A Case Study of Heavy Rainfall Associated with Weak Cyclogenesis in the Northwest Gulf of Mexico

Abstract

This paper describes a case of unexpected weak cyclogenesis over the northwestern Gulf of Mexico from 16 to 19 September 1984 based upon manually prepared and European Centre for Medium-Range Weather Forecasts (ECMWF) gridded analyses. Noteworthy aspects of the case include: 1) upward of 50 cm of rain along the extreme southern coast of Texas and 2) the brief occurrence of minimal strength tropical-storm conditions in a weak baroclinic marine environment. A crucial antecedent condition to rainstorm formation was the creation of a low-level baroclinic zone over the northwestern Gulf of Mexico due to the southward advance of drier and slightly cooler air behind a cold front that penetrated into northeastern Mexico. Four factors were responsible for rainfall concentration along the coast: 1) a northward-moving 700-mb trough and embedded vorticity maximum in the easterlies over the western Gulf of Mexico, 2) an eastward-propagating upper-tropospheric disturbance in the midlatitude westerlies over the southern United States to the north of a subtropical ridge line over Texas and Louisiana, 3) the formation of a weak midtropospheric baroclinic zone over the extreme north-western Gulf of Mexico, along which cyclonic-vorticity advection by the thermal wind contributed to a favorable environment for deep convection and cyclogenesis, and 4) the existence and maintenance of a weak north-south-oriented baroclinic zone along the Mexican coast in the lower troposphere. The coastal baroclinic zone was associated with a quasi-stationary axis of ascent that maximized at 700 mb and lay 200?300 km to the cast of a persistent band of frontogenesis along the Sierra Madre Oriental Mountains of Mexico. Frontogenesis (?2?4?10?10°C m?1 s?1) was dominated by the twisting term as relatively cool air over coastal Mexico was forced to ascend in the 700-mb easterly flow, favoring the northward movement of an area of cyclonic vorticity along the coast. The results from this study are compared and contrasted with a similar September (1979) heavy-rain event that also occurred in southern Texas. In both cases, cyclogenesis occurred in a weakly baroclinic environment with embedded convection concentrated along the boundary of the surface baroclinic zone. The paper concludes with a discussion of tropical-storm formation in a baroclinic environment. It is speculated that the apparent, but short-lived, minimal strength tropical-storm development in this case could not be sustained because of the absence of a significant upstream cyclonic-vorlicity maximum aloft, despite otherwise favorable indicators for tropical cyclogenesis.