January 1999 was the 13th wettest such month since 1895, based on preliminary data. 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 January 1999 as the 18th wettest such month on record. This standardized z-score is estimated to be accurate to within 0.256 index units. The January 1999 value was ten times the 1953-1998 average January tornado count of 16 and three times the previous January record of 52 tornadoes which was set back in 1975. The weather pattern took on a spring-like flavor during parts of January with a series of fast moving storms coming in from the Pacific which spawned the record number of January tornadoes. Three significant tornado outbreaks occurred with a preliminary total of 121 tornadoes reported resulting in 19 deaths.