The Difference Between Tornado Watches and Warnings

May 11, 2013; 7:12 AM
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University of Oklahoma storm chasers and observers are anticipating the annual tornado season as it approaches the central part of the country. Adding to the anticipation is the fact they observed the 78-tornadoes, including this May 3, 1999 funnel that became the F-5 storm. (AP Photo/J. Pat Carter)

While the peak occurrences for severe weather events in the United States happen between March and October, severe weather can occur at any time. In order to save lives, branches of the National Oceanic and Atmospheric Administration (NOAA) will issue public watches and warnings.

Knowing the difference between the two can prepare individuals for the necessary steps to take when considering the threat of severe weather. Watches and warnings issued to the public are based on different criteria.

Watches are issued by the NOAA's SPC, and warnings are issued by local offices of the National Weather Service (NWS).

"A watch is issued when conditions are favorable, for example, either for a severe thunderstorm or tornadoes," AccuWeather.com Senior Meteorologist Dan Kottlowski said. "It doesn't mean severe weather is imminent."

"Typical watches cover about 25,000 square miles, or about half the size of Iowa," according to the SPC.

Kottlowski said there are no set criteria for issuing watches, but if the conditions seem consistent with a developing severe weather pattern, watches can be changed and altered by monitoring ongoing developments.

"It can vary," he said. "There is not just one set of ingredients; every watch may have a different set of perimeters from one day to the next since it is based on a synoptic situation that may change within several hours."

Warnings mean that severe weather is imminent and is based on specific criteria and existing reports received by the NWS.

(Photo/NOAA Photo Library)

The criteria include hail that totals more than 1 inch in diameter and wind speeds of 55 mph.

"Lightning is not a criteria for a severe thunderstorm warning," Kottlowski said. "Heavy rain is not either."

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Warnings must follow the two main criteria, he said, adding urban flood and stream advisories, flash flood watches and warnings, and flood watches and warnings, may accompany a storm with heavy rain.

Warnings are issued through the efforts of individuals working for the NWS.

"The way a warning is issued is that a meteorologist will monitor the weather by radar and look for particular areas where there could be high impact damage," Kottlowski said. "They will issue a warning and there will be a signature for an existing storm or developing tornado."

Trained NWS spotters will verify reports of rotation or storm damage.

"This gives the meteorologists confidence in what they are seeing on radar," he said.

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