Dear Tom,
Are we able to forecast droughts in the same manner as other major weather events?
—Brendan A., Lombard
Dear Brendan,
The definition of “drought” varies greatly from one region to another in the United States. In the Chicago area and the Midwest in general, a drought consists of at least several months of severely subnormal precipitation, but that vague definition is inadequate. For example, a drought during the winter months may not be noticeable, but the same period of subnormal precip during the growing season will be. A few weeks of sharply subnormal precipitation at the outset of a drought will not be classified as a drought until the significant deficit persists well beyond that time. In general, a Midwestern drought is not forecastable, but a definition of drought is difficult to arrive at.