According to the JTWC, Kathy was both the largest and longest-lived typhoon in 1964, with the agency issuing warnings for 13.5 days and the storm's circulation reaching a diameter of 1575 km (850 mi). The interaction of a polar trough and easterly wave led to the genesis of a vortex southeast of Japan by August 11. This system developed into a tropical storm the next day east of Iwo Jima based on ship observations. Maintaining a west-northwestward heading, Kathy reached typhoon strength on August 13, passing well south of Tokyo on approach towards the Ryukyu Islands. The typhoon's winds peaked at 165 km/h (105 mph) on August 14 before tapering as the storm curved towards the west-southwest. For the next four days, Kathy and nearby Typhoon Marie began a Fujiwhara interaction, causing both storms to rotate around one another, ending when Marie was absorbed into Kathy's circulation. Between August 15–16, Kathy briefly fell to tropical storm intensity before regaining typhoon status southeast of Amami Ōshima. During this period, an airplane investigating the storm identified multiple wind circulations at the center of Kathy and the storm's clouds were asymmetric.