Rainfall late this week into the weekend will further delay U.S. corn plantings that already have fallen to a record slow pace, an agricultural meteorologist said on Thursday.

"We're about to see delays in plantings once again," said Don Keeney, meteorologist for MDA Weather Services.

Keeney said the warmer and drier weather early this week is being replaced by rain and cooler temperatures later in the week, with the heaviest downpours of up to 3.5 inches expected in the northwest Midwest.

"They will have four or five days of delays," he said. "Then it will be drier and warmer late next week, but showers will move into the eastern Midwest."

Warmer and drier weather in many areas of the U.S. Corn Belt early this week allowed farmers in the fields, and very rapid planting progress is expected to have been made. Some observers are expecting seedings to be half or 60 percent complete by the end of this week, but still at a record slow pace.

As of Sunday, farmers had seeded 28 percent of their intended corn acres, up from 12 percent a week earlier but far behind the five-year average of 65 percent, the U.S. Department of Agriculture said in a weekly report on Monday.

The planting pace for corn was the slowest for this point in the year in USDA records dating back to the 1980s, lagging 1984, when farmers had seeded 29 percent of their corn.

The figure fell below the average estimate of 29 percent from analysts' surveyed by Reuters ahead of the report.

For soybeans, the USDA said planting was 6 percent complete, up from 2 percent a week earlier. But the pace was the slowest for the 19th week since 1984, when soybeans were only 4 percent seeded. The five-year U.S. average is 24 percent.

U.S. corn yields are unlikely to reach their full potential this year as the slowest planting pace on record shortens the growing season, increasing risks that plants will pollinate under peak summer heat, agronomists said on Tuesday.

"We have taken some off of our yield potential," said Emerson Nafziger, extension agronomist at the University of Illinois. "Our preference is to have it in the ground by May 1."

Nafziger said that based on the last six years of the university's lab results for Illinois, corn planted after May 10 in the state, which ranks second in production of the crop, may see a yield loss of 6 percent. The yield losses increase to 12 percent after May 20 and 20 percent after May 31, he added.

Corn grown in the U.S. Midwest grain belt typically starts pollinating in July. Plant growth and yield potential can be reduced if plants are forced to devote energy to staying cool during the hottest days of summer.

Chicago Board of Trade corn futures were trading firm on Thursday due to the slow seeding pace of the U.S. crop and related concerns about production declines due to the hampered plantings. (Additional reporting by Julie Ingwersen, Karl Plume, Michael Hirtzer and Tom Polansek in Chicago; Editing by Lisa Von Ahn)