In May 1964, the western Pacific was characterized by anomalously high geopotential heights towards the northern part of the basin and low geopotential heights in the tropical regions. This configuration was favorable for tropical cyclogenesis and led to the development of typhoons Tess and Viola, the first storms of the 1964 typhoon season. Shower activity in the tropics west of Hawaii was above average between May 10–20. The first half of June marked a reversal of this pattern as a large area of low pressure became established over the mid-Pacific. The average sea-level pressure for the month was below normal for most of the northern Pacific. Typhoons Winnie and Alice formed in the second half of the month when the initial height patterns returned. July 1964 featured more tropical cyclones than any other July on record, though this was superseded by the 1971 Pacific typhoon season. A strong high pressure area south of Japan caused most storms during July to take slow and westward paths.