Multidecadal Changes in the Frequency and Ambient Conditions of Warm Season Convective Storms over the Northeastern United States

Abstract

ong-term changes in warm season (April?September) convective storm frequency over the northeastern United States (NEUS) and the environmental conditions favoring such storms are explored from 1979 to 2010. Linear discriminant analysis (LDA) was used to create thresholds for predicting annual warm season convective storm frequency over various small regions of the NEUS by relating the convective precipitation fields from the North American Regional Reanalysis (NARR) and the Climate Forecast System Reanalysis (CFSR) along with reflectivity data from the National Operational Weather Radar (NOWrad) archive at 2-km grid spacing from 1996 to 2006 to convective parameters in the reanalyses. On average, convective frequency is greatest across inland areas of the NEUS, particularly southern Pennsylvania, with a sharp decrease along the immediate coast. Across western Pennsylvania convective storm frequency has significantly (p < 0.01) decreased from 1979 to 2010, while closer to the coast convective frequency has increased slightly. There has also been a corresponding trend in warm season convective precipitation amounts, with decreasing amounts over inland Pennsylvania and increasing amounts near the coast. This general pattern of inland decreases and coastal increases is largely related to trends in low-level instability, which are attributable mainly to changes in low-level moisture. Analyzing convective parameters over small regions is an important consideration for future climate studies of convection, since using a single LDA threshold over a region encompassing a large portion of the NEUS failed to capture significant spatial differences in convective frequency and was substantially less accurate than using separate thresholds for smaller regions of the NEUS.