This study diagnoses the seasonal frequency distribution of daily precipitation and related variables over the United States using the North American Regional Reanalysis. Regions where the seasonal mean precipitation is dominated by heavy and extreme daily events or by more consistent lighter events were identified and related to the variability of the vertically integrated moisture flux convergence (MFC) and precipitable water. The ENSO effects on the winter frequency distribution of precipitation are also discussed, and considered at the scale of U.S. hydrological basins.

Assuming that the frequency of daily precipitation can be approximated by a gamma distribution, the scale and shape parameters are useful proxies to estimate the observed frequency distribution of precipitation. During winter, most areas of the country with large mean precipitation have a strong contribution from extreme events, particularly California, and the West and Gulf Coasts. Over those same regions MFC winter patterns resemble closely those of precipitation, suggesting a close link between the two. During summer, the wettest areas of the country are Florida, where the mean precipitation is dominated by more frequent light and moderate rainfall days, and the central Plains, where the dominant contribution to the mean is from variable rains and more extreme events. Throughout the year, MFC mean and frequency distribution patterns strongly resemble those of precipitation, and areas with more heavy and extreme precipitation have more strong MFC daily values.

Changes in the winter total precipitation between the phases of ENSO are largely attributable to changes in the heavy and extreme events. Areas exhibiting differences in mean precipitation between the warm and cold ENSO phases, such as the Lower Missouri River basin (higher mean during warm phase) and the Ohio River basin (lower mean during warm phase), show an even greater change in extremes. Similar to precipitation, strong values of MFC are more sensitive to the ENSO phase than is the mean. While the ENSO variability of the frequency distribution of MFC shows a strong relationship to that of precipitation, the variability of precipitable water does not.