The swollen Pecatonica River spills into downtown Darlington, Wis., on March 14. (Dave Kettering/Telegraph Herald/AP)

While a historically snowy winter begins to wind down across parts of the Plains and Midwest, what threatens to be an equally historic spring flood season is now underway. From Nebraska to Wisconsin, as well as up and down rivers in the Plains and Midwest, over 10 million people are under flood warnings into the weekend in the wake of this week’s “bomb cyclone.”

Some of the most significant flooding thus far has hit eastern Nebraska. “Widespread and extremely dangerous flooding will continue today and tonight,” the National Weather Service office in Omaha wrote in an update Friday morning.

Since issuing that statement, the Weather Service itself has had to evacuate because of a dike failure on the Platte River.

In northeast Nebraska, the city of Norfolk remains under a large-scale evacuation on its east side.

Flooding there is caused by rivers like the Elkhorn rising to near-record levels and breaching levees in some spots. Weather.com reported that one-third of the city of 24,000 was evacuated. Water in that location is now subsiding, but only after a portion of the city was submerged.

Columbus, Neb., which sits at the meeting point of the Platte and Loup rivers in southeastern parts of the state has also been particularly hard hit. A farmer was killed there when his tractor was swept away while attempting to help rescue others from floodwaters. Several others are missing in the region.

Major ice jams on both rivers near Columbus led to a flash flood emergency in the area on Thursday. Officials were planning to place coal ash on the ice to help it melt and allow the rivers to recede once the natural and temporary dams were removed, but the jams broke up on their own before that plan was set into motion.

In recent days, as the bomb cyclone dispensed heavy rainfall, flooding focused on creeks and streams and small to intermediate-size rivers. Through the weekend, the most significant flooding is forecast to shift to the larger rivers in the region.

In the section of the Missouri River meandering through southeastern Nebraska to Iowa border, the Cooper Nuclear Station declared a “notification of unusual event” early Friday morning, signaling that the river had surpassed a height that causes concern.

According to a news release, the level of Missouri River at the location of the nuclear power plant had climbed to 42.5 feet, or 899.05 feet above sea level. The plant itself sits at 903 feet above sea level. The alert triggered initial precautions, and if the river reaches 45 feet the plant will be taken offline.

Forecasts from the Weather Service indicate that the water level at the plant may rise to about 45.5 feet this weekend, which would require a shutdown. In that event, electricity would be delivered from other sources. The last major threat to the plant occurred during historic summer flooding in 2011, but it was able to operate throughout.

Flooding on this portion of the Missouri River has already neared and surpassed record levels. While additional rises are not expected to be massive, the river may not crest until Saturday or Sunday in this area.

Water levels on many of the major rivers are expected to stay near record values into early next week before slowly subsiding thereafter.

Flooding is also common across Iowa, with numerous roads across the state closed because of high water. In central portions of the state, some homes have been evacuated near Otho, where some ice jams have also been reported.

Flooding is also common if somewhat less widespread in parts of Wisconsin, Minnesota, Michigan and South Dakota in particular.

The towns of Lodi and Darlington in Wisconsin have seen particularly impressive flooding. In Lodi, the mayor has said it’s “the worst he’s seen in his 71 years in the city.” Darlington is in the midst of its third flood in a year, which is expected to be the worst since 1993 in that location.

The flooding has been caused by a combination of factors.

Heavy rain associated with the bomb cyclone that moved through in recent days was a big catalyst. It was exacerbated by a now-melting hefty winter snow pack thanks to record winter precipitation. Ice jam blockades on rivers and still-frozen ground, which maximizes runoff, are only making matters worse.

Given the abundant precipitation through winter, and an active spring storm track through the nation’s midsection, the events of this week probably mark just the beginning of a long flood season.