MISSOURI VALLEY, Iowa (Reuters) - Unwelcome waves of thunderstorms dumped rain and hail in eastern Nebraska and western Iowa on Thursday and heavy rain fell in Montana this week that could make historic Missouri River flooding worse.
At Omaha, Nebraska, the Missouri River reached 30.4 feet overnight, the second highest level behind the historic 1952 flood, and federal officials said a levee near Hamburg, Iowa, sustained a third partial breach on Thursday morning.
Iowa officials said they were preparing to close parts of Interstate 29 from Sioux City to Council Bluffs by late Friday, including a section that runs by Missouri Valley, Iowa.
"Nobody knows how much we'll get," said Jeff Ellis, owner of several gas stations/convenience stores in Missouri Valley, Iowa, and across the river in Blair, Nebraska, of the flooding. "Two feet, zero feet, nobody knows. It's all a guess."
Slow moving storms dumped rain across Missouri Valley on Thursday afternoon, causing street flooding along U.S. Highway 30, which runs east-west through town. At an Ellis station, 15 people soaked to the skin heaved hundreds of sandbags.
"I grabbed a bunch of college kids and high school guys from Blair, good guys," Ellis said, shaking his head as rain poured down. "We're going to sandbag the front and the side."
Rain is forecast for six of the next seven days in the area around Omaha, but more across the river in Iowa.
"I'm sure to those who are monitoring how much water is being stored and released from reservoirs ... I'm sure any rainfall would be considered a concern to them," said Jim Meyer, meteorologist in charge for the National Weather Service in the Omaha area.
Meyer said a lot of the precipitation has already been factored into forecasts given to federal officials.
On Wednesday, the U.S. Army Corps of Engineers added to planned water releases from the Fort Peck Dam in Montana for a second consecutive day due to hard rains.
About 260 miles of the Missouri have been closed to boaters from Gavins Point Dam south to where the borders of Iowa, Nebraska and Missouri meet. That is near where sandbags were dropped by helicopter to patch a partial levee breach Sunday that threatens the town of Hamburg, Iowa.
The Corps does not expect the levee repairs to withstand the full weight of the Missouri and contractors are racing to raise a secondary levee to protect Hamburg by June 14, when peak water releases are planned from Gavins Point.
From Montana through Missouri residents have been shoring up levees along the Missouri River ahead of planned record water releases from its six major dams to relieve pressure on reservoirs swollen by heavy rains and melting snow.
Jail inmates and volunteers were placing sandbags around Ameristar Casino Hotel in Council Bluffs, Iowa, on Thursday and the parking lot of the former riverboat casino across the Missouri River from Omaha was flooded.
The river has six dams from Fort Peck in Montana to Gavins Point on the South Dakota-Nebraska border. It runs freely below Gavins Point the more than 800 miles to the Mississippi River, making that dam a focus for people along the Lower Missouri.
All of the dams are planned to reach maximum release rates by about mid June and hold them through at least mid August.
Up to five inches of rain fell in parts of Montana from Monday through Wednesday, forcing the Corps to increase planned releases to 60,000 cubic feet per second at Fort Peck and warn that the rate could be increased from there.
The Montana rains forced fresh evacuations due to overflows on tributaries, including in Nashua downstream of Fort Peck, and in flood-battered Roundup on the Musselshell River.
The Corps said the Garrison Dam reservoir above Bismarck, North Dakota, had capacity to handle the additional releases from Fort Peck and has left intact its planned maximum releases at 150,000 cubic feet per second from the other five dams.
The Missouri River forms the northwest portion of the Mississippi River basin that stretches from Montana to western New York and funnels water south into the Gulf of Mexico.
Thousands of South Dakota and North Dakota residents have voluntarily left homes in communities protected by hastily built sandbag barriers and levees ahead of peak releases.
On Thursday, officials cautioned that the sustained record flows could still threaten levees in the Dakotas now holding.
Corps officials also have warned that more breaches like the one near Hamburg are likely given the varying strengths of the flood protections and the height, speed and duration of planned releases of water from reservoirs.
Additional reporting by David Bailey in Minneapolis, James Kelleher in Chicago and Laura Zuckerman in Salmon, Idaho; Editing by Greg McCune and Jerry Norton