With spring officially underway, it’s a good time to review some severe weather basics. The United States has more severe weather than any country in the world because of its unique geography. “Severe weather” refers to a thunderstorm that contains any of the following: 1) hail 1” in diameter or greater; 2) wind gusts 58 mph or greater; or 3) a tornado. Severe thunderstorms often have more than one of these elements. Although the heavy rain and lightning associated with all thunderstorms is potentially dangerous, its presence does not make a thunderstorm “severe” per se.
A weather “watch,” such as for a severe thunderstorm or tornado, is issued when conditions are favorable for said condition in a given region in a given time period. A “warning” is issued for a much smaller region for a shorter period of time when a particular type of weather is either occurring or is imminent. A watch lasts for a period of several hours, while a warning typically lasts less than one hour. If a thunderstorm is also producing a tornado, then a tornado warning is issued instead of a severe thunderstorm warning.
Widespread severe weather can occur under two primary scenarios. The first is more common in the spring and early summer when areas of low pressure draw energy from large temperature gradients, thus creating favorable conditions for showers and thunderstorms. Such spring storms have distinct warm and cold sides, with thunderstorms likely in warm and humid regions. Snow sometimes occurs on the cold side of these mid-latitude storms during the spring. For example, Washington, D.C.’s largest March snowstorm occurred in 1891 when 12” fell on March 28-29.
Tropical storms and hurricanes are more common during the summer and fall months. They can also lead to widespread severe weather in the United States. As the rotation of a tropical storm or hurricane interacts with a landmass, tornadoes often develop. Although Hurricane Ivan made landfall along the U.S. Gulf Coast in September 2004, its remnants caused nearly five dozen tornadoes in the Mid-Atlantic Region from North Carolina to Pennsylvania.
The Mid-Atlantic Region doesn’t have as much severe weather as the Midwest or Southeast United States. However, NOAA data shows that tornadoes have occurred in all 50 states. Florida averages the most thunderstorms in a given year, while Texas has the highest average number of tornadoes. Locally, Maryland averages 10 tornadoes over the course of a year, while Virginia experiences 18. In sum, the Mid-Atlantic Region can see severe weather either from non-tropical, mid-latitude storms or from weakening tropical storms or hurricanes.
Keeping track of the latest weather forecasts and having a safe place you can go to in the event of a severe thunderstorm watch or warning is of paramount importance. You can get the latest weather forecasts from the WUSA9 weather team on-air, online or on the free WUSA9 mobile app for your mobile devices.
