Severe weather, definitions and terminology
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.
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.
The following are the hail sizes that the national weather service equates with various terminology,
Pea..............1/4 inch, not severe Marble...........1/2 inch, not severe Mothball.........1/2 inch, not severe
Dime.............3/4 inch, severe Penny............3/4 inch, severe Nickel...........7/8 inch, severe Quarter..........1 inch, severe Half dollar......1 1/4 inches, severe Walnut...........1 1/2 inches, severe Ping pong ball.. 1 1/2 inches, severe Golf ball........1 3/4 inches, severe Hen egg..........2 inches, severe Tennis ball......2 1/2 inches, severe Baseball.........2 3/4 inches, severe Tea cup..........3 inches, severe Grapefruit.......4 inches, severe Softball.........4 1/2 inches, severe
Some other definitions,
Funnel cloud: a funnel shaped cloud extending downward from a thunderstorm. It is associated with a rotating column of air that has condensed to form a cloud. It is not in contact with the ground.
Tornado: a violently rotating column of air in contact with the ground and extending to the thunderstorm base. It can be a few yards across to a mile wide.
Downburst: a sudden rush of cool air toward the ground that can impact with speeds greater than 70 mph and produce damage similar to that of a tornado. Viewing the damage from the air does not reveal evidence of a twisting motion or convergence toward a central track, like it would for a tornado.