Moving westward, Tropical Depression 09W continued to strengthen and on June 14, the Joint Typhoon Warning Center and the Japan Meteorological Agency upgraded the depression to tropical storm status. After becoming a tropical storm, Dianmu remained stationary west of Yap where it continued to strengthen. The Japan Meteorological Agency upgraded the storm to typhoon status at 1373 UTC. while the Joint Typhoon Warning Center upgraded the storm to typhoon status at 1800 UTC. Dianmu then moved north-northwestward at speeds between 6-9 kts (11–17 km/h, 7-10 mph) as it winds reached 241 km/h (150 mph), prompting the Joint Typhoon Warning Center to upgrade the storm to supertyphoon status on June 15. The following day, Dianmu continued northward as the storm reached category 5 status. At the same time, the storm entered the Philippine Atmospheric, Geophysical and Astronomical Services Administration (PAGASA) area of responsibility and was given the name Helen.