Very heavy precipitation over the contiguous United States: Climatology, trends, and relationship with high streamflow and cloudiness

A comprehensive database of in-situ daily meteorological observations has been used to construct the climatology of very heavy precipitation events (e.g., those above 100 mm per day) and to track their changes during the past century over the contiguous United States. Different definitions of these events are considered and the trends in their frequency assessed. The pattern of these trends will be presented.

Changes in several components of the hydrological cycle over the contiguous United States have been documented during the 20th century including (a) an increase of precipitation and especially heavy and very heavy precipitation and (b) a significant retreat in spring snow cover extent over western regions during the last few decades. These changes have affected streamflow, including the probability of high flow. In the Eastern half of the United States, we found a significant relationship between the frequency of heavy precipitation and high streamflow events, both annually and during the months of maximum streamflow. Two factors contributed to finding such a relation: (1) the relatively small contribution of snowmelt to heavy runoff in the eastern United States (compared to the West) and (2) the presence of a sufficiently dense network of streamflow gauges available for analysis.

An increase in low cloudiness (first of all in the frequency of those types that characterize the convective boundary layer, e.g., Cumulonimbus and Stratocumulus types) during the past several decades (most prominently in the shoulder seasons, spring and autumn) can be considered as an indicator of increasing convective activity and (maybe) a lengthening of the “summer type” weather conditions. This corroborates with an increase of frequency and intensity of very heavy precipitation events over most of the contiguous U.S.