Published: 2014-03-02 09:10:00
Updated: 2014-03-04 12:16:06
Posted March 2, 2014
Updated March 4, 2014
By Mike Moss
Today we kick off Severe Weather Awareness Week in North Carolina, which runs through Saturday March 8th. Each day we'll touch on a related topic both on-air and here in the WeatherCenter Blog, starting with one of the keys to safely navigating those parts of the year when the chance of severe weather is highest, understanding and responding to the watch and warning process. Those of us in the WRAL WeatherCenter stay vigilant for upcoming weather patterns that may pose a hazard of severe storms, to include potential for damaging wind gusts, large hail, tornadoes and flash flooding, and pass that information to you in our forecasts as it becomes apparent. As we approach the time of possible severe weather, the formal responsibility for issuing specific watches and warnings falls to portions of the National Weather Service, as part of their mission to protect lives and property.
The system of watches and warnings begins on a fairly large scale, as severe weather specialists at the Storm Prediction Center in Norman, OK keep an eye out for patterns that suggest a significant threat for severe weather over regional areas of the country. When we reach a time when that threat may materialize within a few hours, they issue a watch, meaning that conditions are supportive of the formation of severe storms or tornadoes. These watches usually cover many hundreds or thousands of square miles, in most cases involving parts of multiple states, and stretch over time periods of six hours or more. When you find yourself within a watch area, it means that you should have fresh in mind what to do, where to be and how to react should an immediate severe weather threat develop near your location, and should stay attuned to the weather. Across central NC, we typically find ourselves under about 5-8 tornado watches in year, and about 10-13 severe thunderstorm watches. It's also an especially good idea then to have access to a means of receiving reliable notification of any warnings that become necessary, which is the next step in the watch/warning process.
Once storms actually develop in the watch area, local NWS offices, like the one located on the Centennial Campus of NC State University in Raleigh, keep a very close eye on radar, looking for signatures that may indicate damaging wind gusts, severe-sized hail, the possible existence or development of tornadoes, and the chance for sufficiently heavy downpours to produce flash flooding, and they also monitor ham radio and phones for any reports of observed severe weather that radar may be missing. When they have reason to believe any of these are happening or are about to, they issue a warning that is restricted to a fairly limited area and a relatively short time period. A warning indicates that severe weather is imminent or underway, and you should take appropriate action to protect yourself!
Until about six years ago, these warnings were issued for entire counties or groups of counties, which led to rather large areas being warned while in many cases a small part of that area would actually be affected by the severe storm. Since that time, however, warnings are "storm-based" and involve projecting a polygon that covers the locations most likely to be impacted. These may cover only a portion of a county or parts of multiple counties along the expected track. Although the warning areas are still usually a good bit larger than the eventual impact area, this has reduced the potential for false alarms and means that when your location is warned, it is a good idea to take that warning seriously. The third image I included here is a graph that shows how many warnings the Raleigh NWS office issued for the 31-county area they serve across central NC in 2013, along with the average numbers for the five years covering 2009-2013.
Here at WRAL, we play a role in the warning process by running crawls on the bottom of the TV screen, cutting in over programming when possible tornadoes or especially intense wind gusts are present, and providing a range of other means to receive warning information, which my colleague Nate Johnson will address in more detail in a later post as part of this Severe Weather Awareness Week.