Summertime Convective Storm Environments in Central Arizona: Local Observations

Abstract

The daily evolution of local surface conditions at Phoenix, Arizona, and the characteristics of the 1200 UTC sounding at Tucson, Arizona, have been examined to determine important meteorological features that lead to thunderstorm occurrence over the low deserts of central Arizona. Each day of July and August during the period 1990?95 has been stratified based upon daily mean, surface moisture conditions at Phoenix, Arizona, and the occurrence of afternoon and evening convective activity in the Phoenix metropolitan area. The nearest operational sounding, taken 160 km to the southeast at Tucson, is shown to be not representative of low-level thermodynamic conditions in central Arizona. Thus, Phoenix forecasters? ability to identify precursor conditions for the development of thunderstorms is impaired. On days that convective storms occur in the Phoenix area, there is a decrease in the diurnal amplitude of surface dewpoint changes, signifying increased/deeper boundary layer moisture. This signal is very subtle and may not have much forecast utility. Additionally, it is found that surges of moist air from the Gulf of California do not occur frequently during the 36?48 h immediately prior to thunderstorm events in the Phoenix area. It is shown that the 1200 UTC Tucson wind profile has a significant northerly flow in low levels on moist days when storms do not occur in the Phoenix area. The forecaster needs information on the local temperature and moisture profile to assess the potential for thunderstorms in the Phoenix area. However, routine upper-air observations are unavailable. Steps are being taken to obtain morning soundings in Phoenix, and the improving capabilities of satellite-derived thermodynamic data and mesoscale models may also provide the forecaster critical information in the future. The findings, although specifically developed for the Phoenix area, may be relevant to thunderstorm forecasting in many regions of the interior West.