Shifting seasonality and increasing frequency of precipitation in wet and dry seasons across the U.S.

Water management, agriculture, and ecological systems are sensitive to the frequency and timing of precipitation. Here we document historical trends in these characteristics for station-specific wet and dry seasons over the U.S. from 1930 to 2009. Simulations based on Markovian precipitation occurrence models are used as a null against which to test observed trends without resorting to area averaging. Most regions display increases in precipitation frequency during both wet and dry seasons accompanied by a decrease in length of dry spells. Prominent increases (decreases) occur over the Central and Great Plains during the dry season. An exception is the Atlantic Plains, which experienced a decrease in frequency and an increase in dry spell length, especially during the wet season. Regionally consistent trends in the timing of wet and dry seasons are also evident, particularly over the Ohio (Missouri) River valleys where the dry season now arrives up to 2–3 weeks earlier (later).