The threat of an earlier than usual frost looms for the Midwest this month, some weather forecasters say, adding another potential headache to corn and soybean growers already hampered by a cold, wet spring and extreme heat over the summer.

Following a brief hot spell before the end of this week, the eastern half of the U.S. will shift into a much cooler pattern that “may very well be our first step towards an eventual frost during the month,” according to Craig Solberg, a meteorologist with Freese-Notis Weather, Inc., in Des Moines.

An “extraordinarily” early frost, such as the one that occurred Sept. 3, 1974, “is not going to happen even with the coming cool weather,” Solberg said in a report. “But something occurring sometime during the last 20 days of the month certainly looks possible.”

Temperatures in Des Moines are expected to reach highs of 88 degrees Fahrenheit Sept. 2 then drop as low as 48 degrees two days later, according to the Weather Channel’s website. Indianapolis is expected to hit 99 degrees Sept. 2 before temperatures tumble to lows around 48 degrees Sept. 5.

Cooler Midwest conditions starting next week mean that crops will start to run short of “growing degree days” needed to fully fill out kernels and soybeans, Solberg said. “We are certainly not going to be pushing crops to maturity after the Labor Day weekend.”

In the grain markets, an early frost that brings the growing season to a premature halt is often a concern during this time of year as corn and soybean crops complete final stages of development. In the Midwest, the first killing frost, when temperatures drop below freezing for at least a few hours, typically arrives in early- to mid-October.

This year, an early frost may have less-damaging effects on corn because July heat speeded crop development, helping offset delayed spring planting, Solberg said.

“Ramifications of a frost in September, if it were to occur, are likely going to be less notable than it looked like at the beginning of the growing season, given that our summer heat has really pushed the crops quickly towards maturity,” Solberg said.

As of Aug. 28, 53 percent of the U.S. corn crop was in the “denting” stage, compared to the five-year average of 54 percent for that date, according to the Agriculture Department. A year earlier, the crop was 70 percent dented.

At CME Group in Chicago, a late-week slide in grain futures indicated there was little concern over the prospect of an early frost would cut the harvest, though the corn market remains near all-time highs hit earlier this summer.

In trading Sept. 1, December corn futures fell 29 cents to $7.38 ½ a bushel, down 3.7 percent on the week. Barring a sharp rally before the weekend, December corn will post its first weekly decline since the end of July.

November soybeans fell 23 cents to $14.34 ½ a bushel, up 0.8 percent so far this week.

Matt Connelly, a broker in the CME corn options pit, downplayed the potential for an early frost, though he noted colder temperatures could be a problem for soybeans, which aren’t as far along in development as corn.

Corn futures rallied most of August as it became clear the July heat wave curbed the crop’s production potential. Now, with lower yields already factored into the market, corn futures may have further downside over coming weeks, Connelly said.

Current futures prices indicate traders expect average U.S. corn yields of 147 to 149 bushels an acre, Connelly said. In a report last month, the USDA estimated nationwide yields at 153 bushels an acre.

“We need confirmation of a 145 to 146 (bushels an acre) yield before we have another leg up,” Connelly said. Sept. 1. “We’re running out of bullets. You’ve got to feed a bull.”

Still, analysts and traders will likely keep a wary eye on Midwest weather into the autumn. With U.S. corn stockpiles already on track to shrink to the lowest levels in 16 years in 2012, farmers can ill-afford any more weather difficulties.

“Corn losses are certain this year from the hot and dry weather seen in July and the poor spring weather that hurt planting,” Jack Scoville, an analyst with Price Futures Group, said in a Sept. 1 report.