Additional rainfall late this week into the weekend will further slow corn and soybean plantings in the U.S. Midwest, threatening to reduce yield potential for the 2013 crop season, an agricultural meteorologist said on Friday.

"It's not the best of conditions, there will be more rain for the next two days with the heaviest southeast of a line from Kansas City to Green Bay," said John Dee, meteorologist for Global Weather Monitoring.

Dee said it would turn drier from Sunday through Tuesday but more rain is expected in the Midwest beginning next Wednesday and "we could see more showers next Friday into the weekend." The showers will be widespread and "continue to cause some issues," Dee said.

Commodity Weather Group (CWG) on Friday said showers would favor parts of Missouri, Illinois, Indiana and Michigan over the next two days, with a break from the rainfall in most of the western Midwest until Tuesday or Wednesday of next week.

"This may allow some limited late corn and soybean seeding in wet western parts of the corn and soybean belt, but the mid-week and late-week showers next week will still prevent any huge advances," said CWG meteorologist Joel Widenor.

U.S. farmers are struggling to plant their crops due to rainy conditions that delayed the tail end of corn seeding and pushed soybean planting to its slowest in 17 years, the U.S. Department of Agriculture said in its weekly crop progress report on Tuesday.

The slow seeding of both crops this spring has raised concerns about reduced yields at autumn harvest as key phases of crop development will likely be delayed until the heat of the summer. A late planting also increases the possibility of an early frost inflicting further damage on the crops.

The USDA said that corn planting was 86 percent complete as of May 26, up 15 percentage points from a week earlier.

The corn progress was down from 99 percent a year ago and behind the five-year average of 90 percent. But prospects were much improved from just two weeks ago, when muddy fields led to the slowest start on record for corn planting.

Farmers had finished 44 percent of soybean planting as of May 26, compared with 87 percent a year ago and the five-year average of 61 percent. It was the slowest pace for soybeans since 1996, when farmers had seeded just 35 percent of their crop by the end of May.

"The rains fell again across the state last week bringing planting progress to a halt," the Illinois field office of USDA's National Agricultural Statistics Service said in a report.

Analysts had been expecting corn planting to be 86 percent complete and soybean planting to be 42 percent finished, according to the average of estimates in a Reuters poll. (Additional reporting by Mark Weinraub in Chicago; Editing by Chizu Nomiyama)