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The rain across the Midwest and Great Plains that has closed refineries, flooded streets and snarled Mississippi River traffic shows no sign of abating, adding to the United States' wettest 12-month stretch on record.

Heavy showers will fall from Texas to the Great Lakes as a monthslong pattern of storms continues for at least the next week, said David Roth, a senior branch forecaster at the U.S. Weather Prediction Center. Tornadoes and hail also threaten the region.

"Days and days of rain — the pattern is not really changing," Roth said. As much 21 inches has soaked parts of Kansas, Oklahoma, Missouri, Arkansas and Louisiana in the past month, he said.

The 12 months ending in April have been the wettest yearlong stretch on record in the contiguous United States, according to the National Centers for Environmental Information. Parts of the Mississippi and other rivers have been at flood stage since last fall. The deluges have killed dozens of people, forced others from their homes and had severe consequences for agriculture, transportation and energy.

Only 49% of the U.S. corn crop had been planted as of May 19, trailing the five-year average, according to the Department of Agriculture. In Ontario, just 5% of fields have been seeded, said Trevor Hadwen, an agriculture-climate specialist with Agriculture and Agri-Food Canada in Regina, Saskatchewan.

"Planting across key areas of the Corn Belt has been seriously hampered," said Jim Rouiller, chief meteorologist at the Energy Weather Group. "The near-term to medium-range weather models point to this weather pattern persisting into mid June."

For many farmers, the length of the growing season is starting to enforce a natural deadline that could force them to choose different crops because fields are too wet for corn and soybeans. The forecast doesn't offer much hope.

"There isn't enough recovery time for the ground to dry out," Roth said.

Chicago, St. Louis, Dallas and Topeka, Kan., have all had above normal rain through May 23, according to the National Weather Service.

Flooding also curbed the flow of oil this week at America's largest supply hub in Cushing, Okla. HollyFrontier Corp. began closing two Tulsa refineries with capacity to process a total of 155,000 barrels a day.

Heavy rains have raised the prospect that the lower Mississippi will rise so high that the Morganza spillway in Louisiana will need to be opened in early June to mitigate flooding.