Ensemble-based Analysis of the May 2010 Extreme Rainfall in Tennessee and Kentucky

From 1 to 3 May 2010, persistent heavy rainfall occurred in the Ohio and Mississippi River Valleys due to two successive quasi-stationary mesoscale convective systems, with locations in central Tennessee accumulating more than 483 millimeters (19 inches) of rain, and the city of Nashville experiencing a historic flash flood. This study used operational global ensemble forecasts from the European Centre for Medium-Range Weather Forecasts (ECMWF) to diagnose atmospheric processes and assess forecast uncertainty in this event. Utilizing ensemble members, several methods are used to examine the processes that led to the development and maintenance of this high precipitation system. Differences between ensemble members that correctly predicted heavy precipitation and those that did not were determined, in order to pinpoint the processes that were favorable or detrimental to the system's development. Statistical analysis was used to determine how synoptic-scale flows were correlated to area-averaged precipitation. The heavy precipitation throughout Nashville, Tennessee and surrounding areas occurred ahead of a deep upper-level trough located over the central United States. The distribution of precipitation was found to be closely related to the strength of this trough and an associated surface cyclone. In particular, with a very pronounced upper-level trough digging deep into the central plains, the surface cyclone was much stronger. The strong cyclonic rotation was associated with much stronger winds out of the south, causing the warm front and moisture at low and midlevels to move much farther to the north into the Great Lakes. This caused moisture and precipitation to spread over a larger area across the eastern United States. On the other hand, when the upper-level trough was shallower, the surface cyclone and associated southerlies remained slightly weaker. This caused the conveyor belt of moisture from the Caribbean Sea to be concentrated over Tennessee and Kentucky, where, in conjunction with focused ascent, heavy rain fell. These relatively small differences in the wind and pressure fields led to important differences in the precipitation forecasts and highlighted some of the uncertainties associated with predicting this extreme rainfall event.