Average winter temperature in those five years was above the average for the entire period by 5.8 degrees in Illinois and by 7.1 degrees in Iowa. Average summer temperature in those five years was above the average for the entire period by one degree in Illinois and below the average by 0.5 degree in Iowa. Warm winters were followed by average summer temperatures that were both well above and well below average.
The average trend adjusted corn yield for the five years was below the average for the entire period by 9 bushels in Illinois and above the average by 5 bushels in Iowa. Average yields in individual years were both well above and below the average for the five-year period, particularly in Illinois. Average yields showed less variation in Iowa, but the range was still considerable.
The average winter temperature in Illinois in 2011-12 was equal to the average temperature of the previous five warmest winters since 1960 and the past winter ranks as the fourth warmest since 1960. For Iowa, the average winter temperature was 0.9 degree cooler than the average of the previous five warmest winters and the past winter ranks as the fifth warmest since 1960. Based on relationships in previous warm winters in these two states, the warm winter of 2011-12 provides little indication of likely average summer temperature or state average corn yields in 2012. The lack of a relationship is consistent with the view that, beyond seasonal tendencies, weather is very difficult to predict over time horizons longer than a few weeks.