Report Sees Increase in Extreme Downpours for California, U.S.

Extreme rainfall events are happening more frequently today than 50 years ago and are producing more precipitation on average, according to a new report by the Environment California Research and Policy Center.

The report “When It Rains, It Pours,” looks at precipitation trends between 1948 and 2011, with a focus on extreme events. Nationwide, large rain or snowstorms that happened once every 12 months, on average, in the middle of the 20th century now happen every nine months. Moreover, the largest annual storms now produce 10% more precipitation, on average.

Though the most pronounced changes were seen in New England, the mid-Atlantic region and the Midwest, the report cites California as one of 43 states to see statistically significant increases.

Overall, California experienced a 13% increase in extreme rainstorms and snowstorms between 1948 and 2011. The rise in extreme rain events is not evenly distributed in the state, however. Southern California and parts of the Central Valley are experiencing up to a 72% increase in the frequency of extreme storms with a 7% increase in the intensity of the storms as well. In other words, in the south, storms are happening more frequently and are bigger in size.  

  • Northern California is seeing a decrease in extreme precipitation events showing almost a mirror image to the south with areas like Redding, Santa Rosa, and Yuba City seeing a 29, 34, 42 percent respective decrease in the frequency of extreme rainstorms since 1948.  
  • The report attributed the nationwide rise in extreme storms to global climate change and noted that the increase has costly ramifications for the country, with the potential to cause more flooding that jeopardizes property and lives.

    With scientists predicting even greater increases in extreme precipitation in the years ahead, the report recommends the United States and other nations take action to reduce pollution that contributes to global warming.

    The report also notes that increasingly heavy downpours are expected to be accompanied by an increasing risk of drought. Higher temperatures brought by global warming will increase the rates of evaporation of moisture from the land and the ocean, reducing soil moisture while simultaneously fueling potential downpours with more water vapor.

    Warmer weather will also reduce snowpack and cause spring runoff to happen earlier, increasing the risk of water shortages in late summer, especially in the West, the report notes.