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The James River just east of Mitchell is completely covered in ice and snow on Wednesday, Feb. 27, 2019. Laura Edwards, SDSU Extension’s state climatologist, said the National Weather Service predicted last week that there is more than a 50 percent chance of moderate flooding in the James and Upper Big Sioux rivers and of major flooding in the Lower Big Sioux. (Matt Gade / Forum News Service)

Above-average river runoff expected in southeastern South Dakota

MITCHELL, S.D. — As February ends with average temperatures in the area still well below historical averages, it will likely be at least a few more weeks before snow begins to melt.

But once that melting starts, the eastern and southern parts of the state can expect to see more river runoff than usual, thanks to both the snow that has piled up throughout the winter and the rain that left the ground wet in the fall.

As of last Thursday, Feb. 21, SDSU Extension predicted that abnormally cold temperatures will continue into early or mid-March, and that will likely impact the speed at which snow melts and how severe flooding becomes.

“Ideally, we like to see air temperatures kind of straddle freezing; a little thawing at day, a little melting at night, and a kind of gradual melt,” said Laura Edwards, SDSU Extension’s state climatologist. “As we hold on to the snowpack later and later into March, that kind of compresses, or contracts, our spring season. And the later you go into the spring, the higher the chance that when it does get warm, it’s all going to melt in a hurry.”

According to the National Weather Service, the average high and low temperatures for February in Mitchell are 33.6 and 13.9 degrees, respectively. This year, the average high for the month as of Wednesday, Feb. 27, was 19.3 degrees, while the average low was 3, and there have been 20.7 inches of observed snowfall this month — three times more than the average of 6.9 inches. The temperature has not risen above freezing since Feb. 3.

John Remus, chief of the Missouri River Basin Water Management Division, said that based on a Feb. 1 forecast, the Missouri River Basin north of Sioux City is expected as a whole to see average runoff this year, but that the southern part of the area, between Yankton and Sioux City, will likely get slightly more than average.

Flooding along the main part of the Missouri River, however, is not expected to be much of an issue once snow melts, Edwards said, in part because water can be moved through dams and reservoirs in Pierre, Fort Thompson, Pickstown and Yankton.

“The full flood capacity of the reservoirs is available, so if we do get runoff below the dams, we do have some capacity to cut back flows to lessen the flooding on the mainstream Missouri,” Remus said. “We’re in as good of shape as we can be in this time of the year.”

However, Remus said that the amount of snow on the ground in the eastern Dakotas paired with already wet soil from an abnormally rainy fall and significantly deep frost depths — the depth at which groundwater is expected to freeze in soil — means that there will be a significant amount of runoff in the James, Vermillion and Big Sioux rivers.

According to Edwards, the National Weather Service predicted last week that there is more than a 50 percent chance of moderate flooding in the James and Upper Big Sioux rivers and of major flooding in the Lower Big Sioux.

“It’s really just the plains, the eastern part of our state — Jim River, Big Sioux — that are really going to be our big problems here,” she said. “... There isn’t any control structure to hold back water, and it’s flat as a pancake, so it spreads out quite a bit, too.”

Edwards said that the flooding forecast will likely impede spring planting for farmers in the southeastern part of the state, many of whom are still dealing with last year’s rain.

“I think we’re going to see a lot of flooded fields,” she said. “It certainly will delay planting. There’s not going to be any early activity going on out there.”

While the snow on the ground has to melt eventually, and that can be used to estimate what this year’s flooding will look like, one unpredictable factor that will impact the amount of flooding is how much additional precipitation will fall in upcoming months.

“Our wild card is still what happens with rain in April, May and June,” Remus said. “Right now, the forecast from the National Weather Service is that it’s equal chances of above-normal, normal or below-normal precipitation. Nobody really knows exactly what’s going to happen.”

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