"There is little correlation between winter temperature or winter precipitation and summer weather," Good said. "This is consistent with the view that, beyond seasonal tendencies, weather is very difficult to predict over time horizons longer than a few weeks. There may be some predictability over longer horizons due to so-called ENSO events, such as El Nino and La Nina, but the reliability of these patterns for predicting U.S. Corn Belt growing season conditions is still open to considerable debate."
Good said that in contrast to winter temperature, winter precipitation is likely to have a small impact on corn yields in the following growing season. The available data indicate December, January, and February precipitation in Illinois (6.8 inches) was near average and in Iowa (4.3 inches) above average.
"Preliminary data indicate March was drier than average," Good said. "On balance, this suggests little impact of winter 2012 precipitation relative to trend yield in either state."
The analysis extended to the relationship between total state average precipitation during December, January, and February and the total precipitation in the following July and August and between the total winter precipitation and the trend-adjusted corn yield the following year.
"The correlations between the total winter and summer precipitation indicate a slight negative relationship between winter and summer precipitation in Illinois and virtually no relationship in Iowa," Good said.
The correlations can vary between -1 and +1, with zero indicating no relationship, Good explained.
"Given the slight relationship between winter and summer precipitation in Illinois and Iowa, it would not be surprising to find little relationship between winter precipitation and the state average trend-adjusted corn yield the following year," Good said. "Nonetheless, in contrast to winter temperatures, there is a logical reason to think that winter precipitation should actually be positively related to corn yields. Specifically, winter precipitation contributes to the preseason charging of soil moisture reserves, which ultimately contributes to higher yields.
"It is, of course, important to keep in mind that the impact of precipitation before the growing season is small compared to precipitation during the growing season," Good said.
He said the yield impact of moving from the minimum to the maximum winter precipitation observed over 1960-2012 in both states is at most 5 bushels per acre.