3 Halloween Storm 



Synoptic Analysis 



The genesis of the storm of 28 October to 2 November 1991 (the "Hallow- 

 een Storm") involved three significant meteorological features: (a) the sub- 

 tropical low that evolved into Hurricane Grace; (b) a near-record high-pressure 

 (1046-mb) anticyclone that developed in the vicinity of the Hudson Bay region 

 of Canada, and (c) a low-pressure region that formed over the Great Lakes 

 along the front accompanying the strong anticyclone to the north. Figure 8 

 (National Oceanic and Atmospheric Administration 1992) shows the surface 

 analysis at 0000 hr UTC, 27 October 1993. One day later, at 0000 hr UTC, 

 28 October (Figure 9), the sub-tropical low had intensified to hurricane status 

 and been named Grace, the low-pressure region had moved northeastward to 

 the Canadian Maritimes, and the anticyclone had moved to the southeast, 

 thereby increasing the pressure differential along the northeastern North 

 America coasUine. During the next 24 hr (to 0000 hr UTC, 29 October), 

 interaction of the surface low with an upper level trough located to the west of 

 the low (see Figures 10 and 11) promoted rapid development of the surface 

 low. Indeed, during the 12-hr period from 1200 hr UTC, 28 October to 

 (XXX) hr, 29 October, the minimum surface pressure dropped 14 mb from 

 1,012 mb to 998 mb, a rate of decrease approximating that of a rapidly inten- 

 sifying hurricane. 



As the surface low intensified and moved off the coast, it began to entrain 

 warm, moist air from the outer regions of Grace. The warm moist air from 

 Grace, coupled with the cold, dry air flowing in from the strong anticyclone to 

 the northwest maricedly increased the baroclinic contrast (Figure 12). By 

 0000 hr UTC, 30 October, the minimum surface pressure had decreased to 

 983 mb, a 15-mb drop during the previous 24 hr. Meanwhile, the upper level 

 trough had deepened to a closed low and was positioned near longitude 65°W, 

 still to the west of the surface low (Figure 13). During the next 12 hr the 

 surface low continued to intensify and by 1200 hr UTC, 30 October had 

 reached its lowest central pressure of 972 mb. At this time the 500-mb level 

 low was located just slightly west of the surface low (Figures 14 and 15). 



By 0000 hr UTC, 31 October 1991, the system had begun to fill, with the 

 minimum central pressure rising to 982 mb, but moving to the west, which 



12 



Chapter 3 Halloween Storm 



