The monsoon trough spawned a tropical disturbance over the western Pacific Ocean to the east of Guam around October 7. The system moved steadily westward, organizing enough for the American-based Joint Typhoon Warning Center (JTWC) to issue a Tropical Cyclone Formation Alert early on October 9. After the disturbance passed south of Guam, the JTWC began issuing advisories on Tropical Depression 18W late on October 9, and the Japan Meteorological Agency (JMA) followed suit the next day. While passing north of Yap in the Caroline Islands, the depression intensified into a tropical storm according to the JTWC, which gave it the name Zeb. The JMA again was delayed in upgrading by a day.