Climatology and Trends in the Relationship between United States Tornadoes and Extratropical Cyclones

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 anecdotally to frequently form in the warm sector of extratropical cyclones (ETCs), yet relatively little research exists on the climatological relationship between the two phenomena and their covariability. This work analyzes the climatology of F/EF1+ tornadoes relative to ETCs using historical databases of tornadoes and ETC centers, in conjunction with ERA5 reanalysis data. The majority of tornadoes (73%) occurred to the southeast of an ETC center within 2000km, with a median distance of approximately 500 km. Of those tornadoes, 68% occurred in outbreaks of 6+ tornadoes. The spatial distributions and ETC-relative distributions are similar across all intensity levels, with stronger tornadoes occurring closer to the ETC center. Through the seasonal cycle, tornadoes shift north and south along with ETCs and the jet stream, and they are most strongly linked to ETCs in the winter when ETCs are strongest and least strongly in the summer when ETCs are the weakest. Finally, median tornado location and median ETC location covary strongly with one another interannually, including an eastward shift in ETC location that aligns with the eastward shift in tornadoes found in past work, suggesting that this eastward shift may be driven by spatial shifts in ETC activity. Taken together, this work identifies a strong link between tornado activity and ETC activity over the U.S. that offers a pathway to understand how climate change may modulate tornado activity via changes in the jet stream and the extratropical cyclones it produces.