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 thls paper a n extreme day is deflned 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 Slxteen stations are selected from the Historical Climate Network Daily (HCN/D) recold to represent condlt~ons across a 6-state region Significant interseasonal and interannual variability are found in the time-series of extreme days at all stations In wintel approximately 10 to 15 warm and cold days occur each ycar However, neither type of event perslsts for long penods of time This pattern is indicative of the vigorous atmospheric circulation that moves fronts and air masses through the midlatitudes d u n n g this season Winters wlth a high frequencv of cold days are associated with laige-scale mer~dional 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-llved 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 perlods of time when they do occur Large-scale c ~r c u l a t ~o n differences between summers wlth frequent warm and cold days are weak indicating that regional-scale patterns of subtropical flow and the expansion of the Atlant~c Subtropical High are most likely Important forcing factors in summer Over t h ~s century there IS a long-term trend toward more frequent cold days in all seasons However this t ~e n d may be reversing in the past decade KEY WORDS: Extreme temperature -Climate var~ability .