Tennessee's Record Flooding a Sign of Things to Come

The devastating rain and flooding that recently paralyzed Nashville is just the sort of extreme precipitation event that global warming is expected to make more common.

The same weather conditions that produce such events could indicate a rough hurricane season ahead.

Hydrologists from the U.S. Geological Survey (USGS) were out in force during and after the Tennessee storm trying to gather whatever data they could from stream flow gauges that were sometimes underwater or even washed away by the deluges of May 1 and 2.

"This was such an extreme event," said Rodney Knight, a surface water specialist with the USGS Tennessee Water Science Center. Knight lives in the Nashville area. "It was just surreal to see. We were driving boats in the tree canopies."

Rivers in the middle of Tennessee crested on May 2 and 3, breaking records, reported Knight. The highest flows were observed from Nashville west toward Jackson, extending about 40 miles north and south of Interstate 40 and affecting major streams feeding the Cumberland and Tennessee Rivers.

Flows on the Harpeth River exceeded 46,000 cubic feet per second (cfs) on May 3 -- and that was a day after the stream peaked 4 feet higher. This breaks the previous record of 40,000 cfs, set in 1948. The Duck River near Hurricane Mills flowed at 138,000 cfs on May 4, exceeding the previous high by 17,000 cfs.

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The flows surprised even the experts, Knight said. They were greater than any ever seen before, including the floods of 1975, 1927 and any other event since the records started being kept in the 1880s.

"Several of these estimates were based on the last observations received by satellite before our stream gauges were submerged," Knight said.

While the new heights and breadths of flooding will likely change FEMA flood maps for Nashville and surrounding areas, the extreme conditions may also be instructive in other ways -– specifically what kind of storms to expect in a warmer world.

"The Tennessee storms are just the latest in a sequence (of severe weather events) this year that includes Washington, D.C., and New York," said climate researcher Kevin Trenberth of the National Center for Atmospheric Research.

All the extraordinarily wet snow and rain events match a pattern that has been predicted by climate modelers. That pattern is based on some pretty basic physics: For every one degree Fahrenheit that the atmosphere warms, the water-holding capacity for the atmosphere increases 4 percent, Trenberth explained. That means 4 percent more water vapor over the oceans.

"Now these storms we get in the spring reach out 1,500 to 2,000 miles to get moisture," said Trenberth. They reach right out over the oceans where all that extra water-laden air is. This moisture is fed into the storms that normally come across the country, making them wetter and more intense. "This moisture invigorates the storms."

This temperature/moisture content issue is of special concern for the remainder of 2010, said Trenberth. The air temperatures over the tropical and subtropical Atlantic Ocean are currently reaching all-time highs. "This is something that we're looking at as we come into the hurricane season," said Trenberth.

It's no guarantee of a harsh hurricane season ahead, but it's one powerful ingredient.