The insidious nature of drought makes it hard to predict and monitor. Unlike tornadoes or hurricanes, which can be clearly delineated in space and time, it is difficult to operationally identify when a drought has started or ended. This is due to the many economic and social sectors affected by the phenomenon as well as the different scales of drought.

This paper will set the stage by discussing the different scales and types of drought, drawing distinctions between short-term and long-term drought, and agricultural, hydrological, and meteorological drought. The paper will address the occurrence of flash flooding in the midst of a long-term drought and the resulting question: Was this enough rain to end the drought? The authors will examine how several drought indicators, which have been developed over the past 100 years, measure the development and termination of drought. These indicators include the Palmer Drought Index, Palmer Z Index, Crop Moisture Index, Standardized Precipitation Index, Keetch-Byram Drought Index, precipitation percentiles, percent of normal precipitation, streamflow, soil moisture, and Vegetation Health Index. The paper will illustrate the problem, and potential solutions, by reviewing U.S. drought monitoring in the 21st Century and providing examples drawn from preparation of the operational Drought Monitor product.