Severe thunderstorms are forecast 1-2 days in advance, primarily based on a variety of NWP models and the National Severe Thunderstorm Forecast Guidance System (NTFGS).
Severe thunderstorm warnings follow a two-tiered system. A "state-based warning" is issued for more rural areas of a state which is usually outside the radar coverage area. This type of warning is relatively broad in space and time and relies mostly on an assessment of the near-storm environment rather than a structural interrogation of existing storms.
In the metropolitan areas of some states quite specific cell-based severe thunderstorm warnings are issued. Now a larger component of the evidence supporting the warning decision stems from a detailed examination of the base reflectivity and base velocity structure of the storm. Radar-based algorithms such as the Thunderstorm Identification, Tracking Analysis and Nowcasting (TITAN) algorithm and the Warning Decision Support System (WDSS) are used as alerting mechanisms and safety nets. The radar-based and rain gauge-corrected rainfall algorithm RAINFIELDS is used as additional guidance for flash-flood warning decisions.