National Climate Report - Annual 1998
Preliminary data indicate that the United States average temperature in 1998 was 54.62 degrees F (12.57 deg C), which placed 1998 in a virtural tie with 1934 as the warmest year in records dating to 1895. Ten out of the last 13 years have averaged near to much warmer than the long-term mean.
The national temperature index expresses temperature departure from the 60-year mean in terms of standard deviations. Each year's value is computed by standardizing the temperature for each of 344 climate divisions in the U.S. by using their 1931-90 mean and standard deviation, then weighting these divisional values by area. These area-weighted values are then normalized over the period of record. Positive values are warmer and negative values are cooler than the mean. The national standardized temperature index ranked 1998 as the second warmest year on record.
Preliminary data indicate that 1998 was the fifth wettest year on record for the United States. Annual 1998 precipitation across the contiguous United States averaged 32.61 inches (828 mm.). 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. The gamma statistical distribution takes into account heavy precipitation years and extremely dry years in the historical record (in mathematical parlance, "a zero-bounded skewed distribution"). 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. This index gives a more accurate indication of how precipitation across the country compares to the local normal (60-year average) climate. The national standardized precipitation index ranked 1998 as the fifth wettest year on record.
About 75% of the country averaged much warmer than normal for 1998, while less than 1% had 1998 annual temperatures much cooler than normal.
Annual precipitation was much wetter than normal over about 22% of the country and much drier than normal over about 2% of the nation.
Based on preliminary data from the National Weather Service Storm Prediction Center through late-December, there were 1239 tornadoes observed in the United States during 1998, which is the second largest annual total in the 1953-1998 record. While the value for 1998 is the second greatest annual total, it should be pointed out that the preliminary tornado count is traditionally higher than the final count and that the tornado observations have generally improved with time as better observing practices and instrumentation (especially weather radar and satellites) were utilized.
The winter 1997-98 tornado count was the second greatest December-February total 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. This is above the long-term mean of about 8 hurricanes and tropical storms.
Preliminary data indicate that there were 57 hurricane days during 1998, which is above the long-term mean of 33.
Hurricane Mitch, the strongest October hurricane on record for the Alantic Basin, brought widespread death and destruction to portions of Central America. Based on annual maximum hurricane/tropical storm wind speed, 1998 ranked as fifth strongest.
Mitch had the fourth lowest pressure of Atlantic hurricanes this century, tying with Hurricane Camille in 1969.