A severe thunderstorm watch is issued when atmospheric conditions are favorable for the development of severe thunderstorms, while a severe thunderstorm warning means that severe weather is occurring or is imminent. The Washington area is most prone to downbursts or straight-line winds from severe thunderstorms. A downburst is a small area of rapidly descending air beneath the storm. Downbursts can cause winds in excess of 100 mph. In fact, a powerful, localized downburst called a microburst produced a wind gust of 130 mph at Andrews Air Force Base back in 1985.
Severe thunderstorms can also spawn tornadoes.
Most of these twisters are rated F0 or F1 on the Fujita Scale, meaning they pack winds of 112 mph or less, and stay on the ground for just a few minutes. But even a small tornado is capable of downing large trees, and power lines, ripping the roof off of a home, and hurling debris at very high velocities that can seriously injure or kill someone trapped outdoors. Still, every thunderstorm has the potential to be very dangerous. The reason is that all thunderstorms produce lightning, so that even those storms that seem rather innocuous can be killers. In fact, lightning typically kills more people each year than either tornadoes or hurricanes.