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Weekly Palmer Drought Indices

National Oceanic and Atmospheric Administration

National Climatic Data Center

Week Ending January 10, 2009

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Select a week ending on a Saturday from the options below and click on "Submit" to display your selection.

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Background

The Palmer Drought Index (PDI) and Palmer Hydrological Drought Index (PHDI) were created by Wayne Palmer in the 1960s as a way of integrating water supply (precipitation) with water demand (evapotranspiration as computed from temperature) in a soil moisture model (Palmer, 1965). They are standardized indices where values of zero represent near normal conditions, negative values indicate drought, and positive values indicate wet spell conditions (Heim, 2002). Palmer (1965) developed the indices using monthly climate data, but he noted that they could be computed on a more frequent basis -- weekly or perhaps even daily. Monthly temperature and precipitation data are more consistent, more complete, and extend further back in time than daily data, so the PDI and PHDI have traditionally been computed on a monthly basis. Previous efforts to compute these indices on a more frequent basis (e.g., weekly) have led to results which were inconsistent with the historical record as represented by the monthly indices (Heim, 2005).

The weekly PDI and PHDI shown on this web page represent a way of estimating the monthly PDI and PHDI on an operational weekly basis. If the date of the maps is before the end of the month, then observed temperature and precipitation through the date of the map are combined with 1971-2000 normals of temperature and precipitation for the remainder of the month to create an estimate of the monthly temperature and precipitation values. These estimated monthly temperature and precipitation values are fed into the Palmer model to compute the monthly PDI and PHDI values for the date (week) in question (Heim, 2005).

References

Heim, R.R., Jr., 2002: A review of Twentieth-Century drought indices used in the United States. Bull. Amer. Meteor. Soc., 83, 1149-1165.

Heim, R.R., Jr., 2005: Computing the monthly Palmer Drought Index on a weekly basis: A case study comparing data estimation techniques. Geophys. Res. Lett., 32, No. 6, L06401, 18 March 2005.

Palmer, W.C., 1965: Meteorological drought. Research Paper No. 45. U.S. Weather Bureau. [NOAA Library and Information Services Division, Washington, D.C. 20852]