The Storm Prediction Center (SPC) issues Day 1, Day 2, and Day 3 Convective Outlooks depicting forecast areas of general (non-severe) and severe thunderstorm threats across the contiguous United States, along with a text narrative discussion consisting of a plain-language summary of the threat type(s) and timing focused on areas of highest risk, and a technical discussion written in scientific language that usually includes a synoptic overview of convective patterns as well as, if necessary, a geographically specific narrative of meteorological reasoning and justification for the type of coverage and intensity applicable to the severe thunderstorm threat.

The categorical forecast in the Day 1-3 Convective Outlooks—which estimates a severe weather event occurring within 25 miles (40 km) of a point and derives the attendant risk areas from probability forecasts of tornadoes, damaging winds, and large hail on Day 1, and a combined severe weather risk on Days 2 and 3—specifies the level of overall severe thunderstorm risk via numbers, descriptive labeling, and colors as follows: (The Day 4-8 Convective Outlook assesses the percentile probability of severe thunderstorm activity during that period at the 15% and 30% likelihood.)

Convective thunderstorm risk categories

Risk Category


0 - General Delineates, to the right of a line, where a 10% or greater probability of thunderstorms is forecast during the valid period.
1 - Marginal An area of severe storms of either limited organization and longevity, or very low coverage and marginal intensity.
2 - Slight An area of organized severe storms, which is not widespread in coverage with varying levels of intensity.
3 - Enhanced An area of greater (relative to Slight risk) severe storm coverage with varying levels of intensity.
4 - Moderate An area where widespread severe weather with several tornadoes and/or numerous severe thunderstorms is likely, some of which should be intense. This risk is usually reserved for days with several supercells producing intense tornadoes and/or very large hail, or an intense squall line with widespread damaging winds.
5 - High An area where a severe weather outbreak is expected from either numerous intense and long-tracked tornadoes or a long-lived derecho-producing thunderstorm complex that produces hurricane-force wind gusts and widespread damage. This risk is reserved for when high confidence exists in widespread coverage of severe weather with embedded instances of extreme severe (i.e., violent tornadoes or very damaging convective wind events).