National Climate Report - Autumn (SON) 1999
National Temperature - Autumn
Preliminary data for autumn (September-November) 1999 indicate the mean temperature for the three-month period averaged across the contiguous United States was much above the long-term mean and ranked as the fifth warmest autumn on record. Over 22% of the country was much warmer than normal while about one percent of the country averaged much cooler than normal. The last two autumn seasons have been above the long-term mean.
The national temperature index expresses temperature departure from the 60-year mean in terms of standard deviations. Each year's temperature value is standardized for each of 344 climate divisions in the U.S. by using their 1931-90 means and standard deviations, 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 preliminary national standardized temperature index ranked autumn 1999 as the eighth warmest autumn season since 1895.
National Precipitation - Autumn
larger image Preliminary data for autumn (September-November) 1999 indicate the three-month precipitation averaged across the contiguous United States was much drier than normal and ranked as the 11th driest autumn since 1895. Nearly 39% of the country was much drier than normal while about two percent of the country was much wetter than normal. Autumn 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. 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 preliminary national standardized precipitation index ranked autumn 1999 as the seventh driest autumn on record.
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.
Based on preliminary data from the National Weather Service Storm Prediction Center the count of 57 tornadoes ranked autumn (September-November) 1999 as the eighth least active autumn season since 1953. The 1953-1998 average autumn tornado count is 102. The autumn season with the most tornadoes was 1992 with 265 while the least active autumn season for tornadoes was 1953 with 23 documented tornadoes. larger image
Current data are based on preliminary reports from River Forecast Center stations and First and Second Order airport stations obtained from the National Weather Service (NWS) Climate Prediction Center and real time Global Telecommunications System (GTS) monthly CLIMAT summaries. THE CURRENT DATA SHOULD BE USED WITH CAUTION. 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.
The following NCDC datasets are used for the historical U.S. data: the climate division drought database (TD-9640), and the hurricane datasets (TD-9636 and TD-9697). It should be noted that the climate division drought database consists of monthly data for 344 climate divisions in the contiguous United States. These divisional values are calculated from the 6000+ station Cooperative Observer network.