7 April 2015
The Storm Prediction Center in Norman issues all of the weather watches for severe thunderstorms, and tornadoes, across the U.S. In addition, they also put out an outlook several times per day that indicates the threat level across the country over the next several days. In the past this outlook had three levels for severe storm risk: slight, moderate, and high, but this has now been revised, and the graphic above shows what they mean.
A high risk is quite rare, with only a few of these issued in a year for anywhere in the U.S. If a high risk is in effect, you’re likely to see violent tornadoes somewhere in the risk area with widespread damage reports. In places like Oklahoma and Kansas, a high risk can be expected one or two days per year on average, but across most of the eastern U.S. a high risk is something that may not be issued at all for several years in many places. The exception would be for something like the great derecho wind event on June 30, 2012 in that stretched from Chicago to Washington DC.,and on into Maryland and Delaware.