U.S. corn futures are expected to start higher Tuesday on concerns declining condition ratings for the crop will translate into a smaller-than-expected harvest.

Traders predict corn for December delivery, the most actively traded contract, will open up 2 cents to 4 cents a bushel at the Chicago Board of Trade. In overnight electronic trading, the contract rose 2 3/4 cents, or 0.4%, to $6.77 1/4 a bushel.

Driving prices higher is a four percentage point cut in the U.S. Department of Agriculture's good-to-excellent rating for the corn crop from the previous week. The government, in a weekly report issued Monday, said 62% of the crop was in good or excellent shape, down from 72% a year earlier.

The decline, attributed to last week's intense heat, followed a three percentage point cut the previous week, raising concerns that poor weather will reduce output. Traders said USDA may need to cut its yield forecast of 158.7 bushels an acre, which is up from last year's yield of 152.8 bushels.

"The crop rating is a full 10 [percentage points] behind last year and the USDA is projecting a better yield this year, which is a bit hard to reconcile," said Tomm Pfitzenmaier, analyst for Summit Commodity Brokerage in Iowa.

Traders are paying close attention to the weather because farmers need favorable conditions to grow a large crop to replenish low inventories. Corn prices have pulled back nearly 15% since reaching an all-time high in early June on supply concerns. Yet, users of the grain remain uncertain about the outcome of the crop, which will be harvested in the autumn.

In Pennsylvania, the USDA slashed its good-to-excellent rating by 30 percentage points from last week, to 28%, because of recent heat and dryness. The rating has dropped 42 points in two weeks, a massive fall. Pennsylvania typically accounts for just 1% of national corn output, but its decline shows poor weather could derail farmers' attempts to harvest a big crop.

"While the worst weather is likely behind us, the potential for upside to the USDA's 158.7 bushel an acre forecast appears low," wrote analysts at Susquehanna International Group.

Milder weather across the Corn Belt this week should prevent condition ratings from dropping as steeply in the upcoming condition report, traders said. This week will not be as hot as last week, and some of the region has seen an improvement in soil moisture from rain, according to Freese-Notis Weather, a private forecaster.