Published: 2011-03-04 15:44:00
Updated: 2011-03-04 15:44:00
Posted March 4, 2011 3:44 p.m. EST
Collectively referred to as the severe weather "Alert Process," watches and warnings are at the heart of how the National Weather Service and its media partners (that would be us) attempt to protect life and property when thunderstorms present a threat of damaging winds, large hail, flooding and/or tornadoes.
The process often begins with the Storm Prediction Center (SPC) in Norman, Oklahoma, which is responsible for monitoring weather patterns nationwide and identifying those that are or may become favorable for producing severe thunderstorms or tornadoes. The forecasters at SPC routinely produce "convective outlooks" that identify areas of concern in some detail for the first day or so in advance, a bit less detail for days two and three, and a very general outlook for days 4-8. These products identify broad areas that are assessed as having a slight, moderate or high risk of severe weather. We often highlight these areas in our weathercasts when they overlap our state or are of particular interest somewhere upstream.
When SPC believes conditions are favorable for a growing threat of severe weather over a more defined region within the next 6-8 hours or so, they may issue a severe thunderstorm watch or a tornado watch if the atmosphere appears supportive of the rotation and shear that indicate twisters are possible. Watches usually cover several hundred to several thousand square miles, and when you find yourself under one, it simply means that you should stay especially aware of weather conditions and prepared to seek appropriate shelter to stay safe if such a storm does develop in your local area.
The next step in the alert process is handled by your local National Weather Service Forecast Office. For most of our viewing area, that responsibility falls to the Raleigh office located on the NCSU Centennial Campus. When forecasters there either see radar indications of severe storms or a tornado, or receive eyewitness reports of the same, they issue severe thunderstorm or tornado warnings that cover a much more localized and specific area, confined to a much shorter time period than a watch. When a warning is issued, it means the storm is underway and imminent for locations in the warned area, and that those in the storm's path should take protective actions immediately, typically in the form of finding your way to the center of a home on the lowest floor, with as many walls as possible between yourself and the outdoors.
There are a variety of ways for you to receive these warnings when they are issued. Of course, a major pathway is through television and radio. Here at WRAL, we have automated processes that will generate and run a crawl at the bottom of your screen, and of course in the case of tornadoes or especially destructive and life-threatening severe thunderstorms, we will also cut in over regular programming in order to track the location and progress of the storm as it moves from one community to the next. We also offer free e-mail and text messages that will pass along these warnings, and the WRAL WeatherCall program, which for a nominal fee will call your telephone to let you know that your specific address falls within a warned area. This can be especially valuable in the case of overnight storms, or anytime you are not otherwise in the presence of a TV or radio conduit for the information.
Another way to receive warnings is through a NOAA Weather Radio, available at most larger electronics retailers or via web merchants. Weather radios that have SAME (Specific Area Message Encoding) technology can be set to filter the warnings so that an alarm goes off only when the county you live in is affected. This prevents you from receiving alarms for storms in nearby counties, and serves as an excellent way to stay informed. At this time, however, the technology used for weather radio sets off all radios in your county, even if the tornado or storm may only affect a small portion of the county that doesn't include your location. This is one of the advantages of the WRAL WeatherCall system. Instead of the "county-based" warnings utilized by weather radio, it selects addresses to call using "storm-based" warnings that are much more specific to the path the storm is expected to take. These are outlined by the "polygons" you sometimes see us use on air to indicate the area to be affected by a storm. As an example, suppose a tornadic cell races eastward across the eastern edge of Wake County as occurred on April 25th in 2010, and illustrated in the second image above. When the warning is issued, weather radio alarms will sound off for anyone in the county who has one, while the telephone alert from WeatherCall will only dial those addresses in the vicinity of Zebulon and surrounding communities within the polygon, but will not ring up people in Raleigh, Morrisville, Cary, Wake Forest, etc who are outside of the storm-based warning area.
Regardless of how you receive the end product of the Alert Process, it's a good idea to think through how you'll react, and to have a plan for making sure your family stays safe.