The growth of a convective outbreak into an organized, long-lived mesoscale convective system (MCS) presents additional challenges and responsibilities to forecasters beyond those associated with isolated convection. Since an MCS itself is composed of a range of convective cell types, MCSs present the same severe weather threats as isolated cells. However, the range of severe weather types may be broad and several types may coexist within the system. Furthermore, the system organization enhances the potential for widespread, long-lived, damaging winds, and increases the likelihood of broad-scale heavy precipitation and flash flooding. MCSs occur as frequently as 25 per convective season in some regions of the country (particularly in the northern Midwestern states) and they can occur in any region. Approximately one-half of these can be considered severe events. MCSs can range in size from 50 to 500 km, making forecasting a challenge in terms of monitoring and coordinating between forecast offices. Fortunately, research has uncovered explanations for the behavior of MCSs based on the physical processes underlying their evolution. The results of this research offer the forecaster strategies for identifying the potential for severe MCSs before they become mature systems. An understanding of the physical processes and conceptual models of MCSs also helps forecasters predict the most likely locations of severe weather within existing systems and forecast the longevity, areal extent, and path of the system. These skills are directly related to the issuing of severe weather watches and warnings. MCSs may originate from either a line of cells triggered along a mesoscale boundary such as a dryline or cold front, or may evolve from an isolated cell or a cluster of cells. Both means of MCS initiation result in the same physical processes within the system as it evolves. While the forecaster may be able to examine the morning sounding and hodograph and anticipate the types of convective cells that may form, it is not easy to anticipate with acceptable reliability the evolution into a coherent MCS. However, once cells begin to exhibit linear organization, the forecaster should be able to examine the environment to predict whether the system is likely to become severe and long-lived. Accompanied by conceptual models, numerical simulations, and case studies, these training materials provide explanations of the physical processes involved in the production of MCSs and present strategies with which the forecaster can identify the potential for long-lived MCSs and attendant severe weather. |
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