BLOOMINGTON, ILL. — Corn plantings in the 18 major states advanced quickly after a start slowed by cold, wet weather, adding 30 percentage points to the total planted during the week ended May 11, the U.S. Department of Agriculture said in its latest weekly Crop Progress report.

Gains from 29% planted by May 4 as the 18-state aggregate to 59% planted in the latest week even surpassed the five-year average of 58%, showing producers can make incredible progress when weather conditions turn favorable. Growers in Iowa, the nation’s largest corn producing state, planted 47% of their crop in the latest week, boosting planted area even with the five-year average at 70%. No. 2 Illinois at 78% planted was ahead of its average pace of 53%.

Planting progress, though, was not evenly distributed across all states. The very northern tier states of Minnesota, Michigan, North Dakota and Wisconsin were extremely delayed in their corn plantings because of continuing cold and wet conditions. In North Dakota, for instance, corn was only 3% planted as of May 11, down from 16% at the same date a year ago and 33% as the five-year average.

“You tell me the weather and I’ll tell you the story,” said Dale Durchholz, grains analyst at AgriVisor L.L.C. in Bloomington. He said the northernmost states were facing prevent-plant dates at the end of May for North Dakota or June 5 for Michigan, with an unclear picture of whether windows of planting opportunity will be available over the next few weeks.

Weather predictions in the very northern Midwest for the next two weeks tend to show warming into the 60s at least, up from current temperatures that, in some cases, have dipped to the high 20s overnight. Intermittent precipitation also is being predicted. If fields remain too wet or soil temperatures stay too low to plant corn, those states will not be able to catch up on planting corn in time.

That might not matter if these states planted very little corn. But the combined acres planted to corn in those states, reported in the U.S.D.A.’s March Planting Intentions, represent a significant portion of the overall crop: 18,250 acres out of a national total of 91,691 acres — almost 20% of the U.S. crop.

Mr. Durchholz said farmers in the northern Midwest will be making judgments on a week-to-week basis, trying to figure out if they should plant corn, switch to another crop such as soybeans with a later planting window, or take crop insurance, which requires that fields stay fallow.

But he emphasized there is wiggle room of at least 20 to 25 days after the prevent-plant date before actually planting a crop becomes unfeasible. He said that, if weather conditions are favorably warm and dry around the prevent-plant dates at the end of May and first week of June, farmers will go ahead and plant corn despite losing 1% of their crop insurance guarantee each day after the prevent-plant date.

“A producer can’t really take prevent plant until the 25-day grace period is over,” Mr. Durchholz said.