Atmosphere And Weather

Difference between Tornado Watch and Warning



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The United States experienced its first tornado outbreak of 2010 last  week, as tornadoes ravaged large portions of the Deep South and Mississippi River Valley. On average, the United States records more tornado sightings than any other country in the world. Volatile atmospheric conditions, combined with unique topography, provide a fertile environment for tornado formation. Tornadoes have been spotted in each American state, and they can form during any month of the year. However, the apex of American tornado activity runs from mid-March to early August.

Tornadoes unleash incredible destruction, with wind speeds often climbing above one-hundred miles per hour. The potential for mass structural damage and vast loss of life prompted the National Weather Service (NWS) to create a warning system that alerts Americans of potential and actual tornado development. The system has evolved through the years, initially relying on loud sirens to warn the public of pending danger. Technology advanced the warning system onto television and computer screens.

The screens emanate bright red before and during a severe weather event. Red boxes typically precede a tornado event and cover areas affected by an issued tornado watch. NWS meteorologists in Norman, Oklahoma monitor atmospheric conditions, and when the atmosphere is primed for severe thunderstorm development, the meteorologists may issue a tornado watch for the affected areas. The NWS in Norman commonly issues tornado watches for periods up to twelve hours in length. Meteorologists issuing tornado watches predicate their analysis on a number of factors: contrasting jet stream winds, relative humidity/dew points, barometric pressure gradient differences, wind shear, contrasting air masses, and jet stream wind flows.

A tornado watch means conditions are favorable for severe storm and subsequent tornado development. In addition to issuing a tornado watch, the NWS in Norman may also classify the watch area by severity. In the event of a prospective major tornado outbreak, a “particularly dangerous situation” designation may be included with the official weather alert. The Weather Channel (TWC) designates high probability events as “high risk.”

The NWS issues a tornado warning when a trained spotter views a rotating cloud or meteorologists indicate a tornado’s signature on Doppler Radar. Tornado spotters receive intensive multi-week training by NWS personnel that includes field work during severe storm events. Spotters call in their location after spotting a tornado, and they provide information on the tornado’s size, shape, and direction. Recent Doppler Radar enhancements have improved tornado warning accuracy.

The NWS issues tornado warnings to indicate people living in the affected area should treat the warning as a life-threatening situation. Local NWS offices issue tornado warnings based on a storm’s path, although the NWS issues warnings for entire counties if the storm’s path wavers or its swath is inordinately large. Moreover, the NWS in Norman issues a tornado emergency when a large, violent tornado bears down on a densely populated area.

In the simplest terms, a tornado watch means the time has come to devise a plan of action in case a tornado drops from the sky. On the other hand, a tornado warning means the time has come for you to enact your plan. A tornado warning plan must include a place for you and your family to safely hunker down, preferably a basement or underground cellar. Prepare an emergency kit that includes first aid materials, portable lights, and enough food to ride out a few days trapped inside your home.

Tornadoes are the most violent and unpredictable of weather phenomenon. It is imperative to take the watches and warnings seriously and follow local media outlets for instructions.



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