A climatology of derecho-producing mesoscale convective systems in the eastern United States, 1986-1995

In 1888, Iowa weather researcher Gustavus Hinrichs gave widespread convectively induced windstorms the name derecho. Refinements to this definition have evolved after numerous investigations of these systems; however, to date, a derecho, climatology has not been conducted.; This investigation examines spatial and temporal distributions of derechos and their associated mesoscale convective systems that occurred in the eastern U.S. from 1986 to 1995. The spatial distribution of derechos revealed four activity corridors during the summer, five during the spring and two during the cool season. Evidence suggested that the primary warm season derecho corridor was located in the southern Great Plains. During the cool season, derecho activity was found to occur in the southeast states and along the Atlantic seaboard. Temporally, derechos are primarily late evening or overnight events during the warm season and are more evenly distributed throughout the day during the cool season.; Synoptic-scale environments favorable for producing derechos in the Northern Plains and cool season were examined with the goal of providing pattern-recognition techniques for identifying features within derecho, activity corridors. Nineteen derechos were identified across the northern Great Plains and 14 events during the cool season. The synoptic environment at the initiation, midpoint, and decay of each derecho, was then evaluated using surface, upperair and gridded datasets.; Results of the analysis suggested that the low-level synoptic environment was critical in initiating and maintaining warm season derecho producing mesoscale convective systems (DMCSs). Circulation around surface low pressure increased the instability gradient and maximized leading edge convergence in the initiation region of nearly all events regardless of DMCS location or movement. Other commonalities in the environments of these events included: the presence of a weak thermal boundary, high convective instability, and a layer of dry low to mid-tropospheric air. The synoptic environment in place downstream of the MCS initiation region determined the movement and potential strength of the system.; Marginal instability and strong synoptic-scale forcing characterized the environments of cool season events in both corridors. The overall synoptic patterns associated with cool season DMCSs resembled environments found with cool season tornado episodes.; Identification of key synoptic-scale features in warm and cool season DMCS environments should lead to improvements in prediction and further insights into their formation and sustenance. Results from this investigation also revealed spatial and temporal commonalities in DMCS occurrence that appear to be controlled by elements in the synoptic environment.