Hydrological droughts are much more difficult to quantify, especially when they span several consecutive years. It may take several years of above normal moisture to undo the damage of a single, intense drought year. Conversely, because the annual precipitation of the central and eastern Corn Belt may be two or more times greater than that in the western Corn Belt, a hydrological drought of equal magnitude can be eradicated in half the time as one in the western Corn Belt, he says.
"Our long range forecasting ability is weak at best," Dutcher says. "However, the long-range models that we rely on work best when La Nina or El Nino conditions develop across the eastern and central equatorial Pacific. Weak El Nino conditions are now prevalent in the eastern Pacific and expected to strengthen slightly this fall and winter. These events usually dissipate as we move through spring, unless they are moderate to strong."
Several areas of the country respond to La Nina or El Nino events with enough regularity that they can be described with an accuracy of 70% or greater. During an El Nino event:
In the southern third of the U.S. precipitation is usually above normal during the fall through spring and temperatures are below normal. (During La Nina years this same region will typically see below normal moisture and above normal temperatures.)
The Pacific Northwest and the eastern Corn Belt will be drier and warmer than normal. (During a La Nina they will have above normal moisture and below normal temperatures.)
For the remainder of the country, fall and spring weather is entirely a function of where the mean jet stream pattern develops, even in the absence of El Nino or La Nina conditions, according to Dutcher. In the western Corn Belt, if precipitation patterns this fall are similar to last fall, the drought risk will remain elevated into next spring. If precipitation is close to normal, the risk of drought really won't be definable until next March, at the earliest."
For the central and eastern Corn Belt, Dutcher believes that the drought risk for 2013 will be no different than any other year. Most of the eastern and central Corn Belt droughts develop during the growing season and not during the fall/spring period as happens in the western Corn Belt.