Over four percent of the country was much wetter than normal while about seven percent of the country was much drier than normal. 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 preliminary national standardized precipitation index for June 1999 averaged near the long-term mean. The map immediately below, based on approximately 250 First Order airport stations, shows June 1999 total precipitation as a departure from 1961-1990 station normals. The departure map, in general, shows the less coherent spatial pattern characteristic of summertime precipitation. However, certain areas do stand out. For example, the Northeast to Ohio Valley was unusually dry during June, and unusually wet conditions occurred over much of the Southeast and southern and central Plains.