Genesis and evolution of the 4 June 1999 derecho

A cluster of thunderstorms formed over South Dakota and northeastern Nebraska during the early morning of 4 June 1999, within an area of persistent warm air advection just north of a stationary surface front. The convection moved southeast across Iowa through the morning, before quickly evolving into a large, forward-propagating mesoscale convective system (MCS). The system accelerated southeast across the mid Mississippi Valley during the afternoon and evening, before finally diminishing over northern Alabama. The resulting derecho produced a series of small scale damaging winds events along its path, and for several hours was characterized by a continuous arc of storms that extended for more that 400 miles (650 km).

The MCS's intensity, degree of organization and motion were not well forecast by either human or numerical forecasts. The data suggests the development of a well-defined cold pool at the surface, in conjunction with the availability of an increasingly favorable thermodynamic environment for the downshear development of new thunderstorms, fostered the genesis and longevity of this event.