Corn for December delivery jumped more than 3 percent Wednesday morning to a new nine-month high, after Tuesday's 5 percent gain took it to $6.24 per bushel. The price for new crop corn futures has jumped about 24 percent since the first of the month and 14 percent this week alone.

“We need some good, soaking rains within the next 10 days, or we really are talking some yield reductions,” said Randy Mittelstaedt, director of research at RJ O’Brien.

While other grains are also impacted, Dennis Gartman of the Gartman Letter points out that the corn crop is most at risk.

“For corn is tasseling…or soon shall be…and that is its most vulnerable period of the year,” he wrote. “That is when the crop is first ‘made.’ If tasseling does not occur properly, it shall matter not what happens to the corn crop thereafter; the crop will be lost, or greatly diminished.”

So the next two weeks are central to this crop. Right now the weather for the next five-ten days looks to be hot and dry; the worst possible conditions for making but the best conditions for driving prices higher,” he wrote. Gartman also warned that investors should be wary of a weather trade, as it can quickly reverse.

The latest USDA data, released Monday, shows that conditions have deteriorated with heat and drought impacting the key Midwest growing states. The report said 56 percent of U.S. corn is now rated at good to excellent, down from 63 percent last week. There had been expectations of a bumper crop, needed to replenish U.S. stocks. Soy beans were 53 percent good to excellent, down from 56 percent the week earlier.