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NOAA's Drought CalculatorJune 12, 2001 — Several parts of the nation, from the Pacific Northwest to the Southeast, are suffering from severe, ongoing droughts. Many water managers, journalists, and weather-watchers have all asked how much rain would be required to end these droughts—NOAA has the answer.

NOAA's National Climatic Data Center in Asheville, N.C., has calculated the amount of rain required to end droughts in the contiguous U.S. and the statistical probability that a given area would receive the needed rainfall. Calculating these figures is a complex procedure.

"The severity of the drought, as defined by the Palmer Hydrological Drought Index, is the essential starting point for determining the needed precipitation," said Jay Lawrimore, who heads the center's Climate Monitoring Branch. "Knowing the typical conditions that a region experiences during each month and season of the year—what we call that region's climatology—is also essential. A normally wetter region requires more precipitation than a drier region to end the drought." The season in which the precipitation falls can also greatly influence the amount of precipitation required to end a drought.

The quantity of rain needed to end a drought says nothing about the probability that a region will actually receive that amount of precipitation. A region, such as the West Coast, that seldom experiences excessively heavy precipitation during the summer, may be less likely to receive a quantity sufficient for ending a drought than a region that has a record of extreme precipitation events during the same season.

The Asheville center has developed a Web site that takes these and other factors into account to answer the questions in a given area. The Web site provides information on the amount of rain needed to end drought in a given area, the percentage of normal precipitation that would end the drought, and the statistical probability that the area will receive the precipitation.

"This last piece of information, the statistical probability, is based on past data, and is not meant to be a forecast," Lawrimore said. "NOAA's National Weather Service provides current forecasts."

Relevant Web Sites
Drought Termination and Amelioration

NOAA's Drought Information Center

Media Contact:
Patricia Viets, NOAA, (301) 457-5005