A PRESEASON HURRICANE OF SUBTROPICAL ORIGIN

Abstract

The occurence of the May 1951 hurricane of subtropical origin in the western Atlantic before the beginning of the usual tropical storm season was precedent-setting. Through an analysis of the hurricane an attempt is made to explain (1) the unusually early occurrence, (2) the difference between this hurricane of subtropical origin and the usual tropical storm, and (3) the movement as related to vertical structure, upper air flow, and distribution of ocean surface temperatures. The analysis suggests that the following factors contributed to the intensification of the incipient storm which began in connection with a cold high-level Low: (1) superposition of a divergent upper-wind field; (2) heating of the surface layers of the air mass by the Gulf Stream; (3) occurrence of unusually low temperatures at high levels. The movement of the surface center is found to be in accord with the stream flow at the top of the warm core between the 700- and 500-mb. levels. A possible influence of the ocean surface temperature distribution is suggested on the basis of a striking coincidence between the Gulf Stream axis and the storm track.