finding 7.2 : key-finding-7-2

Heavy precipitation events in most parts of the United States have increased in both intensity and frequency since 1901 (high confidence). There are important regional differences in trends, with the largest increases occurring in the northeastern United States (high confidence). In particular, mesoscale convective systems (organized clusters of thunderstorms)—the main mechanism for warm season precipitation in the central part of the United States—have increased in occurrence and precipitation amounts since 1979 (medium confidence).

This finding is from chapter 7 of Climate Science Special Report: The Fourth National Climate Assessment: Volume I.

Process for developing key messages: Based on numerous analyses of the observed record in the United States there is high confidence in the observed changes in heavy precipitation events, and medium confidence in observed changes in mesoscale convective systems.

Description of evidence base: The key finding and supporting text summarize extensive evidence documented in the climate science peer-reviewed literature. Numerous papers have been written documenting observed changes in heavy precipitation events in the United States, including those cited in the Third National Climate Assessment and in this assessment. Although station-based analyses (e.g., Westra et al. 2013e941a5b9-10b7-462b-9042-5760a82fc415) do not show large numbers of statistically significant station-based trends, area averaging reduces the noise inherent in station-based data and produces robust increasing signals (see Figures 7.2 and 7.3). Evidence of long-term changes in precipitation is based on analysis of daily precipitation observations from the U.S. Cooperative Observer Network ( and shown in Figures 7.2, 7.3, and 7.4.

New information and remaining uncertainties: The main source of uncertainty is the sensitivity of observed precipitation trends to the spatial distribution of observing stations and to historical changes in station location, rain gauges, and observing practices. These issues are mitigated somewhat by methods used to produce spatial grids through gridbox averaging.

Assessment of confidence based on evidence: Based on the evidence and understanding of the issues leading to uncertainties, confidence is high that heavy precipitation events have increased in the United States. Furthermore, confidence is also high that the important regional and seasonal differences in changes documented in the text and in Figures 7.2, 7.3, and 7.4 are robust.

This finding was derived from figure -.2: Confidence / Likelihood

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