Rainfall remains the wild card when it comes to worsening flooding along the Missouri River.
The river is so choked with water that worst-case scenarios are pointless to discuss, top officials of the U.S. Army Corps of Engineers have said.
“One thing that we've deliberately avoided here is talking about ‘What's the worst case?' Nobody knows what's the worst case,'' said Brig. Gen. John R. McMahon. “There's a ‘most likely' case and a ‘most dangerous' case. Those two scenarios keep evolving as we go through this.''
Widespread, heavy rain fell Monday and was expected again Tuesday across a significant portion of the Missouri River basin, said Bruce Terry of the U.S. Hydrometeorological Prediction Center. In the short term, this week's storms are forecast to drop 1 to 2 inches of rain in some areas and 5 inches or more in others, Terry said.
The full effect of that rain will be factored into the flood forecasts once hydrologists have a better sense of where and how much rain falls.
Drier weather is expected to prevail across most of the basin by midweek and then continue into the foreseeable future, he said. Scattered storms could occur, but nothing as widespread as during the first part of this week.
McMahon said releases from Gavins Point Dam could be increased but won't go below 150,000 cubic feet per second. Gavins Point is the dam that affects Missouri River flows in Nebraska, Iowa and points southward. Already, record releases from dam have led to significant flooding: Fort Calhoun Nuclear Station is in a cold shutdown because it is surrounded by water, portions of Interstate 29 and some bridges across the Missouri River are closed, and a half-dozen levee breaches and failures have occurred.
Climatically, the odds favor above-normal rainfall through the summer in the upper portion of the Missouri River basin, according to the U.S. Climate Prediction Center.
Corps of Engineers officials say there's no longer any point in talking about “how much rain the river can handle” because it's already out of its banks.
“When you fly over it, it's bluff to bluff (in some places),” McMahon said. “The historic flood plain has water in it.”
Flood forecasts by the Corps of Engineers take into account two definitions of normal rainfall, said Dave Pearson, a hydrologist for the National Weather Service. Pearson issues flood forecasts for eastern Nebraska and western Iowa.
At Omaha, for example, the corps projects a summer crest of between 34 feet and 36 feet. As a point of comparison, at 6 p.m. Monday, the Missouri River at Omaha stood at 33.89.
The lower number, 34 feet, is based on average rainfall, basinwide.
The higher number, 36 feet, is based on average rainfall at every point within the basin, thus a higher total rainfall amount than would normally occur, basinwide.
Neither projection takes into account extraordinary rainfalls.
Pearson said the big concern would be a series of heavy rains over a wide area.
Up and down the river, local officials have begun preparing for rain-related flooding. The City of Omaha is publishing maps on its website of where rainfall would pool, given that the city's drainage system is no longer fully functional.
Pearson said the metropolitan area's vulnerable drainage system has been taken into account in flash flood watches and warnings. Surprisingly, urban areas already are so prone to flash flooding that the impaired drainage system doesn't dramatically worsen things, he said.
“The flash flooding threat in places like Omaha is pretty similar. It's an urbanized area. It doesn't take much rain for it to start causing a problem.”
People should heed those warnings, he said.
“We don't issue those just to cover ourselves. We're definitely seeing things that raise the threat of flooding.”
In Omaha, homeowners and business owners have been advised to plan for flooding if their properties normally takes on water when it rains.
Pearson said he knows what he would do about his basement if he lived in a low-lying neighborhood in the metro area.
“I would take everything out; I just would,” he said. “With a high water table this long, you just don't know what's going to happen. ... Everything has a point where it can't take it anymore.”
It's clear that some of the City of Omaha's improvements to drainage are paying off.
Tim Barry, managing partner of the Hot Shops Art Center at 13th and Nicholas Streets, had a relaxed look about him Monday night as he watched the rain fall and sipped on a beer.
The gushing sewer water that was a problem last week — when it wasn't even raining — was not a problem Monday. No puddles, no sewer odor.
“Whatever they did, seems to be working. We're much more optimistic,” he said. “The smell is gone, and that's the best part.”
Regionally, the big rain-related flood concern is successive rounds of heavy rain, especially over a wide area, Pearson said.
“In terms of how high can it go, we've seen areas come up fairly quickly in some cases,” he said.
In Nebraska City, the river rose about 6 inches in a 24-hour period over the weekend, after a 1-inch rain fell in the immediate area, Pearson said.
At Rulo, the river shot up a foot after heavy rain fell in the vicinity of the gauge.
The worst-case rain-related threat in the Omaha metro area comes from flash flooding on the Papillion Creek system. The system feeds into the Missouri River at Bellevue.
The Papio-Missouri River Natural Resources District has projected that a storm over Omaha would result in $679 million in damage to building, roads and infrastructure — because of Papio flooding, not Missouri River flooding.
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Video from Nebraska City