Friday, May 08, 2009
Weather columnist Kevin Myatt: What is a tornado watch?
Kevin Myatt is The Roanoke Times' weather columnist.
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@roanoke.com
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Three times in the past three weeks, a tornado watch has been issued for much of Virginia, generally east of the Blue Ridge.
Franklin, Henry and Bedford counties have been in all three of the watches, coming Monday and Wednesday this week and back on April 20. Roanoke, Salem and Roanoke County were even included in Monday's watch. That's a year's worth of tornado watches for our region -- Southwest Virginia averages one tornado watch a year along Interstate 81 and two just east of the Blue Ridge, according to Storm Prediction Center statistics from 1999 to 2008.
No tornadoes occurred in the region during any of these watches. However, a tornado occurred Sunday near Indian Valley in Floyd County and another happened near Galax late Wednesday night without a watch or a warning in effect.
So what's the purpose of a tornado watch?
A tornado watch is a broad area, covering a large portion of a state or multiple states, where atmospheric conditions are favorable for the development of tornadoes. A watch typically lasts about six hours.
A tornado warning is a small focused area -- covering part of a county upward to a few counties -- where a tornado has either been sighted or where enough rotation is detected on Doppler radar to suggest that a tornado is occurring or may soon develop. A warning usually expires within an hour of its issuance.
Tornado watches are issued by the Storm Prediction Center in Norman, Okla. Forecasters there detect areas where factors are coming together for severe thunderstorms or tornadoes.
Tornadoes most often occur in moist, unstable environments (warm at the surface rising into colder air aloft) with strong winds shifting with height (shear) near a boundary between differing air masses.
Storm Prediction Center forecasters draw a "box," about 200 to 300 miles long and about 100 to 150 miles wide, over the areas where those factors overlap to the greatest degree for possible tornado formation.
A tornado watch also denotes a heightened risk of severe thunderstorms with large hail and high winds. Severe thunderstorm watches are issued when the risk for severe thunderstorms is high but the chance that they will spawn tornadoes is minimal.
Warnings are issued by local weather service offices, such as the one in Blacksburg for Southwest Virginia. Forecasters there depend on Doppler radar and trained spotters.
A tornado watch does not always lead to a tornado warning or a confirmed tornado, and not every tornado occurs within a tornado watch.
A watch indicates strong potential, not absolute certainty. Tornadoes can also form when conditions are right only within a very small area, far smaller than one that would lead to a tornado watch.
When a tornado watch is issued, it may not be necessary to cancel all activities. Tornado watches are often issued on otherwise sunny, warm days.
But it would be advisable to pay more attention to the sky and media weather reports than you might otherwise, as watches often precede ugly outbreaks of severe weather.
