Extreme temperature days in the south-central United States

Extreme temperature days in the south-central United States can have significant impacts on energy demand, agriculture, and human comfort. In this paper an extreme day is defined as a day that exceeds 1 standard deviation of the long-term average temperature for that day. Using this methodology, similar numbers of extreme days are found to occur in each season. Sixteen stations are selected from the Historical Climate Network Daily (HCN/D) record to represent conditions across a B-state region. Significant interseasonal and interannual variability are found in the time-series of extreme days at all stations. In winter approximately 10 to 15 warm and told days occur each year. However, neither type of event persists for long periods of time. This pattern is indicative of the vigorous atmospheric circulation that moves fronts and air masses through the midlatitudes during this season. Winters with a high frequency of cold days are associated with large-scale meridional circulation over North America, while zonal flow in winter produces more frequent warm days. In summer cold days occur regularly each year but cold events are short-lived. Warm events, however, follow a very different pattern. Warm day frequency is much more Variable in the summer, and warm events tend to persist for longer periods of time when they do occur. Large-scale circulation differences between summers with frequent warm and cold days are weak, indicating that regional-scale patterns of subtropical flow and the expansion of the Atlantic Subtropical High are most likely important forcing factors in summer. Over this century there is a long-term trend toward more frequent cold days in all seasons. However, this trend may be reversing in the past decade.