Severe Thunderstorms Phenomena
A severe thunderstorm is defined as a thunderstorm that produces one or more of the following phenomena:
Hail the size of pennies or larger
Wind gusts of at least 58 mph
A severe thunderstorm watch is issued by the Storm Prediction Center in Norman, Oklahoma, for large portions of the region when they feel that the combination of several factors, such as wind shear and instability, could support the formation of severe thunderstorms. A severe thunderstorm watch usually includes several states and will frequently go into effect for 4 to 8 hours. Likewise, the Storm Prediction Center will issue a tornado watch when they feel that atmospheric conditions are ripe for tornadoes. Whenever a watch is issued, it is recommended that residents keep a close eye on the latest statements and forecasts issued by the national weather service and be ready to take cover when a warning is issued.
A severe thunderstorm or tornado warning is issued by the local National Weather Service forecast office, such as in Taunton, Massachusetts, when severe thunderstorms or tornadoes are imminent based on radar or spotter reports. When a warning is issued residents are urged to take cover immediately inside a sturdy building, preferably in a basement. Keep in mind that all thunderstorms produce deadly lightning, even though a storm may not be deemed severe.
Flood & Flash Flood Warnings
Note that torrential downpours of rain that cause flooding are not part of the definition of severe thunderstorms. They would prompt the issuance of flood or flash flood warnings, but not severe thunderstorm warnings. Flash floods are the number one weather killer in the united states.
Source: National Weather Service, Taunton, Massachusetts