Prolonged Severe Tornado Outbreaks in the United States in the Context of Atmospheric Blocking

Prolonged severe tornado outbreaks that last for longer than a few days have drawn notable attention in light of the remarkable outbreak of May of 2019. Other notable outbreaks distinguished by their longevity include those in May 2003, May 2008, and April 2011. Specific parameters determine whether any one individual thunderstorm or set of storms may form tornadoes, but the general circulation of the northern hemisphere can set up the environments that allow tornadic thunderstorms to occur over the same region over a prolonged period. Tornado frequency has not changed significantly over the past 50 years, though recent studies suggest that the pattern of frequency and the duration of tornado outbreaks may have. If the planet’s overall temperatures have increased, the circulation patterns tend to slow down and become more meridional which would make it more likely for low and mid-level blocking to occur and lead to prolonged periods of severe weather. The objective of this study is to determine if prolonged tornado events in the United States are associated with the preferred patterns of blocking in the northern hemisphere westerly wind belt. We analyzed tornado touchdown points and intensities that occurred during prolonged outbreaks to determine what, if any, changes in the pattern and frequency of prolonged outbreaks have taken place. We then compared these tornado outbreak patterns to the patterns of blocking. Preliminary results indicate that prolonged tornado outbreaks tend to occur during periods of blocking in the Pacific and to a lesser extent over and near Europe. They also seem to be associated with troughing over the west coast of North America.