The two years 2010 and 1983 have a lot in common so far, at least at Ames, Iowa. "I haven't analyzed other locations, but Ames was not typical of the region either year," says Elwynn Taylor, Iowa State University Extension climatologist.

This year the growing degree day (GDD) accumulation has climbed from normal in mid-May to about 100 GDD above normal at the last week of June, he says. In 1983 the GDD accumulation was about 200 GDD behind usual by mid-June then it rapidly increased to slightly above normal by mid-August.
 
"The 1983 heat in July was accompanied by greatly diminished rain after the first week of July until the last week of August," notes Taylor. "The temperature stress, hours of temperature exceeding 86 F, reached 260 by the last week of August. Normal temperature stress is 65 for the period."

Switch to La Nina would heighten weather risk in July and August

Are the years 1983 and 2010 alike to date? There are three considerations in answering this question, says Taylor

First, neither year had significant periods of abnormally high temperature in the spring. Second, both years had above normal precipitation reaching to more than 16 inches between January and the end of June (normal is less than 14 inches) and both years had above usual moisture during the previous fall season. Third, a winter El Nino condition rapidly dissipated during May and June.
 
Taylor also points out that the May through June 2010 switch from an El Nino to a La Nina weather pattern, as he looks at the 90-day Southern Oscillation Index values, does present an historical probability of an increased incidence of stressfully hot days during the 2010 season. The statistical chance that it could result in a U.S. corn yield average below the trend (below 160 bushels per acre) is near 70% as opposed to a 50/50 chance on having an average yield year. 

Based on past history, an El Nino weather pattern is favorable to corn yields and weather conditions in the U.S. Corn Belt. However, a switch to a La Nina weather pattern brings conditions that create drought and lower yields in the Corn Belt.