This paragraph  from unpublished paper,

The (PDSI) is the most prominent index of meteorological drought used in the United States (Heim 2002). The PDSI was created by Palmer (1965) with the intent to measure the cumulative departure (relative to local mean conditions) in atmospheric moisture supply and demand at the surface. It incorporates antecedent precipitation, moisture supply, and moisture demand [based on the classic work of Thornthwaite (1948)] into a hydrological accounting system.  PDSI values are distributed in 9 categories using a comprehensive classification depending of the severity of dry or wet conditions (Table.2). The PDSI is calculated based on precipitation and temperature data, as well as the local Available Water Content (AWC) of the soil.

(Table. 1): Classification of PDSI categories defined by Palmer (1965)

     PDSI has been criticized for a variety of reasons of which most significant is perhaps that is not comparable between diverse climatological regions (Alley, 1984 and Karl, 1986). This problem has been solved by Wells et al. (2004) creating Self-Calibrated PDSI (Sc-PDSI) which improves the “original” PDSI. A detailed description of the modifications to this algorithm to obtain the Sc-PDSI is given by Wells et al., 2004 and van der Schrier et al. (2006a).

Here is the paper that should help you Palmer (1965) at - http://www.ncdc.noaa.gov/temp-and-precip/drought/docs/palmer.pdf

This paragraph  from unpublished paper,

The (PDSI) is the most prominent index of meteorological drought used in the United States (Heim 2002). The PDSI was created by Palmer (1965) with the intent to measure the cumulative departure (relative to local mean conditions) in atmospheric moisture supply and demand at the surface. It incorporates antecedent precipitation, moisture supply, and moisture demand [based on the classic work of Thornthwaite (1948)] into a hydrological accounting system.  PDSI values are distributed in 9 categories using a comprehensive classification depending of the severity of dry or wet conditions (Table.2). The PDSI is calculated based on precipitation and temperature data, as well as the local Available Water Content (AWC) of the soil.

(Table. 1): Classification of PDSI categories defined by Palmer (1965)

     PDSI has been criticized for a variety of reasons of which most significant is perhaps that is not comparable between diverse climatological regions (Alley, 1984 and Karl, 1986). This problem has been solved by Wells et al. (2004) creating Self-Calibrated PDSI (Sc-PDSI) which improves the “original” PDSI. A detailed description of the modifications to this algorithm to obtain the Sc-PDSI is given by Wells et al., 2004 and van der Schrier et al. (2006a).

It will help you :

http://onlinelibrary.wiley.com/doi/10.1002/wrcr.20342/epdf

otherwise use sc-PDSI algorithm.

Hi

You can use this software:

http://www.agrimetsoft.com/Drought%20Monitoring%20And%20Prediction.aspx