A good explainer from the National Weather Service's Storm Prediction Center on the difference between a "slight", "moderate", and "high" risk of severe weather.
A slight risk implies that well-organized severe thunderstorms 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, with 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.
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