Created by Roger Edwards, Storm Prediction Center
At right is a velocity display, showing a pronounced couplet of bright yellow and blue in the same place as the "hook." The Doppler radar, which in this case is west-southwest of the storm, detects motion in a storm moving toward and away from it, as it "looks" down the radar beam. Here, the color couplet shows strong motion away from (yellow) and toward (blue) the radar, indicating that the storm is rotating where the radar beam cut through it. This circulation, called a mesocyclone, was also present at higher levels in the storm, and lasted over an hour. Thunderstorms containing mesocyclones are known as supercells, and sometimes produce destructive tornadoes.
Supercells in hurricanes often have small, very subtle, short-lived, and/or very shallow mesocyclones, forcing the radar observer to be exceptionally diligent. Very destructive tornadoes have come from weaker- looking mesocyclones than this in tropical cyclones; and the Cape May storm has an unusually large and prominent one. This supercell was also uncommon in that it was quite isolated -- very far from any other thunderstorms in the circulation of the hurricane. Most supercells detected in tropical cyclones occur within their spiral rain bands.
Had this been over land, a tornado warning may have been needed. It barely stayed offshore, moving from Atlantic coastal waters northwestward to a few miles off Cape May, then parallel to the New Jersey shoreline, before dissipating over the mouth of the Delaware River. Since this supercell avoided land, we will probably never know if it contained a tornado.
