Extreme Long-Range Forecast— More Heavy Rain

7 years after Irene, Dartmouth researchers see a disturbing trend

From Maine to West Virginia, the Northeast has seen a larger increase in extreme precipitation than anywhere else in the U.S, according to a paper recently published by Dartmouth professor Jonathan Winter and Huanping Huang, a graduate student and the lead author of the paper. Prior research found that these heavy rain and snow events, defined as a day with about two inches of precipitation or more, have been 53 percent higher in the Northeast since 1996. The Dartmouth study finds that hurricanes and tropical storms are the primary cause of this increase, followed by thunderstorms along fronts and extratropical cyclones like Nor'easters.

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"Our study provides insight into what types of extreme storms are changing and why. We found that hurricanes were responsible for nearly half of the increase in extreme rainfall across the Northeast. A warmer Atlantic Ocean and more water vapor in the atmosphere are fueling these storms, causing them to drop more rain over the Northeast," explains Winter, an assistant professor of geography at Dartmouth and co-author of the study.

 "Other research has demonstrated that these two conditions have been enhanced in our warmer world," added Huang.

“The months of September and October contributed the most to the overall extreme precipitation increase, due primarily to an increased frequency of extreme events caused by tropical cyclones.Extreme events caused by fronts and extratropical cyclones also contributed substantially to the overall extreme precipitation increase, especially during the months of June, July, and March. Increased extreme precipitation from tropical cyclones is associated with warmer Atlantic sea surface temperatures and more water vapor in the atmosphere. Enhanced extreme precipitation from fronts and extratropical cyclones is coincident with a wavier jet stream and potentially connected to weaker west‐to‐east steering winds. By describing how, when, and why extreme precipitation has increased over the Northeast, this study can help a variety of stakeholders, including towns, emergency responders, and utilities, adapt to the current regime of extreme precipitation, as well as inform future work attributing this extreme precipitation shift,” conclude the authors.



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