Potential vorticity disturbances as a trigger of southwest U.S. severe weather

During the North American summer monsoon (NAM) the American Southwest 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). The purpose of this presentation is to show that transient subsynoptic-scale potential vorticity (PV) disturbances (PVDs) contribute significantly to the formation and propagation of severe weather-producing MCSs during the NAM.

A PVD climatology constructed from NCEP/GFS 1.0 degree gridded analyses during the NAM (1 June through 15 September 2004-2009) will be presented to identify which subset of PVDs can trigger severe weather-producing MCSs across the Southwest based upon their location and propagation direction across the region, and the environmental deep-layer shear, convective available potential energy and precipitable water amounts. The results indicate that westward-propagating PVDs located to the east of the Phoenix and Tucson metropolitan areas are mostly likely to trigger severe weather-producing MCSs in populated regions of Arizona. When northeasterly flow in the middle and upper troposphere to the west of westward-propagating PVDs overspreads low-level moist southwesterly flow in the late afternoon and early evening surface to 6 km shear values can approach 30-45 kt in the vicinity of Phoenix and Tucson, sufficient to sustain severe storms that formed earlier in the afternoon over the higher terrain. Flash-flood producing MCSs can be triggered by PVDs associated with weaker deep-layer shears and ample deep-layer moisture.