Zoe Schroder Searcy , James B Elsner

An outbreak can occur as a single day or a multi-day event characterized by many tornadoes associated with specific regional scale environmental factors. The objective of this research is to quantify the relationship between environmental factors and tornado activity using big tornado days that occur in the largest multi-day groups in the United States. First, the largest groups across space and time are identified as those with at least 30 tornadoes. Then, all days with ten or more tornadoes are extracted from the largest groups. Seasonally big days in large outbreaks occur most often during April, May, and June. Accumulated tornado power (ATP) is defined as a metric of big day severity. Finally, linear mixed effect models are used to statistically examine the relationship between ATP and environmental factors including convective available potential energy and shear. Results show an upward trend in ATP at 5% per year and an increase of 124% for every 10 m/s increase in the magnitude of bulk shear. Results show an increase in ATP of 29% for every 1000 J/kg increase in CAPE. Residuals from the regression model show no regional difference. However, the number of tornadoes per unit area is larger on days when the model under-predicts ATP.