The Categorical Outlook is a five-tier summary of the SPC Convective Outlook that translates the underlying probabilistic maps into one of five named risk levels: Marginal, Slight, Enhanced, Moderate, and High. It is a heuristic layer built on top of the numeric probability forecasts, designed to communicate severe weather potential in a single glance.
How the categories are defined
Each category is defined by fixed probability thresholds. If the tornado, wind, or hail probability at a point exceeds the value for a given category, that point qualifies for that category. The highest-qualifying category is the one plotted. A Moderate Risk area, for example, requires that at least one of the probabilistic hazards meet Moderate-level probability values.
The tiers also carry qualitative meaning. Marginal represents isolated organized severe potential. Slight indicates well-organized severe storms in small numbers or low coverage. Enhanced expects more numerous or higher-coverage severe events. Moderate represents a widespread severe weather event with an increased tornado or long-duration wind concern. High is reserved for outbreaks with the expectation of either strong tornadoes or a widespread damaging wind event.
How to read a categorical map
The category summarizes what the atmosphere is capable of within the outlined region, not what will occur at any single spot. It is a good tool for pre-event planning. Staffing, chase targeting, and public messaging can all key off the category.
Comparisons across events are useful. Two Enhanced Risk days will not look identical, but the SPC forecasters have standardized the meaning enough that Enhanced consistently sits between Slight and Moderate in expected severity and coverage.
What the category does not tell you
A category compresses several separate hazard probabilities into one label, and a lot of information is lost in that compression. A Slight Risk driven mainly by hail probabilities looks very different in the field from a Slight Risk driven by tornado probabilities. Reading only the category, and ignoring the underlying hazard-specific maps, is a common way to be surprised.
Category boundaries are also lines on a map, not fences the atmosphere respects. Storms routinely occur just outside an Enhanced Risk. Being outside the outlined area is not a reason to stand down if the environment supports storms.