Evolution of warm-season heavy rain systems over the Great Plains during late-morning hours

Thunderstorms within mesoscale convective systems often produce heavy rains that can produce flash flooding and other kinds of severe weather during nighttime hours in the warm season over the Great Plains and Mississippi Valley of the United States. These systems often dissipate or decline in intensity during the four hours or so before local noon, but some are maintained or regenerate and continue into the afternoon. Forecasters are therefore faced with the decision of whether to forecast an end to the activity during late morning or to indicate a continuation. Relatively little is known of the factors that are primary in determining evolution of these systems during this period of the day. The climatology of such systems says that they will most likely dissipate, so the forecaster often looks for factors that might allow for continued activity.

A new research program called the Morning Convection Project has begun with the goal of better understanding the factors that influence evolution during this period of the day. One component of this is a climatological study that will include the summer months of 1995-2000 and will cover the areas of forecast responsibility for the Norman, Oklahoma and Dodge City, Kansas National Weather Service Offices. The results of such a study are expected to benefit other areas facing this forecast problem. The climatology will look at system origin, tracks, evolution, and occurrences of flooding rains and other severe weather phenomena based upon radar, satellite, and surface and upper air information. Particular attention will also be given to relating system evolution to environmental influences, such as system relative wind and wind shear, as well as atmospheric stability ahead of these systems. Numerical model output will also be assessed to determine whether any factors that are found to important in determining evolution are reliably forecast by the models.

Results of a preliminary climatology covering the 1995-97 period will be described. This study has revealed a preference for system motion from the north-northwest over the lifetime of such systems, and has verified that the majority of systems decline or dissipate during the late morning. The project is a collaborative effort among the National Severe Storms Laboratory, the NWS offices in Norman and Dodge City, and the University of Oklahoma. Over the next three years the project will receive support from the NWS Cooperative Projects program of the Cooperative Program for Operational Meteorology, Education and Training (COMET).