How does National Weather Service predict river’s crest?

Published 12:02 am Wednesday, May 4, 2011

NATCHEZ — The process of coming up with the number that sent half of the Miss-Lou into a near panic Monday was no easy task.

The Lower Mississippi River Forecast Center, with an office in Slidell, La., used information from the U.S. Geological Survey, the U.S. Army Corps of Engineers, the National Weather Service forecast stations from Minnesota to Arkansas and the Tennessee Valley Authority to predict that the Mississippi River would crest at 65 feet in Natchez on May 22, NWS Service Coordination Hydrologist Jeff Graschel said.

Hydrologists looked at water levels in the Great Lakes, the Tennessee River, the Arkansas River, the White River, the Yazoo River and several other tributaries to the Mississippi.

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And finally, they looked at predicted rainfall amounts.

“Doing a forecast for the Mississippi River takes a tremendous number of agencies,” Graschel said.

The NWS forecast center then looks at rainfall gauge information on the ground to factor in what has already fallen and account for runoff.

All the numbers ultimately go into a hydrologic model to calculate the expected crest.

But the crest only accounts for rain that has already fallen and that predicted for 24 hours out. Additional rain can raise a crest, Graschel said.

“Rainfall really plays a key role,” he said. “If we don’t get rain, we feel like the numbers work out well.”

The NWS doesn’t boast a perfect record when it comes to predicting crests, but river watchers know they are usually pretty close.

For water levels near Natchez, it’s the Cairo, Ill., crest that matters, Graschel said, until this year.

When the U.S. Corps of Engineers blew out levees Monday night, re-routing the Mississippi around Cairo, it took away the trusted benchmark for the lower Mississippi Valley.

Now what matters most is what happens at New Madrid, Mo.

It’s too early to say how accurate the Natchez crest predication may be, Graschel said.

“As you get closer in time to the crest, then the accuracy should improve,” he said.

The forecast center reviews the crest every 12 hours during times of flooding, more if needed. They did push back the Natchez crest by two days late Monday.

The current crest — the highest in history and well above the 1937 record of 58.04 — is due to a number of factors, he said.

“On the Mississippi River we had snow melt in Minnesota and Wisconsin that had finally melted,” Graschel said. “It was making its way past the St. Louis area. There was a pretty good flow from the Ohio River, and then 10 to 12 inches of rainfall in the last (two weeks).”