A Hard Rain's A-Gonna Fall

Researchers at the University of New Hampshire have analyzed 60 years' worth of National Weather Service rainfall records in nine Northeastern states and found that storms that produce an inch or more of rain in a day are coming more frequently. An increase in the frequency of extreme precipitation events
is one of the predicted impacts of a world warmed by heat-trapping
gases.

The researchers looked at several indicators of changing incidence of heavy rain events:

• Frequency of 24-hour periods when one inch of rain fell at a particular weather station site (a "one-inch event")
• Similarly, the occurrence of "two-inch events" and "four-inch events", when two or four inches fell at a site in 24 hours
• The frequency of extreme precipitation events, defined as the top one percent of 24-hour precipitation measurements for each year. "Changes in the threshold of the 99th percentile of daily accumulations exemplify changes in precipitation intensity" (how much rain has to fall in 24 hours to put an event in the 99th percentile for the year?)
• A third method was to define extreme precipitation events using recurrence intervals. They looked at the change in the amount of time between storms of a given magnitude.

According to each of the indicators studied extreme rainfall events have increased over the 50-year period. For 11 stations the records go back far enough to track such events from 1900 to 2007. For all of the indicators the  increases at those stations since 1948 were faster than for the whole period 1900 to 2007.

The increase in more-intense rainfall was correlated with increases in temperature seen over the period. This suggests that further increases in temperature will correlate to further increases in the occurrence of heavy rainfall events.

They also found that over the whole 50-year study period rainfall in the Northeast has an overall increasing trend of about three-quarters of an inch per decade.

The study concludes that communities are likely to experience increased flooding due to intense storms (as they have this year, for instance) and that planning and expenditure to minimize the impacts of flooding will be increasing drains on the public purse.

The report, Trends in Extreme Precipitation Events for the Northeastern United States 1948-2007, is available in PDF here.