The Palmer Drought Index is updated weekly by the Climate Prediction Center. It is based on rainfall, temperature and historic data, and is computed based on a complex formula devised by W.C. Palmer in 1965.
The Crop Moisture Index is updated weekly by the Climate Prediction Center. It is a derivative of the Palmer Drought Index, designed to reflect quickly changing soil moisture conditions.
Standardized Precipitation Index
The Standardized Precipitation Index (SPI) is a probability index that considers only precipitation. The SPI is an index based on the probability of recording a given amount of precipitation, and the probabilities are standardized so that an index of zero indicates the median precipitation amount (half of the historical precipitation amounts are below the median, and half are above the median). The index is negative for drought, and positive for wet conditions. As the dry or wet conditions become more severe, the index becomes more negative or positive. The SPI is computed by NCEI for several time scales, ranging from one month to 24 months, to capture the various scales of both short-term and long-term drought.
Issued every Thursday by the CPC along with the U.S. Department of Agriculture, and the National Drought Mitigation Center in Lincoln, Nebraska. These assessments review national drought conditions and indicate potential impacts for various economic sectors, such as agriculture and forestry.
National Integrated Drought Information System (NIDIS) US Drought Portal
The Drought Monitor was introduced as an operational weekly product in 1999 to provide an overview of conditions averaged across a broad array of time scales and impact indicators, leaning toward those that seem most relevant to observed impacts.
Find Additional information about Drought at the National Drought Mitigation Center (NDMC)