Over the past century (1910-2010) the United States has observed an increase in the occurrence of heavy precipitation events, based on rain gauge data collected by the National Oceanic and Atmospheric Administration (NOAA). One measure of this is the percentage of land area in which a greater than normal percentage of total annual precipitation has occurred from single day storm events. This percentage has risen from about 8-10% between 1910 to about 1990 to roughly 15% since 1990. (Figure 1)  More evidence of this is the steady increase since the 1950’s in the number of 2-day precipitation events that are exceeded only once in a five year period. The largest increases in precipitation seem to be in the Midwest and Northeast, while many parts of the West and Southeast have had decreases (Figure 2). The causes for this observed increase are believed to be the greater amount of water vapor that can be contained in the air as average temperatures increase, and the occurrence of more intense weather systems.
Naturally, one would think that as a result of more extreme precipitation events in the U.S., flooding, especially flash-flooding, would also show an increase. Even though stream flows in the U. S. have increased since the 1940’s, peak stream flows that would be associated with flooding events have not had a similar increase. Further research is needed to reconcile this difference, but research is handicapped by the short period of record for stream flow gauges (for example, only 30% of stream flow gauges have been in existence beyond 25 years in the south and Middle Atlantic regions).
Figure 1. Percent of land area in the 48 contiguous states affected by extreme one-day precipitation events from 1910-2015.
Figure 2. Percent change in precipitation in the United States from 1901-2015. (Source: NOAA 2016)
A rising trend in total precipitation has also been observed globally over land since 1901 using surface observations. (Figure 3)  A shorter period of observations derived from both microwave and infrared satellite data used for the Global Precipitation Climatology Project (GPCP) has shown a similar increasing trend in precipitation over the wettest regions of tropical oceans (30N to 30S) since 1979, while drier Subtropical oceanic regions have seen the opposite trend.  Over tropical land areas however, there was no significant tendency. A more recent study that used precipitation reports from a long period (1930-2005) of island surface observations showed good agreement with the satellite (GPCP) studies over tropical oceans. 
Figure 3. Precipitation anomalies (inches) over land worldwide (1901-2015).
Projections of precipitation extremes through the year 2100 have been obtained from a global 20 km general circulation forecast model, assuming an approximate doubling of the CO2 concentrations during the period. The results show that many regions that are currently wet (i.e., South Asia, the Amazon, West Africa, and the northeast U.S.) will experience a significant increase in heavy precipitation, while regions that are currently dry (South Africa, south Australia, and the southern Amazon) will suffer even further decreases in rainfall.  (Exceptions are extremely dry areas such as Antarctica, Sahara, and Tibet where the length of dry spells will decrease). These results are consistent with the trends seen over the last century in surface observations and more recent (since 1979) satellite data described above.
Recent analysis of trends in inundation frequency for four National Weather Service flooding categories using twenty years of data from more than 2,000 river gauges suggests that flooding will increase in the northern U. S., while decreasing in the southern U. S. . Another source of data used in the study is from a NASA satellite that measures ground moisture or “basin wetness.”
In summary, while there will be significant regional differences in the observed response of precipitation to an increase in global temperatures (and thus, SST), it is expected that there will be an increase in rainfall extremes through the end of this century. These tendencies have already been observed in surface and satellite observations within the past 100 years.
- U. S. Environmental Protection Agency (EPA), 2016: Climate change indicators in the United States, 2016. Fourth edition. EPA 430-R-16-004. http://www.epa.gov/climate-indicators
- Takahashi, K. and co-authors, 2006: Trends of Heavy Precipitation Events in Global Observations and Reanalysis Datasets. Scientific Online Letters on the Atmosphere (SOLA), Vol. 2, pages 96-99.
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- Kamiguchi, K., and co-authors, 2006: Changes in Precipitation-based Extreme Indices Due to Global Warming Projected by a Global 20-km-mesh Atmospheric Model. Scientific Online Letters on the Atmosphere (SOLA), Vol. 2, pages 64-67.
- Slater, L. and G. Vallarini, 2016: Recent trends in U. S. flood risk. Geophysical Research Letters. 43. doi:10.1002/2016GL071199
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