at 43° N, 50° W. At that point the storm was caught by the strong zonal wester- 

 Hes which then prevailed at higher latitudes and suddenly moved into the Icelandic 

 low, covering as much distance in two days as it had in the previous thirteen days, 

 and losing its hurricane characteristics at the center. 



It should be pointed out that also the disturbance centered over Nova 

 Scotia on August 29 followed a low index behavior pattern during the following five 

 days. As the front moved slowly eastward the disturbance developed retrogres- 

 sively on the front so that the center was displaced actually southeastward in the 

 western Atlantic. By September 3 the center had nearly disappeared in the south- 

 western Atlantic. The behavior of this disturbance was obviously determined by 

 the weak circulation pattern which continues to dominate this portion of the 

 Atlantic. The fact that the southwestern portion of the North Atlantic should 

 frequently be the last portion to change from a weak to a strong circulation pattern 

 is consistent with the statistical evidence that index changes tend to progress from 

 east to west over Europe and the Atlantic, and that the reestablishment of strong 

 zonal westerlies occurs first at high latitudes, thence progressively southward. 



By September 1 a typical summer strong circulation pattern is well estab- 

 lished north of latitude 45°, as is evidenced by the two strong cyclonic centers at 

 high latitudes, by the absence of polar anticyclones, and by a strong Azores high. 

 The extension of the subtropical highs into the western Atlantic and the eastern 

 United States is the final step needed to complete the typical summer high index 

 pattern from North America to Europe. This completion of the strong circulation 

 pattern is delayed for more than a week while the tropical hurricane which was 

 discussed above moves slowly northward. Meanwhile, at high latitudes, the pre- 

 vailing circulation pattern during the first half of September is exceptionally 

 strong. A sequence of strong lows move rapidly eastward, near latitude 60° N 

 from North America to northern Europe. This tendency to rapid eastward motion 

 of strong lows at high latitudes is quite characteristic of a strong circulation pat- 

 tern, both summer and winter. It normally extends eastward into northern Europe, 

 as observed in this case on the map of September 3, unless the blocking action of a 

 strong polar anticyclone is present in northern Eurasia (cf. p. 49). 



In this case the rapid eastward movement of strong lows continues through- 

 out the first half of September. During this time cyclogenesis recurs repeatedly in 

 the vicinity of Hudson Bay, and between Iceland and northern Europe. Conse- 

 quently, the mean circulation pattern continues throughout the period to show one 

 principal cyclonic cell in each of these two regions, within a trough of low pressure 

 near the sixtieth latitude circle. This strong circulation pattern terminates quite 

 abruptly about September 15, with a return to a markedly weak state of the 

 general circulation. 



55 



