Report: Extreme Downpours Up 33% in Virginia

July 31, 2012 5:21 pmBy: 
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Flooding near Rts. 123 and 50 in Fairfax in 2011 (Photo by Nick L’Italien)

Less than one month after extreme storms devastated Northern Virginia, an Environment Virginia report confirms that extreme rainstorms are happening 33 percent more frequently in Virginia since 1948.

“As the old saying goes, when it rains, it pours—especially in recent years as bigger storms have hit Virginia more often,” said Laura Kate Anderson, Field Organizer for Environment Virginia. “We need to heed scientists’ warnings that this dangerous trend is linked to global warming, and do everything we can to cut carbon pollution today.”

Based on an analysis of state data from the National Climatic Data Center, the new report found that heavy downpours that used to happen once every 12 months on average in state now happen every 9 months on average. Moreover, the biggest storms are getting bigger. The largest annual storms in Virginia now produce 11 percent more precipitation, on average, than they did 65 years ago.

Scientists have concluded that the rise in the frequency and severity of heavy rainstorms and snowstorms is linked to global warming. Warming increases evaporation and enables the atmosphere to hold more water, providing more fuel for extreme rainstorms and heavy snowstorms.

Anderson pointed to Hurricane Irene in August of 2011 as an illustration of what more extreme rainstorms could mean for the state. That storm led to four deaths in Virginia and billions of dollars in damage.

The new Environment Virginia report examines trends in the frequency of and the total amount of precipitation produced by extreme rain and snow storms across the contiguous United States from 1948 to 2011. Using data from 3,700 weather stations and a methodology originally developed by scientists at the National Climatic Data Center and the Illinois State Water Survey, the report identifies storms with the greatest 24-hour precipitation totals at each weather station, and analyzes when those storms occurred. The report also examines trends in the amount of precipitation produced by the largest annual storm at each weather station.

30% nationwide increase

Nationally, the report found that storms with extreme precipitation increased in frequency by 30 percent across the contiguous United States from 1948 to 2011. Moreover, the largest annual storms produced 10 percent more precipitation, on average. At the state level, 43 states show a significant trend toward more frequent storms with extreme precipitation, while only one state (Oregon) shows a significant decline.

Key findings for Virginia and the Mid-Atlantic include:

  • Extreme rainstorms and snowstorms are becoming more frequent. Virginia experienced a 33 percent increase in the frequency of extreme rainstorms from 1948 to 2011. In other words, heavy downpours or snowstorms that happened once every 12 months on average in 1948 now happen every 9 months, on average.
  • Storms with extreme precipitation increased in frequency by 55 percent in the Mid-Atlantic during the period studied. The Mid-Atlantic region ranks 2nd nationwide for the largest increase in the frequency of storms with heavy precipitation.
  • The biggest rainstorms and snowstorms are getting bigger. The amount of precipitation released by the largest annual storms in Virginia increased by 11 percent from 1948 to 2011.

Environment Virginia was joined by Gem Bingol, Field Officer with Piedmont Environmental Council, at a press event at Raflo Park in Leesburg to release today’s report.

“Local decision-makers in Loudoun and elsewhere can help reduce the impact of those storms by focusing growth in locations that allow residents to reduce their need to drive everywhere,” said Bingol. “They can also help reduce the impacts of stormwater on residents by encouraging preservation of existing trees and vegetation, ensuring adequate set-back along streams to reduce flooding impacts, and requiring Low Impact Development, green roof, and other improved stormwater management practices.

“The insurance industry is aware of and responding to the increasing risks of extreme storms; local decision-makers and the general public also need to be informed and engaged in reducing the devastating impacts.”

But not a drop to drink

Anderson was careful to note that an increase in the frequency and severity of extreme rainstorms does not mean more water will be available for human use. Hotter temperatures fuel extreme rainstorms by increasing rates of evaporation. At the same time, however, that evaporation increases soil dryness.

Moreover, she said scientists expect that, as global warming intensifies, longer periods with relatively little precipitation will tend to mark the periods between heavy rainstorms. As a result, droughts are likely to become more frequent and severe in some regions of the United States. Currently, more than half of the lower United States is suffering through prolonged drought, aggravated by the fact that the last six months have been the hottest January-June period on record.

According to the most recent science, the United States must reduce its total global warming emissions by at least 35 percent below 2005 levels by 2020 and by at least 85 percent by 2050 in order to prevent the most devastating consequences of global warming. Environment Virginia highlighted two proposals from the Obama administration—carbon pollution and fuel efficiency standards for cars and light trucks through model year 2025, and the first ever carbon pollution standards for new power plants—as critical steps toward meeting these pollution reduction targets.

“How serious this problem gets is largely within our control – but only if we act boldly to reduce the pollution that fuels global warming,” said Anderson. “We applaud the Obama administration for their proposals to cut carbon pollution from vehicles and new power plants, and urge them to move forward with finalizing these critical initiatives this year.”

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