Case Study of a Severe Mesoscale Convective System in Central Arizona

Abstract

A mesoscale convective system (MCS) developed over central Arizona during the late evening and early morning of 23?24 July 1990 and produced widespread heavy rain, strong winds, and damage to buildings, vehicles, power poles, and trees across northern sections of the Phoenix metropolitan area. Although forecasters from both the National Weather Service and National Severe Storms Laboratory, working together in the 1990 SouthWest Area Monsoon Project (SWAMP), did not expect thunderstorms to develop, severe thunderstorm and flash flood warnings were issued for central Arizona between 0300 and 0500 local standard time. This study examines the precursor and supportive environment of the mesoscale convective system, drawing upon routine synoptic data and special observations gathered during SWAMP. During the evening of 23 July and the early morning of 24 July, low-level southwesterly flow developed and advected moisture present over southwest Arizona across south-central Arizona into the foothills and mountains north and northeast of Phoenix. The increase in moisture produced substantial convective instability in an environment that had been quite stable during the late afternoon. Thunderstorms rapidly developed as this occurred. Outflow from these thunderstorms likely moved downslope into the lower deserts of central Arizona, helping to initiate additional convection. The most persistent convective activity developed within a region of low-level convergence between a pronounced mesoscale outflow boundary and the low-level southwesterly flow. The resultant MCS moved to the south-southeast and weakened just south of Phoenix, while its outflow apparently forced new thunderstorm development north of Tucson. The operational sounding and surface observation network in Arizona failed to detect important mesoscale circulations and thermodynamic gradients that contributed to the occurrence of the severe weather over central Arizona. In this case, conditions favorable for severe thunderstorms developed rapidly, over a period of a few hours. Large-scale analyses provided little insight into the causes of this particular severe weather event. Higher time and space resolution observational data may be needed to improve forecasts of some severe weather events over the Phoenix area.