Shifts in Precipitation Accumulation Extremes During the Warm Season Over the United States

Precipitation accumulations, integrated over precipitation events in hourly data, are examined from 1979 to 2013 over the contiguous United States during the warm season (May-October). As expected from theory, accumulation distributions have a characteristic shape, with an approximate power law decrease with event size followed by an exponential drop at a characteristic cutoff scale s(L) for each location. This cutoff is a predictor of the highest accumulation percentiles and of a similarly defined daily precipitation cutoff P-L. Comparing 1997-2013 and 1979-1995 periods, there are significant regional increases in s(L) in several regions. This yields distribution changes that are weighted disproportionately toward extreme accumulations. In the Northeast, for example, risk ratio (conditioned on occurrence) for accumulations larger than 109 mm increases by a factor of 2-4 (5th-95th). These changes in risk ratio as a function of size, and connection to underlying theory, have counterparts in the observed daily precipitation trends. Plain Language Summary Extreme accumulations of rainfall over a precipitation event can damage infrastructure, impact transportation, and be hazardous to human lives. For each region, there is a characteristic accumulation size that controls the probability of the most extreme accumulations. We show that this characteristic size has increased in several U.S. regions over recent decades. This implies an increase in the probability of extreme accumulations, which is expected to further intensify under future global warming.