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.
"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.
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.
Storm Prediction Center Introduces New Storm Intensity Categories
In efforts to better inform the public when severe weather looms, NOAA's Storm Prediction Center altered the risks associated with different expected storm intensities. Starting October 22, 2014, storms are now preemptively classified into five categories compared to four. The scale ranges the storm risk levels from marginal to high.
The most notable change is the addition of the new enhanced category. Previously included within the moderate risk, an enhanced storm risk is described where "numerous severe storms [are] possible" and capable of producing a few tornadoes, several reports of wind damage and hail ranging from 1 to 2 inches in diameter.
The next step up is moderate, a category classified by a concern for where "widespread severe storms [are] likely" with strong tornadoes, widespread wind damage and destructive hail at 2 inches in diameter or larger. NOAA said in a press release that the changes were made based on customer feedback.
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