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University of Oklahoma: "The lower Mississippi Valley rainstorm and severe weather event of 15-16 November 1987"

Final Report

On 15-16 November 1987 a heavy rain and severe weather event caused havoc in the lower Mississippi Valley. The event was noteworthy for the exceptionally heavy rains (in excess of 20 inches in Louisiana) and a long-lived mesoscale wake low behind the active convective line in which surface pressure falls of more than 10 mb were observed in one hour. A comprehensive synoptic and mesoscale analysis of the storm has been conducted by the university, in collaboration with the National Meteorological Center. Gridded initialized fields from the NMC Regional Analysis and Forecast System were used to diagnose the synoptic scale circulations associated with the rainstorm. High resolution grids from the European Centre for Medium Range Weather Forecasts (ECMWF) were used to perform a similar task.

An important research finding was that the episodic convection which resulted in the two enormous mesoscale convective complexes observed during the event could be traced to the passage of subsynoptic scale disturbances along the tropopause. These disturbances were clearly revealed in maps of tropopause pressure and potential temperature where the tropopause was defined dynamically by the 1.5 potential vorticity surface. Another important finding was that the wake trough behind the main convection area could be tracked from western Texas to the Atlantic coast over a 48 h period, a remarkably long-lived event. The wake trough was triggered beneath a region of vigorous deep ascent in a slantwise neutral atmosphere. It appeared to be maintained in an environment of previously rain-cooled air in which descending motion behind the squall line contributed to excessive local pressure falls in the statically stable boundary layer air. Investigation continues into the relationship between long-lived wake troughs and equally impressive long-lived, large-amplitude gravity waves as both features appear to share common formation and maintenance mechanisms.