Summertime convective storm environments in central Arizona: Local observations

The daily evolution of local surface conditions at Phoenix, Arizona, and th e characteristics of the 1200 UTC sounding at Tucson, Arizona, have been ex amined to determine important meteorological features that lead to thunders torm occurrence over the low deserts of central Arizona. Each day of July a nd August during the period 1990-95 has been stratified based upon daily me an, 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 nor 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 c onvective storms occur in the Phoenix area, there is a decrease in the diur nal amplitude of surface dewpoint changes, signifying increased/deeper boun dary layer moisture. This signal is very subtle and may nor have much forec ast utility. Additionally, it is found that surges of moist air from the Gu lf of California do not occur frequently during the 36-48 h immediately pri or to thunderstorm events in the Phoenix area. It is shown that the 1200 UT C Tucson wind profile has a significant northerly flow in low levels on moi st days when storms do not occur in the Phoenix area. The forecaster needs information on the local temperature and moisture profile to assess the pot ential for thunderstorms in the Phoenix area. However, routine upper-air ob servations are unavailable. Steps are being taken to obtain morning soundin gs in Phoenix, and the improving capabilities of satellite-derived thermody namic data and mesoscale models may also provide the forecaster critical in formation in the future. The findings, although specifically developed for the Phoenix area, may be relevant to thunderstorm forecasting in many regio ns of the interior West.