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Home > Severe Weather Awareness: Focus on Severe Thunderstorms
Severe Weather Awareness: Focus on Severe Thunderstorms
May 2, 2012
Tragically, last years tornadoes in the southern and central United States have highlighted the deadly threat caused by nature’s most violent storms. The National Weather Service encourages the public to become more aware of those threats so they can act appropriately when severe storms threaten.
Severe Thunderstorms: Downbursts, Microbursts, Meso-cyclones and Hail
By definition, a severe thunderstorm is one which produces wind gusts of 58 mph or more, or hail 1 inch in diameter or greater. Severe thunderstorms can also produce tornadoes.
In New England, severe thunderstorms are not uncommon during the summer. Every year, the National Weather Service gets numerous reports of wind and hail damage throughout Maine. Severe thunderstorm winds down trees and branches onto homes, buildings, vehicles, and power lines. Scattered power outages are often the result of lightning or wind-fallen trees and branches.
Also, wind-driven hail from thunderstorms flattens and/or damages crops in the states. On rare occasions, large hailstones damage homes, buildings, and vehicles. In addition to the lightning, falling trees and large hail also pose a significant threat to people, as well.
During the development of a thunderstorm, warm air rises upward in the atmosphere (an updraft) causing the formation of clouds and precipitation. As a thunderstorm matures, cool, precipitation-laden air sinks downward through the atmosphere (a downdraft). When a downdraft reaches the ground it spreads out causing the cool, gusty wind that often accompanies a thunderstorm.
In some thunderstorms, intense downdrafts develop. When these downdrafts reach the ground, they spread out very quickly, causing strong and often damaging winds at the ground. These intense downdrafts are called downbursts and can cause significant wind damage over large areas.
In the case of downbursts, the damage is generally referred to as straight-line wind damage since fallen trees generally line up in the same direction. In Maine, most thunderstorm wind damage is caused by downbursts.
A special type of downburst is the microburst. Microbursts get their name because they generally affect a much smaller geographical area, but the winds in a microburst can be very intense. Like the general downburst, most of the damage with microbursts lines up in one direction, although, there may be a tendency for the damage to radiate outward. Microbursts are usually accompanied by heavy rain and/or hail and can have winds as strong as those in a small tornado.
Under certain atmospheric conditions, thunderstorms can begin to develop a circulation within the thunderstorm cloud. These storms are often called meso-cyclones because of the counter-clockwise circulation that develops within the storm. The updrafts and downdrafts in these storms can persist for hours as the storm moves along its path.
Severe winds and hail are also more likely with meso-cyclones. If the rotation within the storm becomes more intense, there is an increasing possibility that the storm might produce a tornado. National Weather Service Doppler radar allows meteorologists to monitor air movement within these storms and to see the development and strength of any circulation within the storm.
Recent impact in Maine
During the summer of 2011, Maine had numerous thunderstorms that produced damaging straight-line winds. Unfortunately, one person was killed by a falling tree caused by these winds.
Falling trees caused by thunderstorm winds were also responsible for two deaths in the area in 2006. The first death occurred in Fryeburg, Maine on June 19 when a tree fell on a tent in which people were camping. The second occurred in Waterboro, Maine on September 9 when the top of a tree fell on a vehicle, killing the driver.
The circulation that accompanies a meso-cyclone is also a factor in hail formation. Hail initially forms when liquid water droplets are carried upward by the updraft to a level where the droplets freeze. Eventually, the small hail stone may begin to fall downward, only to be caught once again by the persistent updraft of a meso-cyclone.
Each time the hailstone goes through this process, it gets larger and heavier. Eventually, the hailstone will be blown away from the updraft or will become too heavy for the updraft to support and the hailstone will fall to the ground.
In Maine, hail is fairly common during well-developed summertime thunderstorms. Although most hail that reaches the ground in northern New England is an inch or less in diameter, occasionally hailstones over 2 inches in diameter will fall. Large hailstones can fall at speeds faster than 100 mph and can do considerable damage to cars, homes, and buildings. They can be a significant threat to people, as well.
During 2011 in Maine, the most significant hail report was 2.75 inch hail in Kingfield in Franklin County. During 2010, the most significant report was 2-inch hail in South Paris in Oxford County.
Here are some of the larger hailstones reported in northern New England since 1995.
Pay attention and be safe
For both severe winds (58 mph or greater) and large hail (1 inch or greater in diameter), the National Weather Service issues Severe Thunderstorm Watches and Warnings. A Watch indicates that the atmospheric conditions are favorable for severe weather to develop. A Warning indicates that severe weather is imminent or is already occurring. If you hear a Warning for your area, be prepared to seek a safe shelter if you are in the path of the storm.
Severe Weather Awareness Week ...
The National Weather Service has declared the week of April 30th through May 4th Severe Weather Awareness Week in New England. Today's message is presented in partnership with the National Weather Service Forecast Offices in Maine:
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