simulation of HUR5 was estimated during the later stage of the 

 simulation for which the forced component of tjq probably has been 

 damped out. The variation of the period of tjq, from one simulation 

 (of the original synthetic storms HURl through HUR25) to the other 

 may be caused by the variation of the period of the forced component 

 which is subjected to different forcing. There is some indication 

 that the period of t)q is smaller for a faster moving storm. Based on 

 the Gulf mean tidal response in the diurnal band, the GOMT model of 

 Reid and Whitaker (1981) used a 28.5 hour Helmholtz mode. Their 

 volume mode period was adjustable because the radiation boundary 

 condition employed in the GOMT model effectively takes into account 

 the added mass of the Cayman Sea (including the Atlsintic Ocean) by 

 means of a complex ocean impedance. The 28.5 hour period used by 

 Reid and Whitaker (1981) is close to the average period of the free 

 component of tjq obtained from this study. 



It is likely that Gulf hurricanes in general elicit the 

 Helmholtz mode and the longer period modes, but only certain storms 

 generate a forerunner. With the definition of a forerunner as the 

 initial rise of water level before the peak surge, storms traversing 

 PATH4 do not generate forerunner surges but certainly excite »?q. The 

 synthetic storm HUR25 (along PATHS) serves as another example of the 

 situation where tjq exists but with no forerunner. The presence of a 

 forerunner in local hydrographs is therefore dependent upon the path, 

 but is also dependent on the landfall position and the time of the 

 excitation of tjq relative to the peak surge. This would explain why 

 every hurricane of record does not have an associated forerunner. 



195 



