Snowpack melt is a concern this spring for the Platte River
June 6, 2011
LINCOLN, Neb. – The large snowpack over the Rocky Mountains is a big concern this spring for the Platte River, the University of Nebraska-Lincoln state climatologist says.
But what makes that concern even bigger is that not much of it has started to melt yet and when it does start to melt, it’s difficult to predict how quickly it will happen. Al Dutcher, state climatologist at UNL, says runoff from Rocky Mountain snowpack is now projected at nearly 2 million acre feet of water – an increase of 500,000 acre feet from a projection a month ago.
“If it remains cool over the Rockies, snow melt will be slow. However, a major fear is that an extended rain event falls on the snow or conditions turn hot over the mountains and ramps up runoff,” Dutcher said.
If that happens, there could be some strong inflows and flooding, he said.
This week the Central Nebraska Public Power and Irrigation District plans to step up releases from Lake McConaughy to 4,000 cubic feet per second.
Currently inflows are expected to come in at 7,000 cubic feet per second with outflows from 2,200 cubic feet per second to 4,000 cubic feet per second. The Platte River height at North Platte is at 7 feet – the local flood level is 6 feet.
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“This most likely will exceed record stage set in 1971, which was 7.1 feet,” Dutcher said.
In addition, reservoirs upstream in Wyoming – Glendo, Pathfinder and Seminoe – are releasing water. While reservoirs upstream were full last fall, Seminoe currently is down to 38 percent of capacity in anticipation of the snow melt, Dutcher said.
Inflows into Seminoe are now averaging 5,000 cubic feet per second and will likely increase to 15,000 cubic feet per second within the next 20 days. At that rate, it would take less than two weeks to completely fill Seminole at its current level.
While everything is being done to minimize flooding, there is a concern with South Platte River flows.
“We expect some substantial inflows to materialize within portions of that basin within the next month,” Dutcher said. “Those increased flows will only compound the flood potential east of McConaughy.”
Another concern is how much rain the state will see this spring. Dutcher said models have shown an above normal precipitation trend.
However, precipitation trends have been spotty. For example, last week’s storm brought Lincoln only about an inch, while Waverly saw around 3 inches.
Heavy rains are always a concern for crops, especially during planting season.
Right now farmers are hoping rains hold off for planting.
“Rains are important, but we’d like to see them spaced out a bit,” he said.
In addition, there still are some very significant long-term deficits in soil moisture, especially in areas across south central and southeast Nebraska.
“We tend to forget how dry it was last fall and in March,” he said.
While forecasts are showing above normal precipitation, Dutcher cautions that there has been significant variance with forecasts so far this spring.
“It seems we’ll get two to four week stretches with well below normal precipitation only to be followed by a wet period,” he said.
Precipitation balance is a major concern. However, May and June typically are the state’s wettest months, representing one-fourth of the state’s annual precipitation.
“We need to see normal precipitation in this time period,” Dutcher said. “That can help alleviate lingering drought concerns in regards to long term dryness impacting soil moisture reserves now that the majority of the state’s corn crop is in the ground.”
Another concern this spring is La Nina. While the event is still expected to reach neutral status in June, it could happen within the May/June/July period.
“That being said, there could still be residual impacts even after sea temperatures return to normal,” Dutcher said. “Because the atmosphere is still showing signs of moderate La Nina, conditions at it could take up to a month for the atmosphere to return to neutral conditions.”
High winds also are likely to continue, but weaken as spring progresses. There also will remain a trend for severe weather this spring with large tornadic outbreaks.