Over eight percent of the country was much wetter than normal while over 14 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 ranked Winter 1998-99 as the 30th driest winter since records began in 1895. This standardized z-score is estimated to be accurate to within 0.519 index units. These preliminary data are useful for estimating how current anomalies compare to the historical record, however the actual values and rankings for the current year may change as the final data arrive at NCDC and are processed. It should be noted that the climate division drought database consists of monthly data for 344 climate divisions in the contiguous United States. The 1953-1998 average winter tornado count is 55.