Thunderstorm Rainfall in the Conterminous United States

Abstract

Thunderstorm rainfall amounts during the 1950?94 period were determined for 51 first?order stations distributed across the United States, and these values were assessed for seasonal and annual variations in both space and time. Thunderstorms produce 48% of the average annual precipitation received in the Mississippi River basin, which embraces 41% of the United States. Hence, the temporal and spatial variations in thunderstorm rainfall are major factors affecting most of the nation's water cycle. However, thunderstorm rainfall is only a small part of the total precipitation on the West Coast, typically less than 10% of the annual total. Thunderstorms maximize in the summer in most areas and produce 72% of the total summer rainfall that occurs east of the Rocky Mountains. Thunderstorm rainfall in the spring exceeds summer values in the southern plains and portions of California with 40% of the annual average storm precipitation, but spring values rank second in most areas of the nation. Thunderstorm rainfall amounts explain at least 50% of the variation found in annual total precipitation across large portions of the nation. Thunderstorm rainfall departures below average in the driest five years were found to closely match those of total precipitation deficiencies at all stations, revealing that the absence of storm rainfall is a major factor in droughts. Similarly, the magnitude of thunderstorm rainfall departures in the five wettest years of 1950?94 matched the magnitude of the total precipitation departures, revealing that thunderstorms have a sizable influence in producing extremely wet as well as dry years across the nation. The temporal distribution of thunderstorm rainfall during 1950?94 showed 10%?55% increases over time in most parts of the nation except the upper Midwest and a small portion of the Southeast. Increases were statistically significant in the northern high plains and intermountain area of the West. Trends of storm days with heavy rainfall were also upward across the entire nation, being sizable on the West Coast, Intermountain West area, and Northeast. The national pattern based on temporal shifts in thunderstorm rainfall is in agreement with that based on shifts in storm frequencies indicating that the temporal increases in storm rainfall were a result of more thunderstorms over time and more storm days with heavy rainfall.