Synoptic conditions associated with genesis, evolution and tropical transition of Subtropical Storm Alberto

Abstract

Cuba is often hit by tropical cyclones, but the direct impact of a subtropical cyclone had not been previously reported. However, in May 2018, the western and central regions of the Cuban archipelago experienced the influence of heavy rainfall, associated with the subtropical storm Alberto. This system contributed significantly to the national accumulated of May, qualified as the second largest for any month of the year since 1961; consequently, it had an important impact on water resources and agriculture in Cuba. Therefore, this article has as main objective to analyze the particular synoptic situation that favored Alberto's genesis, evolution, and its tropical transition. For this purpose, there were used the synoptic maps of wind field, as well as the variables: geopotential height, air temperature, and sea-surface temperature. The available information was complemented with satellite images and phase diagrams. As a result, it was evidenced that the intrusion of an upper trough in the westerlies and its permanence over the southeast of the Gulf of Mexico and the western Caribbean, generated the favorable synoptic environment for the genesis of this system. In addition, it was found that the high-temperature gradient between the ocean surface and the 500 hPa level established the condition of instability that, in combination with the upper difluence and vertical wind shear generated by the trough, led to the formation of the subtropical cyclone and the development of it.