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Hurricane Isaac storm surge reversed flow of Mississippi River

Hurricane Isaac was only a Category 1, but its storm surge and slow pace led to inland flooding and reversed the Mississippi for 24 hours. Scientists are working to better forecast these effects. 

By Staff writer / August 31, 2012

High winds from hurricane Isaac toppled signs and caused flooding and power outages in New Orleans Wednesday.

Ann Hermes/The Christian Science Monitor

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New Orleans

As hurricane Isaac reached southeastern Louisiana as a Category 1 storm earlier this week, it did something unusual to the Mississippi River: It threw the river into reverse.

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For nearly 24 hours, according to the US Geological Survey, Isaac's storm surge drove upriver at a pace nearly 50 percent faster than the downstream flow. This backflow produced a crest some 10 feet above the river's prestorm height at Belle Chasse, La., in flood-beleaguered Plaquemines Parish southeast of New Orleans. The surge added eight feet to the river's height at Baton Rouge, father north.

Isaac had help. A scorching, rain-starved summer in the middle of the country sent river levels to lows not seen since a similar drought struck the region in 1998, easing the Mississippi's flow.

Still, reversing the Mississippi is no mean feat, and it points to the importance of understanding more about the effects of tropical cyclones beyond wind speeds. While many people still tend to focus on hurricane-intensity categories, scientists are working to provide accurate and easily available daily forecasts of other effects, such as storm surges and inland flooding, in regions subject to tropical cyclones.

Isaac has provided a timely reminder of the need for more comprehensive forecasts. Two fatalities in Braithwaite, La., near New Orleans, have now been tied to Isaac after floodwaters topped a levee in Plaquemines Parish. In addition, officials in Mississippi have attributed two deaths to Isaac.

Isaac's reversal of the Mississippi – and the resulting inland flooding – is the kind of effect scientists are striving to capture in new models.

One effort was triggered when hurricane Floyd did the same thing in 1999. Floyd's surge sent water coursing up rivers in eastern North Carolina, even as rain from the wide storm was hitting already saturated ground throughout the watersheds that fed the rivers. When water moving downstream met water flowing upstream, the combination had nowhere to go but out across the coastal plain.

Under development during the decade since, the Coastal and Inland Flooding Observation and Warning Program (CI-FLOW) forecast tool links measurements of rainfall and water flowing downstream with measurements and forecasts of tides, winds, waves, and storm surge. 

A prototype system has been operating in North Carolina since 2010 and checked out reasonably well when "hindcasting" coastal surge and flooding during tropical storm Nicole in 2010, says Heather Grams, a research assistant at the Cooperative Institute for Mesoscale Meteorological Studies at the University of Oklahoma in Norman and a member of the CI-FLOW team. 

When Irene traveled up the East Coast last year, the researchers ran it during the storm, and "we were pretty happy with its performance," she says.

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