North American tornado numbers were obtained from the SPC. The time period used in our study was 1954–2016. EF is the “Enhanced Fujita (EF) scale” which replaced the “Fujita (F) scale” in 2007 and ranges from EF0 (105 to 137 km/hour wind speed) to EF5 (>322 km/hour wind speed). Because extension of the tornado detection network has introduced artificial trends in long-term tornado datasets, particularly for EF0 tornadoes (36), our analysis only focused on the EF1 to EF5 categories, which are less affected by data inhomogeneities. We calculated monthly mean total number of EF1 to EF5 tornadoes per 1° × 1° grid box. Before the analysis, long-term linear trends were removed from all datasets.

The study area is defined here as 30°N to 40°N and 100°W to 90°W and is referred to as the SGP region. The area was chosen to include a representative portion of the Great Plains and “Tornado Alley” regions of the United States, which have a high climatological occurrence of tornadoes. Our region of interest accounts only for a small fraction of North America. However, this small region accounts for 44 and 45% of the U.S. total April and May EF1 to EF5 tornadoes, respectively. Moreover, correlation coefficients between SGP tornado numbers and U.S. total tornado numbers attain values of 0.79 and 0.84 in April and May, respectively. Therefore, variations in the SGP tornado activity can explain a considerable fraction of the total North American tornado variability.

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