The Climatological Relationship between U.S. Tornadoes and Extratropical Cyclones

Abstract Tornadoes have caused billions of dollars in damage and are one of the leading causes of weather-related deaths in the United States each year. Tornadoes are known to frequently form in the warm sector of extratropical cyclones (ETCs), yet relatively little research exists quantifying the climatological relationship between the two phenomena and their covariability. This work analyzes the climatology of F/EF1+ tornadoes associated with ETCs (ETCTORs) using historical databases of tornadoes and hourly ETC centers for 1980–2022. Most tornadoes (72%) occurred broadly to the southeast of an ETC center within 2000 km, with a median distance of approximately 500 km. Of those tornadoes, 69% occurred in outbreaks of 6+ tornadoes. The spatial and ETC-relative distributions are similar across all intensity levels. Through the seasonal cycle, tornadoes shift north and south along with ETCs and the jet stream. They are more strongly associated with ETCs in the winter (76%), when the jet stream and ETCs are strongest, and more weakly associated with ETCs in the summer (38%), when the jet stream and ETCs are weakest. Finally, median tornado and tornado-producing ETC locations covary strongly with one another interannually. A long-term eastward trend in tornado-producing ETCs mirrors the eastward trend in tornadoes found in recent work, suggesting that this shift may be driven by spatial shifts in ETC tracks. This work identifies a strong link between tornado and ETC activity over the United States that warrants deeper analysis of their role in generating tornadic environments in a warming world. Significance Statement Tornadoes are known to often form in the warm sector of the low pressure systems called “extratropical cyclones” that produce much of the rainfall over the continental United States. The relationship between the two has not received much research attention though. This study finds a strong relationship between the two spatially, seasonally, and interannually. This relationship may explain why tornado activity has been found to be shifting southeastward over the past few decades, and it may offer a pathway to better predict how tornado activity, which is small scale and difficult to model directly, may change with global-scale climate change via changes in the jet stream and the extratropical cyclones that form and track along it.