A Lower-Tropospheric Thermodynamic Climatology for March through September: Some Implications for Thunderstorm Forecasting

Abstract

A thermodynamic climatology was constructed to investigate source regions of low-level moist airmasses and lower mid-tropospheric dry airmasses that contribute to the production of convective instability, and thunder-storms, over the United States. Mean monthly 1200 UTC values of potential temperature, mixing ratio, and geopotential height were computed at the surface and 750 mb for March through September (1966?1980) for 69 upper-air stations in the continental United States and Mexico. Analyses indicated that the Gulf of Mexico is the most significant source of low-level moisture east of the Continental Divide for all months, but the Atlantic Ocean also becomes a significant source in summer. The Gulf of California was found to be the most important source of low-level moisture west of the Continental Divide in summer. A distinct synoptic scale dryline was found over west Texas from March to June and analyses suggested that the strongest drylines occur in April and May. Northwest Mexico and the desert southwest were confirmed to be the source regions of the dryline. At 750 mb, source regions of dry airmasses were found to be the Baja California region and desert southwest in spring, the tradewind easterly dry layer in summer, and the dry, cool airmasses over the northern Pacific and northern Great Lakes regions in spring and summer. The moist, summer, tropical Pacific airstream was found not only to be deeper than that from the Gulf of Mexico, but also to be the primary source of moisture west of the Continental Divide. In summer, significant mean horizontal advection of warm, moist air from higher terrain into the Missouri Valley was noted.