Large-Scale Patterns Associated with Severe Summertime Thunderstorms over Central Arizona

Abstract

Severe thunderstorms are relatively rare over Arizona and occur most frequently during the summer monsoon period, that is, July, August, and early September. Forecasting in Arizona during the summertime is quite difficult and skill scores are low for both precipitation and severe thunderstorm watches and warnings. In the past, due to the sparse population of Arizona, severe thunderstorms usually impacted few people and were considered relatively insignificant events. However, over the last 20 years, the population of central Arizona has grown dramatically, and the impact of severe thunderstorm and flash flood occurrences has also increased. Synoptic conditions associated with 27 severe thunderstorm events that occurred in central Arizona during the summer monsoon have been examined systematically and compared to long-term mean July conditions. The period of study covered 1978 to 1990, and cases selected were limited to the high population area of central Arizona. McCollum subjectively identified three distinct large-scale patterns (types I, II, and III) that were associated with the severe thunderstorm events. Significant large-scale departures from mean conditions are used to characterize the Arizona severe weather environment for these three pattern types. Significant pattern anomalies tend to be far removed from the state, typically by 1000 to 2000 km. Thus, even though the summertime environment may seem locally stagnant, a large-scale perspective is required to monitor the day to day evolution of the severe weather environment in the Southwest. The key factor affecting convective instability at lower elevations, that is, in the deserts of central Arizona, is the amount of low-level moisture present. Severe storm conditions are distinctly more moist and unstable than average from the surface to 700 mb. The standard level charts for the severe weather patterns indicate that the Gulf of California plays an important role in providing a source for this moisture. The summertime severe thunderstorm environment over the southwest United States is distinctly different than central and eastern United States storm settings, which are well known based upon years of study of substantial numbers of events. In general, the environment in which central Arizona severe monsoon thunderstorms occur is one of weak synoptic-scale flow, significant lower- to midtropospheric moisture, and moderate instability. The nature of subsynoptic circulations that initiate and support severe weather over central Arizona is difficult to infer. However, the existence of repetitive, large-scale patterns suggests that forecasting for the general threat of severe summertime thunderstorms can be improved.