Tropical Squall Lines of the Arizona Monsoon

Abstract

Squall lines possessing nearly all the characteristics of tropical squall lines occasionally develop during the summer monsoon over southern Arizona and northwestern Mexico. Initial thunderstorm formation is over the Continental Divide in the late afternoon and the systems become organized within a few hours. Satellite imagery, cloud-to-ground lightning strike data, and surface observations indicate the squall lines move from east to west or northeast to southwest by discrete propagation faster than the environmental flow at all levels below 20 kPa so that most of the anvil clouds lag behind. The synoptic-scale circulation is anomalous with a strong ridge located over the western United States and a deep trough located over the eastern United States. West to northwest winds are found in the boundary layer over southern Arizona and northwest Mexico while a deep layer of east winds are observed above. As a result most of the environmental wind shear is confined to the lowest 2.5 km above the ground with very little shear at higher altitudes. The low-level wind shear seems to be required for the westward propagation of thunderstorms and the formation of the squall lines. Extremely dry midtropospheric air develops in the easterly flow through some combination of advection and subsidence and also appears to be an important factor in the development of the squall lines and in the creation of severe thunderstorms.