Alabama Office of the State Climatologist
The Lawn and Garden Moisture Index measures the capacity of current soil moisture to sustain healthy lawns and gardens in the Southeast U.S. The Index is computed in two stages, according to the Alabama Office of the State Climatologist:
"First, we estimate how much recent precipitation contributes to current soil moisture, assuming that any precipitation over the past 21 days should be included in the computation. The Index also assumes that more recent precipitation is more significant than the less recent: all precipitation during the previous 7-day period is considered equally important, but precipitation before that time is discounted according to a sliding scale. The result is the total effective precipitation during the period. The precipitation we use to compute the index is obtained from NOAA's Climate Prediction Center. It is determined from weather radar images in conjunction with precipitation reported at rain gages.
Second, we find out how much the total effective rainfall for the current day differs from a 'standard' amount of rainfall considered to be adequate for that time of year to sustain healthy lawns and gardens. The difference is the Lawn and Garden Moisture Index. Much less precipitation is needed during the cold months than in the warm ones. ... Positive values of the Index indicate adequate precipitation or better. Negative values indicate a precipitation deficit."