As of this morning, much of the southeastern United States is under a “slight risk” for severe weather according to the Storm Prediction Center (SPC) in Norman, Oklahoma. The SPC is part of NOAA, the National Oceanic and Atmospheric Administration, which is part of the United States Department of Commerce, and they are the federal authority on severe weather. Most of the products you see in the media talking about a “slight,” “moderate,” or “high” risk for severe weather are adapted from the SPC products. So what do these descriptions mean?
A chance for non-severe thunderstorms, while noteworthy, is not risk-worthy. When systems develop that bring the chance for severe thunderstorms (storms with winds of 58mph or greater and/or hail one inch in diameter or greater and/or tornadoes), the vocabulary used by meteorologists gets a bit of an upgrade. A risk for severe weather means that within 25 miles of a point in the risk area, there is a higher than normal chance for a severe storm that day. The probabilities are based on the computer model outputs, but overall decided upon by the outlook forecaster.
The SPC website, www.spc.noaa.gov, gives the best explanation for the risk categories as follows:
A SLIGHT risk implies well-organized severe thunderstorms are expected, but in small numbers and/or low coverage. A MODERATE risk indicates a potential for a greater concentration of severe thunderstorms than the slight risk, and in most situations, the storms and associated severe weather are more intense. A HIGH risk area suggests a major severe weather outbreak is expected, with the expectation of either a concentration of strong tornadoes or an enhanced likelihood of a long-lived wind event (derecho) with the potential of higher end wind gusts (80+ mph) and structural damage.
What it all boils down to is that even the phrase “slight risk” in the forecast means that someone is going to see severe weather somewhere within the risk area. It might not be you or I, but somebody will. With a high risk, many more people will be affected.