7 Western Atlantic 

 Retrograde Storms 



An initial step to estimate the frequency of occurrence of a storm having 

 characteristics similar to those of the Halloween Storm was to focus on west- 

 em Atlantic storms exhibiting retrograde (movement east to west) tracks. This 

 characteristic of the Halloween Storm is an obvious one and is readily and 

 objectively determined. Moreover, using this characteristic minimizes the 

 effect of incorrect depiction in the analysis products of other commonly used 

 storm characterization parameters. The Mariners Weather Log was examined 

 for northwestern Atlantic storms that occurred from 1956 to 1991 with retro- 

 grade tracks. The domain of examination was west of longitude 50°W and 

 south of latitude 55°N. Storms are tabulated by month and year of occurrence. 



Table 3 summarizes the tabulations. There were 1 1 1 storms exhibiting 

 retrograde tracks during the 34-year period, or an average of 3.2 retrograde 

 storms per year. The number of retrograde storms in any 1 year ranged from 

 (1973 and 1981) to 9 (1958). There does not appear to be any particular 

 year-to-year correlation. There does appear to have been significantly fewer 

 storms (21) in the most recent decade, 1980-1989, than in the two previous 

 decades, 35 storms in the 1970-1979 decade and 37 storms in the 1960-1969 

 decade. 



The annual distribution, presented in Figure 33, shows a pronounced sea- 

 sonal variation with more storms occurring in the fall-winter months, as 

 expected. However, the fall-winter distribution appears bimodal with peaks in 

 November (12) and April (14). The statistical significance of this bimodality 

 is questionable given the small sample size. The greatest inter-month differ- 

 ence occurs between October (6) and November (12), which suggests that the 

 strong thermal contrast which occurs when cold continental air masses move 

 south to meet the warmer subtropical maritime air is a significant contributor 

 to the formation of systems which may take retrograde tracks. 



Chapter 7 Western Atlaritic Retrograde Storms 



41 



