Have you ever heard a tornado siren blaring and looked up to see nothing but blue sky overhead? That may be because somewhere in your county there is a real threat for severe weather, but not for your neighborhood. No doubt you have seen severe thunderstorm or tornado warning polygons on television broadcasts or on the internet. So, just what are those polygons, and what do they mean for you?
In the past, severe thunderstorm, tornado, and flash flood warnings were issued for entire counties. On October 1, 2007, the National Weather Service began issuing warnings in the shape of a polygon, which are intended to warn only the locations and people inside the polygon of impending severe weather.
Warning polygons represent state of the art NWS capabilities and understanding of severe weather, which enables them to specify the locations that are most likely to be affected by a severe thunderstorm, flash flood or tornado. Forecasters are continually monitoring the radar, following storms and watching for signs that the storm may be trouble. At the point which most storms become severe, forecasters have monitored them for a while and know where the storms will track and how they will behave. Based on this information, NWS forecasters then draw a polygon that defines the locations that are threatened by the storm.
In the example seen here, the NWS issued a tornado warning at 5:04 PM on April 19th, 2009. Notice that while the warning affects the four counties of Tuscaloosa, Jefferson, Bibb, and Shelby (outlined in red), the area within those four counties defined by the warning polygon (outlined in white) is much smaller. The total warned area was decreased from 3879 square miles to 508 square miles, a reduction of 87%! It just so happens that for this warning, the cities of Tuscaloosa and Birmingham are NOT included in the warning.
So what does this mean for you? Well, when the sirens sound or you become aware of a severe weather warning for your area, you need to act quickly! If it is dark and ominous, go to your shelter immediately. If the sun is out or the weather is benign, tune to your NOAA Weather Radio All-Hazards or your favorite local media outlet to get more details on the storm.
It is our goal that only those inside the polygon should take action. As technology in outdoor sirens and personal warning devices advances, we hope to minimize your personal false alarm rate, or the number of times you are alerted for impending severe weather when it does not actually affect your location. If ever in doubt over whether you are at risk, seek additional weather information immediately.