What Makes a Thunderstorm "Severe?"

The Nation's Capital often features diverse weather

There is an uptick in severe weather in March compared to the winter months.  Due to its unique geography, the United States averages more thunderstorms and tornadoes in a given year than any other country in the world according to NOAA. As the first official day of spring approaches next Friday, it’s important to review some thunderstorm basics in order to be better prepared.

           

A thunderstorm must meet a strict set of criteria in order to be classified as “severe.” To be “severe,” a thunderstorm must have any one of the following: 1) wind gusts of 58 mph or greater, 2) hail 1” in diameter or greater, or 3) spawn a tornado.  In 2010, the severe hail criterion was adjusted upward from 0.75” to hail 1” in diameter. This was done to reduce the number of severe thunderstorm warnings that are issued.  The expectation behind the change was that if fewer severe thunderstorm warnings are issued, then people can be expected to take them more seriously.

The difference between a weather watch and a warning is straightforward. A weather “watch” means a particular type of weather (severe thunderstorm, tornado, etc.) is possible for a particular region over a particular time. A weather “warning” means the aforementioned weather is imminent and to take protective measures. Larger, longer lasting thunderstorms known as “supercells” commonly feature both large hail and tornadoes. When a thunderstorm has a tornado, only a tornado warning is issued. This is done to avoid public confusion despite the fact that a tornado would also make a thunderstorm “severe.”

It is important to note that neither heavy rain nor lightning make a thunderstorm severe. By definition, a thunderstorm features heavy rain along with thunder and lightning. Another common misnomer is to refer to a thunderstorm as “heavy,” since there is no practical way to quantify that distinction. It is correct to say a thunderstorm brought heavy rain since rainfall can be measured. People can also say a thunderstorm was “severe” because specific criteria exists that can be used to quantify that.

Some basic safety precautions should be taken whenever a thunderstorm approaches – whether “severe” or not.  It is critical for people to get to an interior room of a building away from windows. If you’re caught outdoors, find a low lying area away from trees and other tall objects to take cover until the storm passes. People should also crouch down to avoid being the highest outdoor object during a thunderstorm and risk attracting lightning. Shutting down large appliances such as televisions and computers is also a good idea during thunderstorms to prevent damage from lightning strikes or power surges.

           

Additional thunderstorm safety tips along with the latest weather forecasts for the Mid-Atlantic Region can be found both on-air and online from the WUSA9 weather team.