Each of the last nine years has averaged near to much wetter than the long-term mean. 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 national standardized precipitation index ranked 1998 as the fifth wettest year on record. A February tornado outbreak in Florida contributed to this winter total. The spring 1998 tornado count was the fourth greatest March-May total on record. The summer and fall 1998 tornado counts were near the average for the last 10 years. Based on data from the National Weather Service National Hurricane Center , a total of 14 hurricanes and tropical storms developed in the North Atlantic basin during 1998.