Title: Upper-level Potential Vorticity Disturbances as a Source of Severe Weather during the Southwest Summer "Monsoon"




Abstract: During the North American summer monsoon (NAM) the Southwest U.S. frequently experiences damaging severe weather and disruptive flash flood events in conjunction with mesoscale convective systems (MCSs) that typically form over higher terrain in the afternoon and early evening in response to the diurnal heating cycle. MCSs that propagate westward and southwestward off the higher terrain of Arizona can become severe as they approach the Phoenix and Tucson metropolitan areas, especially if they encounter a moist flow of tropical moisture from the Gulf of California (GC). Transient upper-level subsynoptic-scale potential vorticity (PV) disturbances (PVDs) that approach Arizona from the east can enhance deep northeasterly shear and contribute to the organization of severe weather-producing MCSs that form over the Mogollon Rim and the elevated terrain off southeastern Arizona and then propagate toward the populous regions of southern Arizona. The study also prompts questions about whether the NAM is a true monsoon system.




Introduction by Kerry Emanuel