the most severe combination of hurricane parameters that is reasonably 

 characteristic of a region excluding extremely rare combinations." 



The maximum gradient wind speed and maximum wind speed in the belt 

 of maximum winds in miles per hour were determined by the following 

 formulas : 



Vgx = ^(Pn - Pq)^ - R(0-575 f) (29) 



V^ = 0.865 Vgj^. + 0.5 Vp (30) 



where < is taken as 73 for the SPH, Pn and Pq are the peripheral and 

 central pressures in inches of mercury, respectively, R is the radius 

 of maximum winds in nautical miles, f is Coriolis parameter in units per 

 hour, and Vp is the forward speed in miles per hour. Vx is the maximum 

 wind speed 30 feet above the water. 



A knowledge of these variables for a given location allows computa- 

 tion and graphical construction of the storm's entire isovel field. 



3. Probable Maximum Hurricane 



In the past several years, many nuclear-powered electrical generating 

 stations have been constructed or proposed. Many of these have been 

 sited along the Atlantic and Gulf Coasts of the United States where they 

 may become vulnerable to hurricane surge and surface waves. To ensure 

 that these power plants would not be flooded by hurricane surges, includ- 

 ing the wind-generated surface waves, with the exception of extremely 

 rare events, the Atomic Energy Commission (AEC) established a criterion 

 which would provide adequate safety for public health. AEC concluded 

 that adequate safety would be provided if the plant site would not be 

 flooded by the surge and surface waves associated with a Probable 

 Maximum Hurricane. 



The Hydrometeorological Branch of the Weather Bureau in an Interim 

 Report, HUR 7-97 (1968), developed the characteristics of the PMH which 

 is substantially more severe than the SPH. The PMH was defined as "A 

 hypothetical hurricane having that combination of characteristics which 

 will make the most severe storm that can probably occur in the particular 

 region involved. The hurricane should approach the point under study 

 along a critical path and at optimum rate of movement." 



HUR 7-97 not only developed the characteristics for defining the 

 PMH, but updated the experience hurricane data originally carried out 

 by Graham and Nunn (1959) . Development of the isovel fields is essenti- 

 ally the same for the PMH as that given for the SPH. The difference is 

 that Pj^ is taken as the standard sea level pressure of 29.92 inches of 

 mercury for the SPH, while Pj^ is considered as a function of the latitude 



23 



