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