The Climatological Relationship Between United States Tornadoes and Extratropical Cyclones

TL;DR

This study quantifies the climatological relationship between U.S. tornadoes and extratropical cyclones, revealing their spatial, seasonal, and long-term covariability, and highlighting the importance of ETCs in tornado activity.

Contribution

It provides the first comprehensive climatological analysis of tornado-ETC covariability across multiple decades, emphasizing their spatial and seasonal relationships and long-term trends.

Findings

Most tornadoes occur southeast of ETC centers within 2000km.

Tornado outbreaks with 6+ tornadoes are common, especially in winter.

Tornado-ETC association varies seasonally and shows a long-term eastward shift.

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 relative to 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 2000km, with a median distance of approximately 500km. 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, and they are much more strongly (weakly) associated with ETCs in the winter (76\%; summer 38\%) when the jet stream and ETCs are strongest (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 U.S. that warrants deeper analysis of their role in generating tornadic environments in a warming world.