Description
Thunderstorms are caused when air masses of varying temperatures and moisture content meet. Rapidly rising warm moist air serves as the driving force for thunderstorms. These storms can occur singularly, in lines, or in clusters. They can move through an area very quickly or linger for several hours.
Some storms produce a particular type of high wind called a derecho. Derechos are widespread, long-lived, straight-line wind storms associated with severe thunderstorms. They can cause hurricane-force winds, tornadoes, heavy rains, and flooding. Derechos travel quickly, with sustained winds that often exceed hurricane-force. They typically occur in the summer months, though they can occur any time of year and at any time of the day or night. Although thunderstorms generally affect a small area when they occur, they can be very dangerous as, by definition, they contain lightning, and can also produce heavy rain, flash flooding, strong straight-line winds, large hail, and tornadoes.
Location and Extent
The National Weather Service (NWS) estimates that more than 100,000 thunderstorms occur each year across the United States, though only about 10 percent of these storms are classified as "severe". According to NWS, a typical thunderstorm is 15 miles in diameter and lasts an average of 30 minutes. To be classified as a "severe thunderstorms", the storm must be capable of producing either a tornado, straight-line winds gust greater than 58 mph (50 knots), or hail greater than one inch in diameter.
History
Thunderstorm Wind
NCEI Storm Events Database records 883 severe thunderstorm wind events between 1958 and 2019 for the Central Virginia PDC area. There are 507 recorded events that occurred in the past decade (2010 - 2019), most of them reaching 50 - 70 kts in magnitude (i.e. storm and violent storm conditions) and causing damages or injuries. Severe thunderstorm wind occurred every year.
Heavy Rain
There are 62 heavy rain events recorded in the NCEI Storm Events Database between 1996 and 2019 for the CVPDC area, including 26 events that occurred in the past decade (2010 - 2019). The events brought damages but no injuries or deaths.
Lightning
All thunderstorms contain lightning. A lightning strike is an electrical current between the cloud and ground. Each spark of lightning can reach over five miles in length, hit temperatures of approximately 50,000 degrees Fahrenheit, and contain 100 million electrical volts. There are about 25 million cloud-to-ground lightning flashes in the United States per year. According to the VAISALA Global Lightning Dataset, Virginia gets an average of 280,000 cloud-to-ground lightning flashes annually, and 6.9 flashes per square miles. According to the National Weather Service (https://www.weather.gov/phi/ThunderstormDefinition), an average of 300 people are injured and 80 people are killed each year by lightning in the United States.
According to the NCEI Storm Event database, there are 49 documented damaging lightning events for the CVPDC area between 1996 and 2019 which caused property damage, deaths, or injuries. In the past decade (2010 - 2019), 26 events have been reported. The PDC area experiences damaging lightning events every year. In the most recent events, lightning strikes from thunderstorms caused barns, outbuildings, homes, and a church to catch fire, brought down trees on homes and vehicles, and took down power lines across the CVPDC area.
Plan Chapter
CVPDC HMP 2020
