"Nobody's panicking," said Nelson. "We could advance 20 or 25 percent this week by itself." The average amount planted at this time in the last five years was 65 percent, and it was at 85 percent last year.
Nelson expects corn futures to rise on the release, which was made while the market was closed at 4 p.m. ET. Corn settled in Monday's session at $6.55-1/2 per bushel, up 19-1/4 cents.
(Read More: U.S. Corn Soars on Planting Worries, Firm Cash Market )
The latest government forecast is for 14.1 billion bushels of corn to be harvested this year, well above the record 13.1 billion bushels in 2009. Last year's drought hit the corn crop hard, resulting in 10.8 billion bushels in what initially had looked like a record year.
This year, farmers have been racing to get the crop into the ground before May 15, considered to be the prime time to plant. A late crop is expected to yield less per acre, and farmers believe they lose a bushel per acre for every day it is planted after that date.
"Any extreme gains we make this week will only help catch up," Nelson said. "It will not get us back to normal. It will take at least two weeks of clear skies to get us caught back up to normal. Right now next week's weather isn't going to allow that to happen. We have above normal precipitation starting this weekend and into next week."
(Read More: Next 2 Weeks Are'Critical' for Corn)
But Nelson said a later plant means the crop will pollinate later in July than normal. That could be a plus since means the pollination process, which determines yield, would come after the hottest part of the summer, the first week of July.
Nelson said since 1980, there were only two years off to a slower start than this one. One was 1993, when 27 percent of the crop was in the ground, and the other was 1984 when the planting was at 26 percent.
Analyst Dennis Gartman, publisher of the Gartman Letter, said he expects farmers to make up for the late planting, and he still expects a bumper crop.
"I think first of all, if there's one thing you can bet upon in the United States, it is the ability for the American farmer to plant more crop than he ever dreamed possible when you get three or four or five days of decent weather," he said on "Fast Money."
Modern farming technology has made a huge difference and is a big reason why the crop could still be bumper-sized.
"These guys can get in the field and plant 24/7. They have GPS on their planting equipment. They run straight lines. You'll be surprised how much they can get in the ground. Never underestimate the ability of genetically modified corn to do better than anything we ever imagined a mere five years ago," Gartman said.
However, late planting also brings a late harvest, which has its own risks including wet late summer weather or an early frost.
The USDA forecast last week that the domestic corn crop would yield 158 bushels per acre, down from a forecast it made in February due to the slow start to planting.
RJ O'Brien research director Randy Mittelstaedt said the biggest single planting progress week was in 1992, when 43 percent of the crop went in one week.
(Read More: Record world crops on horizon, USDA says)
Analysts said if the corn corp is too difficult to plant in the next week, farmers could decide to switch some acreage to soy beans which can be planted later. The USDA expects a harvest of 3.39 billion bushels of soy beans, up from 3.015 billion bushels last year.
So far, six percent of the soy bean crop has been planted, compared to an average 24 percent at this time of year in the prior four years.
—By CNBC's Patti Domm