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Severe Weather - Summer Storms

Storm

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2011 Severe Weather Information Now Available

Lightning

Lightning is defined as any and all forms of visible electrical discharge caused by thunderstorms.

Source: Minnesota Hazard Mitigation Plan

While windstorms and tornadoes are also a significant hazard associated with severe thunderstorms, lightning is probably the most frequent hazard associated with thunderstorms and the hazard that causes the most loss of life. Lightning occurs to balance the difference between positive and negative discharges within a cloud, between two clouds and between the cloud and the ground. For example, a negative charge at the base of the cloud is attracted to a positive charge on the ground. When the difference between the two charges becomes great enough a lightning bolt strikes. The charge is usually strongest on tall buildings, trees and other objects protruding from the surface and consequently such objects are more likely to be struck than lower objects.

While cloud-to-ground lightning poses the greatest threat to people and objects on the ground it actually accounts for only 20 percent of all lightning strikes. The remaining lightning occurs within the cloud, from cloud to cloud or from the ground to the cloud with in-cloud lightning being the most common.

According to the National Weather Service (NWDS), nationwide, lightning is the number one killer weather phenomena. During a typical year, lightning kills more people than hurricanes, tornadoes, and winter storms combined. NWS-Chanhassen Office estimates that annually, lightning causes two deaths and three injuries in the state.

Wind Storm

A wind storm is any storm that produces winds in excess of 58 miles per hour, excluding tornadoes.

Windstorms can and do occur in all months of the year; however, the most severe windstorms usually occur during severe thunderstorms in the warm months. These include tornadoes and downburst or straight line winds. Winds of greater than 60 mph are also associated with intense winter, spring and fall low pressure systems. These can also inflict damage to buildings and in some cases overturn high profile vehicles.

A downburst is a severe localized downdraft from a thunderstorm or a rain shower. This outflow of cool or colder air can create damaging winds at or near the surface. Winds up to 130 mph have been reported in the strongest thunderstorms. Downburst winds can cause as much damage as a small tornado and are frequently confused with tornadoes because of the extensive damage they cause. As these downburst winds spread out they are often referred to as straight-line winds. They can cause major structural and tree damage over a relatively large area.

Hailstorm

Hail is precipitation in the form of balls or clumps of ice, produced by thunderstorms. Most incidents for which records exist involve hail 3/4" diameter or more. It is formed when strong updrafts within the cumulonimbus cloud carry water droplets above the freezing level or when ice pellets in the cloud collide with water droplets. The water droplets freeze or attach themselves to the ice pellets and begin to freeze as strong updraft winds toss the pellets and droplets back up into colder regions of the cloud. Both gravity and downdrafts in the cloud pull the pellets down, where they encounter more droplets that attach and freeze as the pellets are tossed once again to higher levels in the cloud. This process continues until the hailstones become too heavy to be supported by the updrafts and fall to the ground as hail.

Most hail in Minnesota ranges in size from pea-size to golf-ball size. Larger hailstones have been reported but occur much less frequently. Strong updrafts are necessary within the cloud to form hail. Strong updrafts are usually associated with severe thunderstorms. Area coverage of individual hailstorms is highly variable and spotty because of the changing nature of the cumulonimbus cloud. While, almost all areas of southern Minnesota can expect some hail during the summer months most hail is not large enough to cause significant crop or property damage.

History

Perhaps the most famous tornado that has ever hit central Minnesota was the tornado of 1886. It struck at around 4:00 p.m. on April 14 of that year. The total losses from this terrible storm included 74 dead and 136 injured. Another major tornado struck Benton County in 1909. Between January, 1950 and September, 2001 there was one recorded tornado in Benton County with at least one injury or death. It occurred on August 15, 1954. One person was injured, and approximately $25,000 worth of damage occurred. There was also a major summer storm on July 12, 1978.

Risks

Tornadoes are often viewed as the most damaging summer storm. However, the severe thunderstorm that produces the tornado frequently contains other severe weather elements such as torrential rains, hail, lightning and straight line winds. Unlike floods, none of these elements is confined to any particular local geographic area within the county. No community is without risk; any place in the county is considered to have an equal chance of experiencing a tornado or any other of these severe weather elements.

Damage due to tornadoes can range from minor to major depending on the strength of the tornado and where it strikes. A tornado that occurs in a rural area could cause crop damage and might damage some farm buildings and injure livestock but the damage would typically be less than in built up areas. Several such tornadoes have occurred in Benton County during the past 30 years. No tornado during this period of time has affected any of the cities or urbanized portion of the county. The path of Minnesota tornadoes is typically quite narrow, most less than a quarter of a mile and not very long. Consequently, the total area affected by a tornado is not large. However, should a tornado of moderate strength strike a city, damage could be extensive and risk to human life and property high.

Other violent summer storms are also not confined to any particular geographic area in the county and may occur and inflict damage anywhere they occur. The greatest risk for most people and property is usually confined to urban areas due to the higher density of people and buildings there. However, some storm events, such as hail and straight line winds can cause significant crop damage and damage to farm buildings. Since agricultural use makes up a greater share of the county’s land area than urban areas agricultural areas are somewhat more likely to experience hail storms than urban areas.

Every year in the United States, summer storms kill people. Many are killed by flying debris from homes and other structures. Larger impacts on people would be in the largest municipalities because of higher population densities.

Electric and other public infrastructure could be directly impacted throughout the entire county by severe storms. Specifically, power lines could be knocked down, resulting in loss of electricity for entire areas of the county. Electricity is very important to the community. It operates businesses, homes and other industrial buildings throughout the county. Other major infrastructure facilities such as the waste treatment plant, water plant, roads and bridges could also be damaged by tornadoes. Tornadoes and windstorms can often scatter knocked down trees and other debris over main roads, limiting travel of emergency vehicles.

The county and its cities can mitigate some of the impacts of these deadly storms through strict building code enforcement, proper land use regulations, emergency shelters and improved early warning systems.