Expect 4″ of rain in NYC at least one day a year

The National Climate Assessment came out last week, and used one striking number to distinguish climate changes in the Northeast and elsewhere in the U.S.: “between 1958 and 2010, the Northeast saw more than a 70% increase in the amount of precipitation falling in very heavy events (defined as the heaviest 1% of all daily events).”

from p. 47 of the National Climate Assessment

from p. 47 of the National Climate Assessment














This is a number worth unpacking, since it’s easier to make sense of rainfall as inches than as a percent of a percent. The raw data is not readily available, but I’ve approximated it using data with the same methodology from the same researchers from 2004. (Article here, my excel calculations here.)

Annual precipitation increased slowly over the twentieth century. It used to be that only 8% of that precipitation came in the top percentile of days with rain. Since since there is rain every 3-4 days, this means roughly 8% of the precipitation used to come on the wettest day of the year.

The 71% increase means that now 14% of that precipitation comes on the wettest day of the year.   So here in the Northeast we can expect one day a year with 106mm, or 4 inches of rain.  That’s an average for the reason, and since New York City is right in the middle of that region, it’s probably a reasonable figure for New York in particular.  That’s a lot of rain for city infrastructure to handle.

By comparison, Central Park had 5 inches of rain on April 30th, two weeks ago. And the wettest period of Hurricane Irene, even more rainfall-intensive than Hurricane Sandy, was 4 inches of rain in Central Park on August 28, 2011. These may have seemed like freak events at the time — but if current trends continue, New Yorkers can expect rains that heavy about once a year.

Year Annual precipitation Precipitation in very heavy events % of Precipitation in very heavy events
1955 (averaged) 750 mm 65 mm 8%
2012 (estimated) 780 mm 106 mm 14%
Increase +4% +64% +71%

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