Low Mississippi River Levels to Continue

By , Expert Senior Meteorologist
September 13, 2012; 7:00 AM ET
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A lowering Mississippi River allows sand bars to emerge near Greenville, Miss., Tuesday, Aug. 21, 2012. Officials with the U.S. Army Corps of Engineers say low water levels that are restricting shipping traffic, forcing harbor closures and causing towboats and barges to run aground on the Mississippi River are expected to continue into October. (AP Photo/Rogelio V. Solis)

There is the potential that much of Isaac's rainfall may get "hung out to dry." However, there is some hope of rising levels before the end of the year, thanks largely from lower evaporation rates.

While Isaac's rains did very little to impact Mississippi River levels, it has prepped the soil in some areas for future rain and positive impact on Old Man River.

According to Expert Senior Meteorologist Jim Andrews, "The recent small spikes in river levels in areas that were affected by Isaac and small, lesser rainfall events within recent weeks are a sign that the soil in these locations is ready to shed rainfall rather than continue to absorb most of it."

About 58 percent of the flow over the lower Mississippi is driven by the Ohio River. Most of the remaining flow is split by the upper Mississippi itself and other rivers such as the Missouri and Arkansas.

"At least the Mississippi River level situation shouldn't get much worse into the end of the year, but we need the rains to keep coming over the Upper Midwest and to expand westward over the Plains for agriculture and river navigation for next year," Andrews said.

Delays in traffic, dredging operations and lighter-than-average barge loads will continue among vessels navigating the Mississippi River into the autumn. Alternative, supportive shipments will be made by less efficient, more costly means, such as highway, air and rail.

On average, over much of the Central states, stream and river levels are generally at their lowest point during the late summer and early fall.

Typically, low evaporation rates later in the fall and winter, when combined with average precipitation, begin to increase run off, slowly replenish the ground water and reverse the trend of falling stream and river levels.

Soil moisture conditions have improved over the Midwest, but the drought remains extreme to exceptional over much of the central and southern Plains.

According to Paul Pastelok, head of AccuWeather.com's Long Range forecasting department, "Compared to last winter, we don't expect the extent and magnitude of extreme warmth and lack of snow over the Mississippi Basin this coming winter."

However, exactly how all of this will play out over the individual areas of the basin is challenging.

The Mississippi Basin is huge and tremendous differences are to be expected from north versus south and east versus west. There will continue to be drought-related problems on area rivers into next year.

Pastelok and crew are expecting the southern branch of the jet stream to play more of a role this winter (versus last) over the Gulf Coast and part of the eastern third of the nation, by supplying more moisture to these areas. Some of that moisture would reach part of the Ohio Basin.

"We are not expecting many big, blockbuster storms over the Plains, which would be a huge shot in the arm for low levels on the upper Mississippi, Missouri and Arkansas rivers into next year," Pastelok said, "We do expect near- to perhaps above-average winter precipitation from roughly Louisiana to Kentucky."

Pastelok added that modest storms from western Canada would bring some shots of light to moderate snow over the Upper Midwest.

"This winter, cold shots in the Upper Midwest would not be long-lasting, but they would occur more frequently than last winter," Pastelok said, "That could help to trim ongoing above-average warmth around the Great Lakes during the winter."

Above-average warmth will continue over much of the Midwest this fall.

AccuWeather.com is expecting to release its full Winter 2012-2013 Winter Forecast to the general public during the first week of October.

An early wild card in the river levels would be if another slow-moving tropical system would drift up from the Gulf of Mexico sometime during the next six weeks, or a single strong, non-tropical system were to spend some time over the Central states later in the autumn. Such an occurrence would exploit Isaac's rainfall.

However, to say that another heavy rainfall event in Louisiana and Mississippi within the next six weeks is unwanted would be a huge understatement in the wake of Isaac flooding.

In the short term, spotty rain is forecast to fall over the Plains and the Upper Midwest late this week.

Another rainfall event could gather in southern areas next week.

Both systems by themselves are not likely to bring heavy rain over critical areas and cause significant rises on the Mississippi River. That would take rain on a regular basis across the north.

At least weighing out anticipated precipitation in the various basins and lowering evaporation rates, river levels on the Mississippi should stabilize, if not begin a slight recovery going into the winter.

Based on climatology and the expected weather pattern this winter, there should be a moderate rise on the upper and lower Mississippi and the Ohio rivers next spring.

Low flow and low river levels will continue over the Mississippi River from St. Louis to the Delta well into the fall, but according to National Weather Service hydrologists, record low levels should not be seen in most locations. In other words, the river level is bottoming out.

Individual locations will continue to experience fluctuations, perhaps dipping slightly lower than what was experienced during this summer, but most locations should not reach levels set spanning the 1930s to 1988.

Typical Rise and Fall Cycles of the Mississippi

In the spring, runoff from melting snow up north, combined with rainstorms usually has the major rivers running full-steam.

During the summer, thunderstorms, most notably thunderstorm complexes, over the Plains and Midwest generally keep the rivers flowing, although off their peak from the spring. The large-scale downpours are generally enough to compensate for high-evaporation rates.

During the autumn, the Central states tend to experience diminishing thunderstorm frequency. While winter-style storms, loaded with moisture, tend to increase, they are still scarce. River levels often bottom out during the autumn.

In the winter, northern areas build snow cover. The snow pack can gather more moisture, if rainstorms that follow a particular storm fail to melt all of the snow. In the southern areas, occasional rainstorms and low evaporation rates allow soil moisture to build. River levels tend to recover slowly.

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