The heavy rains that inundated the East Coast during the last week of September were the result of a rare combination of ingredients--namely a wide-open, 3,000-mile-long pipeline of tropical moisture extending from the Caribbean all the way up to Maine and the Canadian Maritimes, a stalled frontal boundary, and multiple waves of low pressure that rode along that boundary. Many cities set new single and multiday rainfall records, and the rain led to severe flash flooding from North Carolina to New York State.Heavy rainfall events such as this one are an example of the sort of extreme events that are already becoming increasingly common as the climate warms due largely to the increasing concentration of greenhouse gases in the atmosphere, scientists say. Wilmington, North Carolina, set new records for one-day, three-day, four-day, and five-day rainfall, and September, which had previously been extremely dry, became the second wettest such month on record there. The one-day total of 10.33 inches, recorded on September 27, was between a 50 and 100-year event, according to a statistical analysis by Climate Central senior research scientist Claudia Tebaldi. This means that, in any given year, there is between a one in 50 and one in 100 chance of experiencing such an event. However, Wilmington’s five-day rainfall total of 22.54 inches was rarer than a 100-year event, Tebaldi said, meaning that there is less than about a one in one hundred chance that such an event would occur each year. Major cities along the East Coast also received very heavy rain, with Washington’s Reagan National Airport recording 4.66 inches of rain on September 30th, which was a new rainfall record for that day of the year. Such rainfall totals--when you look at record rainfall in any day of the year--can be expected to occur about once every ten years, Tebaldi found. In Baltimore, 6.02 inches of rain fell on the 30th, which was a new record for the rainiest day ever recorded during the month of September. This was about a 30 to 50-year event, Tebaldi said. Meanwhile, Philadelphia’s rainfall total of 5.39 inches over two days was about a ten to 20-year event.