the United States by a major outbreak of cold air which followed the last Alberta 

 low. This development represents further the trend toward a low index pattern of 

 the general circulation. 



The map of January 16 shows a continuation of all the trends noted on the 

 preceding day. The Azores high has been largely eliminated in the eastern Atlantic, 

 while the Polar Front and cj^clonic activity extend from the central and eastern 

 Atlantic well into the Mediterranean. The continental anticyclone is beginning to 

 push westward at higher latitudes, as indicated by the rise of pressure westward 

 as far as Iceland and Eastern Greenland. The strong low from North America has 

 reached a position in southern Greenland rather similar to that at which the pre- 

 ceding disturbance of similar origin initiated a marked intensification of the Ice- 

 landic low. But in this case widespread rising pressure over the northerly portions of 

 the Atlantic, the absence of any significant negative pressure tendencies in advance 

 of the disturbance, and the absence of any strong local temperature contrast in the 

 Iceland-Greenland area, all these conditions indicate strongly that no such intensifi- 

 cation of the Icelandic center should occur this time. 



By the morning of January 17 a rather typical low index (weak circulation) 

 pattern of the general circulation extends from North America to Europe. Note 

 the southerly latitudes of the frontal action, with two cells of the Atlantic high 

 centered well south of 35° N, the large number of cyclonic centers which move 

 rather slowly eastward without the development of any one center to dominant 

 size or intensity, and finally the significant westward thrust of the continental 

 anticyclone over the Atlantic at high latitudes while active frontal disturbances 

 continue to move from the eastern Atlantic into the Mediterranean, followed by 

 the occurrence of snow in central Europe. 



No significant new development of the weather conditions is to be noted on 

 the map of January 18. However, it appears that the northern European anti- 

 cyclone has intensified markedly since the preceding day, as it continues to extend 

 its influence westward over the Atlantic at high latitudes to the extent of forcing 

 the Icelandic low westward into Greenland. This intensification and westward 

 thrust of the continental anticyclone at high latitudes reaches its maximum on the 

 following day (map of January 19) after which the anticyclone begins to give 

 ground in the north and to thrust southwestward instead. 



Since in winter the blocking action and westward thrust of a major Asiatic 

 polar anticyclone is the dominant feature of the typical transition from a high to a 

 low circulation pattern over western Europe and the North Atlantic, with all which 

 that imphes for the forecasting of the North Atlantic weather, it is worth noting 

 here the weather sequence over Asia that precedes the change over western Europe 

 and the Atlantic. On January 14 a marked intensification of the Icelandic low 

 combined with a persistent anticyclone over northern Europe established an 

 exceptionally strong southerly current off the northwest coast of Europe. During 

 the next two days, between January 14 and January 16, the northern cell of the 

 Asiatic high, centered near latitude 60° N, longitude 95° E, increased from 1040 

 to 1060 mb. Between January 16 and January 19 the central pressure of this cell 

 remained above 1060 mb., while it moved westward from 95° E to 60° E. This 

 was the anticy clonic development noted over northern Europe between January 

 16 and January 19. Not every equally intense development of the Icelandic low 

 leads to a corresponding development of the Asiatic high. Such anticyclogenesis is 

 apparently most favored by the establishment of strong southerly winds along the 

 northwest coast of Europe, i.e., when a persistent high is already well established 

 to the east. When low pressure prevails over northern Europe the newly intensified 



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