A Damaging-Wind Based MCS Classification System and Formal Definition of a Derecho

The National Weather Service’s Storm Prediction Center is tasked with forecasting severe weather hazards, including 50+ kt surface winds from mesoscale convective systems (MCSs). MCSs can produce severe winds on varying spatial and time scales. Loosely organized MCSs may support localized severe gusts for a couple of hours, while longer-lived, highly organized MCSs may produce widespread severe winds over path lengths of 1000+ km and last nearly a day (i.e. derechos). Since MCSs can produce severe winds on widely varying spatial and temporal scales, a need exists to classify MCSs based on severe-wind production. Such a classification would be useful for establishing severe-wind producing MCS climatologies, and if implemented prognostically, communicate the level of threat an MCS poses in severe weather forecast products (as with tropical cyclone forecasts). Similar to rating tornadoes and tropical cyclones, an MCS classification system should be rooted in simplicity for the public’s understanding, but should also be established with robust, quantitative criteria. A preliminary draft of an MCS rating system is proposed, ranging from a Category 0 (sub-severe MCSs) to Category 5 (super-derecho-producing MCSs). Since Categories 4 and 5 involve derecho classification, a more precise, objective definition of a derecho is also affirmed. Preliminary results show that MCSs can be detected and ranked using an automated algorithm, the details of which are reviewed in a companion presentation. The distribution and climatology of MCSs based on rank is also presented.