Changes in Observed Daily Precipitation over the United States Between 1950-1979 and 1980-2009

Changes in observed daily precipitation over the conterminous United States between two 30 year periods (1950-1979 and 1980-2009) are examined using a 60-year daily precipitation analysis obtained from the CPC Unified Raingauge Database. Several simple measures are used to characterize the changes, including mean, frequency, intensity, and return period. Seasonality is accounted for by examining each measure for four non-overlapping seasons. The possible role of the El Niño Southern Oscillation (ENSO) cycle as an explanation for differences between the two periods is also examined.

There have been more light (1 mm ≤ P < 10 mm), moderate (10 mm ≤ P < 25 mm) and heavy (P ≥ 25 mm) daily precipitation events (P) in many regions of the country during the more recent 30-year period, with some of the largest and most spatially coherent increases over the Great Plains and lower Mississippi Valley during autumn and winter. Some regions, such as portions of the Southeast and the Pacific Northwest have seen decreases, especially during the winter. Increases in multi-day heavy precipitation events have been observed in the more recent period, especially over portions of the Great Plains, Great Lakes, and Northeast. These changes are associated with changes in the mean and frequency of daily precipitation during the more recent 30-year period. Difference patterns are strongly related to the ENSO cycle, and are consistent with the stronger El Niño events during the more recent 30-year period. Return periods for both heavy and light daily precipitation events during 1950-1979 are shorter during 1980-2009 at most locations, with some notable regional exceptions.