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.

At the National Climatic Data Center (NCDC), drought is monitored on weekly (as part of the Drought Monitor effort) and monthly time scales. With the number of drought monitoring indicators in use today, this poster will highlight some of the most recently developed tools utilized to monitor drought conditions and provide information regarding the potential for improving drought conditions across the U.S. These include the weekly Drought Monitor and the precipitation required to end or ameliorate drought.

The operational Drought Monitor product is created through a cooperative effort between NOAA, the USDA and the National Drought Mitigation Center to assess the state of drought in the U.S. Drought indicators used to measure the development and termination of drought 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, snowpack, soil moisture, and the Vegetation Health Index (derived from satellite data).

In addition, NCDC has developed a web resource that provides information on drought termination and amelioration through the use of the Palmer Hydrological Drought Index (PHDI), based on the work of Tom Karl and others. The website provides the amount of precipitation needed to end current drought conditions plus over 2000 maps which show the precipitation totals needed to end or ameliorate drought from periods of 1 month to 6 months based on PHDI values from -2 to -6. Maps showing the probability of receiving the necessary amount of precipitation are also provided.

The Palmer model estimates of the amount of precipitation needed to end a drought are statistical constructs. But the situation is not that simple in practice. Does a heavy rainfall over a short duration (say, 8 inches from an evening thunderstorm) end an ongoing severe drought? In practice, most of this rain will runoff and not contribute to soil moisture recharge. The answer is further complicated if the rain event is isolated in time (i.e., a drought-producing circulation pattern subsequently re-establishes itself). This poster will examine the question from both a statistical (drought model) as well as a conceptual perspective.