Summer 1999 was the first such season since 1988 to be 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 ranked Summer 1999 also drier than the long term mean. Widespread dryness characterized the contiguous U.S. from the southern Plains to the Northeast. The convective nature of summer thunderstorms brought spotty wetness to part of this area, while the remnants of Hurricane Bret caused heavy rains in parts of southern Texas. The pattern in the Northwest was generally dry, with spotty wet areas. The Hawaiian stations indicated significant summer dryness in the Islands.