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.
Total precipitation during the seven-day period ending on Oct. 2. Credit: NOAA.
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.
Rare Atmospheric Setup
The meteorological dynamics that resulted in the heavy rains as well as strong winds in coastal locations were complex and extremely uncommon. In fact, Stu Ostro, a veteran senior meteorologist at The Weather Channel, called it “one of the most fascinating systems” of his career.
The deluge originated deep in the tropics, where a broad area of low pressure — which spawned the short-lived tropical storm Nicole — brought heavy rains to Cuba and Jamaica. An usually sharp dip in the jet stream across the eastern U.S. resulted in southerly winds aloft, blowing up the eastern seaboard — and these winds carried moisture from the region of disturbed weather in the Caribbean all the way northward, and eventually northeastward, so that the moisture plume extended from the Caribbean to Newfoundland. A stationary front draped itself along the eastern seaboard, providing a channel for the tropical moisture to travel along and act a trigger for enhanced precipitation.
Total Precipitable Water over the ocean on September 30. This image shows a plume of tropical moisture (orange/red hues) extending from the Pacific Ocean to the Caribbean Sea, and up the East Coast of the U.S. Blue colors are regions of drier air.
Credit: University of Wisconsin.
The heavy rainfall was especially noteworthy since it occurred in the absence of a land falling tropical storm or hurricane. In Wilmington, for example, most of the other top rainfall records were the result of tropical cyclones. In fact, this rainfall event eclipsed records set in 1999 during Hurricane Floyd, which devastated North Carolina with severe flooding. Thanks to a preexisting drought this year, the flooding was not as nearly as extensive.
Climate Change: More water vapor brings more heavy precipitation events
Could climate change have played a role in contributing to the severity of this extreme precipitation event?
Research into earth’s changing climate shows that as the climate has warmed during the past century, there has already been an increase in intense precipitation events, and computer model projections show this trend is likely to continue in the future. Scientists' confidence in this prediction is very high, because it is supported by simple physical arguments, model results, and observations.
The underlying reason why a warmer climate is causing more extreme precipitation events is actually quite simple: a warmer atmosphere can hold more moisture. Higher moisture content, all else being equal, implies more intense precipitation.
Observational studies show long-term trends towards more extreme precipitation in many regions. In the continental US, for example, overall precipitation in many regions increased during the 20th century, solely as a result of increases in extreme precipitation events (i.e. there was no increase in mild or moderate precipitation events).
There is some controversy, however, regarding how to characterize the relationship between climate change and individual extreme weather events, such as this heavy rainfall event.
Phil Duffy, Climate Central's chief scientist, put it this way: “Despite the robust relationship between climate change and extreme precipitation, it is not possible to definitively attribute this week's storm (or any single storm) to climate change. While climate change may increase the odds of extreme precipitation events, any specific event might have happened anyway (i.e. without climate change).”
However, other researchers, such as James Hansen of NASA and Kevin Trenberth of the National Center for Atmospheric Research, are a bit more bullish on the connection between climate change and certain types of extreme weather events. In an email report yesterday on recent global temperature trends, Hansen wrote:
“The standard scientist answer is “you cannot blame a specific weather/climate event on global warming.” That answer, to the public, translates as “no”. However, if the question were posed as “would these events have occurred if atmospheric carbon dioxide had remained at its pre-industrial level of 280 [parts per million] ppm?”, an appropriate answer in that case is “almost certainly not.” That answer, to the public, translates as “yes”, i.e., humans probably bear a responsibility for the extreme event.“
“In either case, the scientist usually goes on to say something about probabilities and how those are changing because of global warming. But the extended discussion, to much of the public, is chatter. The initial answer is all important. Although either answer can be defended as “correct”, we suggest that leading with the standard caveat “you cannot blame…” is misleading and allows a misinterpretation about the danger of increasing extreme events.”