. An updraft of warm moist air can form when air is forced up the side of a mountain or when air is forced upward at weather fronts, where warm and cool air masses collide. However, updrafts often form simply because warm air rises, without a mountain or front to guide them. Air near the ground heats up during the day as energy from the Sun heats the ground, which subsequently heats the air. The warm air rises hiqher in the atmosphere because warm air has less mass than cool air. 2 Mature As the cumulus cloud continues to grow, the tiny water droplets within the cloud grow larger as more water from the rising air is added to the droplets. The cloud stafts to look dark and gray as more water is added, and the growing droplets that make up the cloud become heavy. Once the rising air cannot hold the raindrops, they begin to fall through the clouds. Meanwhile, cool dry air flows downward in the cloud (i.e., downdraft), resulting in rain. With an updraft, downdraft, and rain, the cloud is now called a cumulonimbus cloud, and the cycling of air up and down is called a thunderstorm cell. The moving air within the cloud builds up electric charges as it slides past other air allowing lightning to form, and as a result thunder when lightning strikes. Thunder often happens after you see the bolt of liqhtninq because sound travels slower than light. Once the downdrafts in the cloud become stronger than the updrafts, the storm begins to weaken. Since warm moist air can no longer rise, cloud droplets can no longer form. The storm dissipates with light rain as the cloud disappears from the bottom to the top. For an ordinary storm, the whole process takes about one (1) hour. However, for severe thunderstorms, which are much larger (e.9., supercells and squall lines) and powerful, tend to last several hours. 3 Dissipating DescriptionStage Thunderstorms can consist of one (1) or multiple convective cells, or one (1) extremely large and convective cell. There are three (3) types of thunderstorms which are described in Table 4-93. powerful 181 17e NOAA, National Severe Storms Laboratory. (n.d.). Severe Weather 101:Thunderstorm Basics. Retrieved from https://www.nssl.noaa.gov/education/svrwxl 01 /thunderstorms/. 180 University Corporation for Atmospheric Research. (n.d.). How Thunderstorms Form. Retrieved from https ://scied.ucar.edu/learn inq rms/how-th u nd erstorms-form 181 University Corporation for Atmospheric Research. (n.d.). Thunderstorms. Retrieved from https://scied.ucar.edu/learn inq-zone/storms/thunderstorms. Chapter 4: Hazard ldentification and Risk Assessment 149