08-12-2019 Council Packet - Laserfiche WebLink

Home Search

Anoka County 2019 Multi-Jurisdictional All Hazards Mitigation Plan 80 Thunderstorms may occur singly, in clusters, or in lines. Some of the most severe weather occurs when a single thunderstorm affects one location for an extended time. Straight-line winds can exceed 100 miles per hour and are responsible for most thunderstorm damage. One type of straight-line wind, the downburst, can cause damage equivalent to a tornado. Thunderstorms are associated with tornadoes and heavy rains that lead to floods. All thunderstorms contain lightning, which is an electrical discharge that results from the buildup of positive and negative charges. When the buildup becomes strong enough, lightning appears as a "bolt." This flash of light usually occurs within the clouds or between the clouds and the ground. A bolt of lightning reaches a temperature approaching 50,000° F in a split second. In the United States, 27 citizens are killed each year by lightning on average. Lightning's electrical charge and intense heat electrocutes on contact, splits trees and ignites fires. Hail is produced by many strong thunderstorms and is a product of the updrafts and downdrafts that develop inside the clouds of a thunderstorm where super cooled water droplets exist. The transformation of droplets to ice requires a temperature below 32° F, and a catalyst in the form of tiny particles of solid matter, or freezing nuclei. Hail can be smaller than a pea or as large as softballs and can be destructive to property, crops, livestock, and people. 87B4.2.1.9 Severe Weather - Tornados Tornados are violent windstorms characterized by a twisting, funnel-shaped cloud. A tornado is spawned by a thunderstorm or hurricane and produced when cool air overrides a layer of warm air, forcing the warm air to rise rapidly. A funnel does not need to reach to the ground for a tornado to be present. A debris cloud beneath a thunderstorm is all that is needed to confirm the presence of a tornado. The damage from a tornado is a result of the high wind velocity and wind-blown debris. Tornados can occur at any time of the year; however, the season is generally March through August. Over 80% of all tornadoes strike between noon and midnight. The most violent tornadoes are capable of tremendous destruction with wind speeds of 250 mph or more. Damage paths can be in excess of 1 mile wide and 50 miles long. Even with advances in meteorology, adequate warning time for tornadoes is short or sometimes not possible. The intensity, path length, and width of tornadoes are rated according to a scale developed by T. Theodore Fujita and Allen D. Pearson. The Fujita-Pearson Tornado Scale is presented below. Tornadoes classified as EF0-EF1 are considered weak, those classified as EF2-EF3 are considered strong, while those classified as EF4-EF5 are considered violent.