Standardized precipitation index forecast June-August 2011 (based on NSIPP 7-mo weather forecast)

The Standardized Precipitation Index (SPI) is a probability index that can be calculated for different time periods to indicate periods of abnormal wetness or dryness.

SPI is derived solely from monthly precipitation and can be compared across regions with different climates.

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).

This dataset shows the average 12-month SPI (in classes ranging from extremely wet to extremely dry) for the three-month forecast period indentified in the dataset title. Negative (i.e., dry) values of the 12-month SPI are closely associated with a high potential for wildland fire.

SPI was calculated from the long-term precipitation record up to the last observed month and from projected monthly precipitation values provided by the weather model identified in the dataset title.

Future climate forecasts are available through cooperation with the International Research Institute for Climate Prediction (IRI) of Columbia University which provides monthly updates of 7-month future climate forecasts from five different general circulation models (GCMs) of the global atmosphere. GCM results come from the University of Maryland (COLA), the University of Hamburg (ECHAM4.5), the National Weather Service’s Climate Prediction Center (NCEP), NASA’s Goddard Institute of Space Studies (NSIPP), and the Scripps Oceanographic Institute (ECPC).