Methodology for Computing Seasons

For this study, an attempt was made to objectively determine the primary severe weather season for each radar coverage area. This was done using the following method:

First, a seven-day running average number of severe reports was calculated for each day of the year. The annual average of these values was also computed. The scheme searched for every period during the year that the daily running average value of severe reports exceeded the annual average value. Any periods that were separated by five or fewer days were merged. Finally, the longest period for the year was determined to be the primary severe weather season for the station.


The plot above is the daily severe weather occurrence chart for Jackson, MS. The lines represent the 7-day running mean values. The maximum daily average number of severe reports for this station is 4.0 on April 23rd, while the average daily value is 1.45. The dates of Feb 12th through Jul 30th were determined to be the primary severe weather season. Notice that there is a period in late January where the average value is above criteria. However, the gap before the primary season lasts longer than five days, so it is not merged with the primary season.

Notice also, that there is a secondary season for this area during the fall and early winter. Since this season is of shorter duration than the spring/summer season, it is not chosen as the primary season.