March 1999 was the 16th driest such month since 1895. The national precipitation index expresses precipitation departure from the 60-year mean in terms of standard deviations. Each year's value is computed by standardizing the annual precipitation in each of 344 climate divisions across the U.S. using the gamma distribution over the 1931-90 period. These gamma-standardized divisional values are then weighted by area and averaged to determine a national standardized value for each year. These national values are normalized over the period of record.
Negative values are drier and positive values are wetter than the mean.
The prelimininary national standardized precipitation index ranked March 1999 as the 15th driest such month on record. This standardized Z-score is estimated to be accurate to within 0.222 index units. The percent area of the country experiencing severe to extreme wetness dropped significantly to less than ten percent. The core dry areas included portions of the Southeast, Southwest, mid-Atlantic, and central and northern Great Lakes. The core wet areas included portions of New England, northern and central Plains, and much of the Pacific Coast.