Hurricane Isbell was first identified as a weak tropical disturbance on October 7, 1964 over the western Caribbean Sea. Situated to the south of a diffuse trough, the system remained weak and relatively disorganized as it moved generally northwest near Honduras and Nicaragua. Despite the presence of an upper-level anticyclone, which promotes favorable outflow for tropical cyclones and aids in tropical cyclogenesis, a lack of distinct low-level inflow inhibited intensification. Additionally, an area of warm mid-tropospheric air was present within the cyclone. Though a disheveled system, it is analyzed to have become a tropical depression by 12:00 UTC on October 8. The following day, the depression skirted the eastern coast of Honduras. Operationally, it was not until October 10 that the Weather Bureau initiated advisories on the depression. On that date, a weather reconnaissance mission into the system found a weak low-level circulation with a barometric pressure of 1007.3 mb (hPa; 29.75 inHg) and winds of 20–30 mph (30–45 km/h) in squalls. Throughout October 11 and 12, the depression slowly executed a tight cyclonic loop over the northwestern Caribbean Sea. It finally organized into a tropical storm and was given the name "Isbell" by 00:00 UTC on October 13 after completing the loop and acquiring a north-northeast trajectory.