Date of Award

Spring 5-2019

Document Type

Honors Thesis

Degree Name

Bachelor of Science


Atmospheric and Environmental Sciences

Advisor/Committee Chair

Andrew C. Winters, Ph.D.

Committee Member

Ross A. Lazear, M.S.


The occurrence of tornado outbreaks are often associated with considerable societal and economic impacts. The U.S. averages nearly 1000 tornadoes per year that result in 1500 injuries and 80 fatalities, many of which are associated with outbreak days. Additionally, one outbreak alone can cause millions of dollars in property damage. The location of these outbreaks can vary temporally throughout the cool season (September–May) and can vary substantially in terms of their severity.

This study focuses on continental U.S. tornado outbreaks during the cool season and their relation to the state and evolution of the North Pacific jet (NPJ) stream 0–5 days prior to an outbreak. Tornado outbreaks are identified as days in which 6 or more (E)F2+ tornadoes were observed during a 24-hour (1200–1200 UTC) period using storm reports available from the Storm Prediction Center. This identification scheme identified a total of 189 outbreak days between 1979 and 2017. Following their identification, outbreak days are classified based on the geographic region impacted by the outbreak, the season, and the Destruction Potential Index ranking of the outbreak (developed by Thompson and Vescio 1998) in order to determine the state and evolution of the NPJ that is most conducive to each class of outbreak.

The state and evolution of the NPJ that is most conducive to each class of tornado outbreaks are investigated using an NPJ Phase Diagram, which is developed from the two leading empirical orthogonal functions (EOFs) of 250-hPa zonal wind anomalies over the North Pacific during the cool season. The first EOF corresponds to a zonal extension or retraction of the exit region of the climatological NPJ, while the second EOF corresponds to a poleward or equatorward shift of the exit region of the climatological NPJ. The projection of 250-hPa zonal wind anomalies at one or multiple times prior to an outbreak onto the NPJ Phase Diagram provides an objective characterization of the prevailing NPJ regime prior to an outbreak. The analysis demonstrates that each NPJ regime can strongly impact the character of the large-scale flow pattern over North America, and that the location and severity of tornado outbreaks can vary as a function of the antecedent NPJ regime.