Thunderstorm Safety and Preparedness

Severe Thunderstorm, Tornado Watches vs. Warnings: What's The Difference?

During severe weather, a number of alerts may flash on the TV screen or on your mobile device. But do you know what they mean, and which ones require immediate action?

Severe Thunderstorm WATCH

A thunderstorm is considered severe when it exhibits one or more of the following:

Large hail (hailstones that are one inch diameter or greater, so hail the size of a quarter or larger fits this criterion).
Wind gusts of 58 mph or greater.
A tornado.

This is an example of a severe thunderstorm watch box. — This is an example of a severe thunderstorm watch area.

A severe thunderstorm watch means that atmospheric conditions are favorable for severe thunderstorm development. The severe thunderstorm watch outlines an area where an organized threat of severe thunderstorms is expected generally during a three- to six-hour period. Severe thunderstorm watches are issued by the Storm Prediction Center after consultation with local National Weather Service offices and can cover several counties to several states at a time.

During a severe thunderstorm watch, it is important to think about where you will be during the lifetime of the watch. If you are outdoors, develop a Weather Ready plan that includes directions to the nearest lightning and hail-proof shelter. Keep in mind that severe hail has the ability to smash car windshields, cause injuries and, in the extreme, punch holes in roofs.

Severe Thunderstorm WARNING

A severe thunderstorm warning means that a severe thunderstorm is imminent: It is occurring or is about to occur. Severe thunderstorm warnings are issued by the local National Weather Service office when it deems a particular thunderstorm has met, or will soon meet, the severe criteria listed above. Severe thunderstorm warnings are usually highly localized – covering parts of one to several counties – and generally last for 30 to 60 minutes.

During a severe thunderstorm warning, it is important to get to safety immediately. Severe winds – gusts of 58 mph or greater – can down trees and power lines, causing injury and potentially death to those hit by large falling objects and other debris. In addition, if a severe thunderstorm is producing large hail, injury and significant property damage can result.

(MORE: Hail's Underrated Danger)

Indoors, stay away from windows and avoid using a landline phone or other device attached to the electrical circuits or plumbing in your home.

Tornado WATCH

During a tornado watch, atmospheric conditions are favorable for tornadoes and severe thunderstorms. High wind shear – changes in wind speed and direction with height – and unstable air (warm, humid air near the surface topped by colder, drier air aloft) particularly in the lowest levels of the atmosphere, are necessary for developing tornadoes.

The tornado watch outlines an area where an organized threat of severe thunderstorms capable of producing tornadoes are expected generally during a three- to six-hour period. Tornado watches are issued by the Storm Prediction Center after consultation with local National Weather Service offices, and the watch areas can span several counties to several states.

During a tornado watch, it is important to think about where you will be during the lifetime of the watch. If you are outdoors, develop a Weather Ready plan that includes directions to the nearest tornado shelter. If you are indoors, think about where you should go in the event of a tornado, and also gather your Weather Ready items in the event a tornado does strikes.

Tornado WARNING

A tornado warning means that a tornado is imminent: A tornado is occurring or is expected to develop. Tornado warnings are issued by the local National Weather Service office when any of the following occurs:

Doppler radar indicates strong rotation within a thunderstorm.
Dual-polarization Doppler radar detects a tornado debris signature, indicative of lofted debris from a tornado.
Trained storm spotters have reported a funnel cloud (a circulation aloft not yet in contact with the ground).
Trained storm spotters have reported a fully-formed tornado (the circulation has made contact with the ground).

This is an example of a tornado warning box. — This is an example of a tornado warning polygon.

Tornado warnings are usually highly localized – span parts of one to several counties – and generally last for 30 to 60 minutes.

During a tornado warning, it is important to get to safety immediately. Strong to violent tornadoes can level buildings and loft vehicles into the air. Even weak tornadoes can cause damage to roofs and other structural damage, causing injury and potentially death to those hit by large falling objects and other debris.

MORE ON WEATHER.COM: Photos of the Moore, Okla. Tornado (May 20, 2013)