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

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

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.

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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.

The project is still about a decade away from turning the tool over to forecasters, she says. Among the steps along the way, the team would like to set up a second prototype in a different geographic setting – perhaps along the Gulf Coast.

Other researchers are looking at the potential vulnerabilities of counties deep inland.

Isaac is forecast to bring heavy rain into the Ohio River Valley and the central Appalachians, according to the National Hydrometeorological Prediction Center in Camp Springs, Md. Isaac's remnants could drop three to five inches of rain along its path, with some locations receiving up to eight inches. Initially, the rain could get a warm welcome from soils desiccated by drought, reducing the initial risk of flash floods, forecasters say. But that risk could increase as the ground becomes saturated.

Purdue University graduate student Dereka Carroll has developed a county-by-county map of the continental US and rated each county for potential tropical-cyclone flood risk. The data include not only meteorological and geographic elements, but also social factors such as income and education levels.

Some of the counties facing the highest potential risk are in Arkansas, largely because of a relatively high level of poverty and poor education in many areas, says James Done, a researcher at the National Center for Atmospheric Research in Boulder, Colo., who is guiding the work. Texas counties stretching along the Rio Grande River are also ranked as high risk because of the large Spanish-speaking population. For now, there are uncertainties about how well hazard information is communicated to and among the US Spanish-speaking population.

The next step, Dr. Done says, is to begin the field work in the high-risk counties to refine the rating. Ultimately, the team hopes to provide a continuously updated map that can help local emergency planners more effectively increase their communities' resilience to tropical cyclones.

For its part, the National Hurricane Center in Miami is working on ways to provide surge warnings independent of any tropical cyclone watches and warnings, according to center spokesman Dennis Feltgen.

Although the center issues advisories that include storm-surge information, many people still tend to focus on wind speeds. As a Category 1 storm with maximum sustained winds of about 80 miles an hour, Isaac was a low-end hurricane. But its large size and slow movement contributed to coastal surges as high as 15 feet.

In addition, surge from a tropical cyclone can inflict damage on coastal areas well outside formal storm watch or warning areas, he explains.

The hope is that breaking storm surge out as a separate forecast will help focus more attention on this related but separate threat.

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