A weekly periodicity in eastern U.S. precipitation and its relation to hemispheric circulation

Recurrence of precipitation during weekends—and of precipitation-free weekdays—was especially prominent over the southeastern quarter of the United States during much of the 1964-65 winter. This paper, with the help of maps averaged by day of the week, points out the relation of the weekly precipitation periodicity to interactive cyclone-scale and long wave systems over North America and the Pacific. Despite the averaging, the continuity in these maps is surprisingly clear. For example, an upper trough over west-central portions of the country in mid-week progresses to a position off the Atlantic Seaboard at the beginning of the week. This progression is associated with periodic (largely diabatic) development of the Gulf of Alaska Cyclone, which, in turn, can be traced to the periodic establishment of a ridge in the Bering Sea and occurrence of cyclogenesis in middle and lower latitudes of the North Pacific. An increase in hemispheric wave number, provided periodically by development of the Gulf of Alaska trough, leads to eastward motion of downstream features over the U.S., but when the Gulf trough fills abrupt retrogression sets in until its redeepening reestablishes the eastward motion. One of the primary circulation forcing elements, cyclogenesis in the Pacific, appears to be related to abnormal sea-surface temperature fields established during the preceding fall and maintained by the overlying atmospheric circulation during winter. While this interaction may offer a clue to the sequence of downstream events, it does not imply a weekly periodicity. Not even the transmission of the periodicity downstream is suggested by present numerical forecasts, but this failure can be attributed to typical errors in these forecasts. Further empirical studies, detecting weather periodicities and relating them to the time-averaged circulations or regimes in which they occur, might help in understanding and eventually predicting the occurrence of the periodicities. DOI: 10.1111/j.2153-3490.1966.tb00294.x