Poor Forecasting of Tennessee Flood Blamed on Bad Data

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River forecasters for the National Weather Service underestimated the flood level that a major river would reach during Nashville’s fatal May floods because they relied on inaccurate data from the U.S. Army Corps of Engineers, the service said in a report.

The report found that forecasters ignored two models that showed more accurate flood predictions for the Cumberland River in favor of a model that relied on inaccurate and untimely information from the corps. The weather service continued to use that data despite observations on the ground that proved their flood predictions were inaccurate.

That meant the weather service failed to warn of major flooding in Nashville until after it had already taken place. The flooding on May 1 and 2 killed 22 people around the state and caused more than $2 billion in damage in Nashville alone.

The report also found that many residents did not respond to its warnings because they did not reflect the urgency of the situation.

“We can’t stop the rain, but we can and must do a better job at warning people of the potential for dangerous flooding,” U.S. Rep Jim Cooper, D-Nashville, said. “A few hours of warning could have saved lives and prevented millions of dollars in damage.”

According to the report, by early Sunday, May 2, and possibly before, officials at the weather service’s Ohio River Forecast Center were concerned about a possible major flood of the Cumberland River with water levels 45 feet and above at downtown Nashville.

Two other flooding simulations predicted major flooding, with a crest possibly reaching 54 feet. But after a conference call with the corps at 8:30 a.m., the weather service issued a public prediction of flood levels reaching only 41.9 feet, just below the “moderate” flood level. That prediction relied on the corps’ estimates of how much water it was releasing from dams.

As a corps assessment previously found, the agency was constantly adjusting the flow from the dams but not updating the weather service about changes.

The weather service report also makes clear that some of the flow data reported by the corps were not accurate even at the moment they were received.

On Sunday morning, the river exceeded the weather services’ predicted 41.9-foot crest in less than 2 hours. But the weather service was slow to update its prediction. It continued to rely on inaccurate and outdated information from the corps even though on-the-ground observations showed the river rising much more rapidly.

The river eventually crested at 51.86 feet in downtown Nashville at 6 p.m. on Monday. A more accurate prediction of a 51.5-foot crest did not come until 4 a.m. on Monday. A 52-foot crest was not forecast until 3:42 p.m., according to the report.