The National Weather Service defines a severe thunderstorm as having large hail, at least 3/4 inches (0.75 inches) in diameter, and/or damaging winds, at least 58 mph, or 50 knots.

Lightning, no matter how frequently it is striking, is not a criterion for determining whether a storm is severe by National Weather Service definitions.

A severe thunderstorm watch is issued when severe thunderstorms are possible. It generally covers a large area, perhaps several states. A severe thunderstorm warning is issued when a severe thunderstorm is occurring or expected to occur within a matter of minutes.  This is a time when the value of a NOAA weather alert radio is at its highest.

It takes a special set of atmospheric conditions to provide an environment favorable for the development of tornadoes. If these exist, then a tornado watch will be issued instead of a severe thunderstorm watch.  A tornado watch means that conditions are favorable for severe thunderstorms, a few of which could produce tornadoes.  A tornado warning means that a tornado has been sighted or is imminent based on radar indications.

Tornadoes can form suddenly, and there may only be a severe thunderstorm warning in effect. Thus, you should pay very close attention to the weather whenever any severe weather threat exists and watches and warnings have been issued.

All severe thunderstorm or tornado watches are issued by the storm prediction center in Kansas City, Missouri. All severe thunderstorm or tornado warnings are issued by local National Weather Service offices.

Source:  National Weather Service, Taunton, Massachusetts