The levels of severe (damaging) thunderstorm and/or tornado risk may mean one thing to a meteorologist… but another to the general public. When you hear “Slight risk of severe thunderstorms or tornadoes,” it has a more ominous connotation than you may think.
There are three levels of risk. The official definitions from the Storm Prediction Center website:
A Slight Risk implies that well-organized severe thunderstorms (or tornadoes) are expected but in relatively small numbers/coverage, or a small chance of a more significant severe event. Not all severe storm events will be covered with a Slight Risk, especially during the summer when short-lived, “pulse-type” severe storms are relatively common during the afternoon.
A Moderate Risk implies a greater concentration of severe thunderstorms, and in most situations, greater magnitude of severe weather and greater forecaster confidence compared to a Slight Risk. A Moderate Risk is usually reserved for days with substantial severe storm coverage, or an enhanced chance for a significant severe storm outbreak. Typical Moderate Risk days include multiple tornadic supercells with very large hail, or intense squall lines with widespread damaging winds.
The High Risk implies that a major severe weather outbreak is expected (think about Alabama on April 27, 2011), with a large coverage of severe weather and the likelihood of extreme severe (i.e., violent tornadoes or very damaging convective wind events). The High Risk category is reserved for the most extreme events with the least forecast uncertainty, and is only used a few times each year.