A Study of 500 mb Vorticity Maxima Crossing the East Coast of North America and Associated Surface Cyclogenesis

Abstract

For assessment of the sufficiency of the approach of an upper-level vorticity maximum as a predictor of explosive surface cyclogenesis in the western Atlantic, a study was made of all 500-mb maxima which crossed the east coast of North America between 5 October 1985 and 4 April 1986. Of 96 such events, 38 produced bombs. This overall likelihood of 40% was greatest (50%) during the period from 19 December through 1 February, when crossings were most frequent. In the 26 cases when the strength of the vorticity maximum was at least 22 ? 10?5 s?1, bomb likelihood was also 50%. When the crossing occurred at or equatorward of latitude 41°N, bombs occurred in 17 of 33 instances (52%). The likelihood rose to 47% when the speed of the upper maximum exceeded 30 kts (15.5 m s?1). The best discrimination was found when this speed was multiplied by a measure of the strength of the vorticity gradients upstream and downstream from the center. On this basis, 16% of the 25 smallest values of this product produced bombs whereas 68% of the 25 largest values did so. Thus it appears that the intensity of the baroclinic forcing influences the probability of an explosive response. The effect of tropospheric stability, static or symmetric, was not examined.