Dianmu formed on June 13 and moved west-northwest where it rapidly strengthened to typhoon status on June 16. Dianmu then strengthened to a 285 km/h (180 mph) typhoon, making it one of nine typhoons to reach that intensity since 1990 and equaled its power with Typhoon Chaba as the strongest typhoons of the 2004 season. After reaching peak intensity, Dianmu turned northwest before skirting the minor Japanese islands and making landfall in Japan as a tropical storm. Dianmu then became extratropical on June 21.