Dynamic Tropopause Mesoscale Disturbances as Triggers of Warm Season Severe Weather Episodes in the Southwest

Transient subsynoptic-scale and mesoscale disturbances can help to trigger the development of mesoscale convective systems (MCSs) over the Southwest during the warm season. Convection associated with these MCSs can sometimes become severe, especially over the lower elevation areas of southwestern Arizona, a region that encompasses the urban areas of Phoenix and Tucson. One common scenario is for convective storms to build during the afternoon and evening over the higher elevations to the east of Phoenix and Tucson, organize into MCS clusters after sundown, and roll westward off the higher elevations across the Phoenix and Tucson metropolitan areas during the nighttime hours. A second, but somewhat less common scenario is for convective storms to form in the lowland valleys during the late afternoon and early evening and organize into MCSs over the Phoenix and Tucson metropolitan areas near and after sundown. The resulting convective mayhem can occur in the form of flash flood-producing rains, damaging winds, widespread blowing dust, windblown hail, and occasional tornadoes.

In this presentation we will document how transient upper-level subsynoptic-scale and mesoscale disturbances, in conjunction with important terrain features, act to create favorable environmental conditions that allow MCSs to form, organize, and persist. A dynamical tropopause (DT) perspective based on potential vorticity (PV) thinking is adopted for analysis and diagnostic purposes. Adoption of the DT perspective shows that transient subsynoptic-scale and mesoscale disturbances can persist for several days and trigger multiple MCS events as they propagate slowly across the Southwest. DT disturbances commonly originate via fracture from northeast-southwest elongated PV tails that extend from higher latitudes into lower latitudes.

DT disturbances (aka “PV critters”) that fracture from PV tails that extend deep into the subtropics and outer tropics will usually propagate westward on the equatorward side of the subtropical ridge axis whereas DT disturbances that fracture from PV tails but at higher latitudes will more commonly propagate eastward on the poleward side of the subtropical ridge axis. Although both westward- and eastward-propagating DT disturbances can trigger significant MCS development over the Southwest, the DT disturbances that propagate westward across the Gulf of Mexico, northern Mexico, and southern New Mexico and Arizona are especially likely to lead to episodes of significant MCS activity over parts of the Southwest. A brief DT disturbance climatology and the results from representative case studies will be used to illustrate how transient subsynoptic scale and mesoscale disturbances can contribute to MCS development and associated severe weather over the Southwest.