Tornadoes Associated with Cyclones of Tropical Origin-Practical Features

Abstract

The frequency of reports of tornadoes associated with tropical cyclones is shown to have increased during the past decade to a level worthy of professional attention. Tornado incidence is demonstrated to be related to the direction of movement of the tropical cyclone, and to the intensity of the cyclone as it approaches the coast. A heading slightly cast of north is most favorable for tornado occurrence. Most tornadoes have occurred with tropical cyclones from the Gulf of Mexico which reached hurricane intensity, or with intensifying tropical depressions. Compact spatial distributions of tornadoes are disclosed upon relating occurrences to five selected stages in the life history of the associated tropical cyclone. The best relationships on location within the parent cyclone are obtained with respect to true azimuth, and are superior to those obtained using an orientation from heading along the tropical cyclone track. By eliminating the tornadoes associated with the extratropical stage of the tropical cyclones, the tornado occurrences were placed in a compact pattern in the northeastern sector and within a specified normalized distance from the center of the parent cyclone. The tornadoes connected with the extratropical stage are found widely scattered within southerly quadrants of the parent cyclone. Rather wide variations are found in the sea level pressure field in the neighborhood of tornado occurrences, but the averages established may have some utility. Tornadoes are shown to have occurred with both inner rainbands and the outer rainband, as well as outer convective cells, with the stronger cells of the outer rainband the most likely spawner of tornadoes. The forecasters' conception of the ?bar? of the hurricane is disclosed as the approximate outer limit for tornadoes. Several parameters normally associated with midlatitude tornadoes are found to be mostly inapplicable to tornadoes within tropical cyclones. Some evidence is presented suggesting convective instability as a contributing factor. Sectional distribution, severity and frequency of tornadoes associated with tropical cyclones are determined to allow comparison with midlatitude tornadoes. Over 80 per cent of tropical cyclone associated tornadoes in the Carolinas and Virginia occurred with parent cyclones which had a Florida coast landfall. Precepts considered of practical value for forecasting tornadoes with tropical cyclones are summarized.