After reaching peak intensity, Dianmu turned northwest before skirting the minor Japanese islands and making landfall in Japan as a tropical storm.

An area of disturbed weather formed on June 11 within a monsoon trough 550 km (340 mi) southeast of Palau.

Satellite imagery showed the disturbance beginning to acquire a low level circulation as the system was within a favorable area for tropical development.

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.

[1] On June 20, the center of Dianmu bypassed Okinawa by 120 km (75 mi) to the east as the storm continued to weaken.

Satellite imagery showed the storm losing its deep convection and significant outflow and its winds dropped to 145 km/h (90 mph).

[6] Because Dianmu was very close to the islands of Yap and Palau, the National Weather Service began issuing advisories.

[1] In Okinawa, the approach of Dianmu caused residents to stock up on supplies as all U.S. military bases were put under a Tropical Cyclone Condition of Readiness level three (TCCOR).

[7] The approach of Typhoon Dianmu resulted in the cancellation of numerous incoming and outgoing flights and ferry service.

[8] In mainland Japan, 130 flights and passenger rail service was canceled, many homes were evacuated, and schools were closed.

[8] In mainland Japan, Dianmu brought strong winds and heavy rainfall across southern, central and eastern portions of the country.

[2] Dianmu's outer rainbands dropped heavy rainfall across southern and eastern parts of South Korea.