Forcing Mechanisms and Other Characteristics of Significant Summertime Precipitation

Abstract

Satellite, radar, surface, and upper-air data from the June?August periods of 1982?83 are examined to determine the mechanisms, and their relative contributions, for producing warm-season precipitation in the United States. Only areas where rainfall equaled or exceeded 12.7 mm during the 24-h period ending at 1200 UTC were considered. Rainfall associated with extratropical cyclones accounted for about half of the warm-season precipitation. Most of the remaining precipitation was produced by mesoscale forcing mechanisms acting independently from the traveling extratropical cyclones. Nearly all the warm-season precipitation was convective; over 80% of the total was directly or indirectly associated with thunderstorms. Nearly three-fourths of the precipitation that occurred between the Rockies and the Mississippi Valley was nocturnal. Conversely, about three-fourths of the precipitation east of the Mississippi Valley fell during the daylight hours. During the 1983 July-August drought, the area of precipitation from extratropical cyclones was significantly smaller than the precipitation area during the corresponding period in 1982. The decrease was the result of a sharp drop in the frequency of cyclone events. Conversely, the frequency of mesoscale events and associated precipitation increased during the drought year. This difference in precipitation mode from one year to the next may help explain the large differences in numerical model forecasting skill from summer to summer. The large proportion of warm-season precipitation produced by mesescale forcing mechanisms suggests that further improvement in forecasting cyclonic storms and fronts will likely result in only limited increases in summertime quantitative precipitation forecast (QPF) skill. Moreover, the historically smaller values of warm-season QPF skill, relative to the cool-season levels, are likely to continue until operational numerical models can predict mesoscale convective systems as well as they predict extratropical cyclones and attendant fronts.