After crossing the Taiwanese strait, Sepat; already weakening into Category 1 status due to unfavorable conditions, made a second landfall in China on August 18, 2007.

On August 11, a low-level circulation center formed in conjunction with a persistent area of convection about 1,280 km (800 mi) southeast of Naha, Okinawa.

With convective rain bands wrapping into the circulation, the Joint Typhoon Warning Center (JTWC) issued a tropical cyclone formation alert on the system late on August 11.

[2] Remaining nearly stationary between ridges to its northeast and southwest, the system developed further, and early on August 12 the JTWC classified it as Tropical Depression 09W.

[3] Shortly thereafter, the Japan Meteorological Agency (JMA) classified it as a tropical depression while located about 1,245 km (774 mi) southeast of Okinawa.

[8] By early the next morning, August 13, Sepat had a banding eye visible on microwave satellite imagery[9] and had remained in an environment conducive for strengthening, including weakening wind shear.

[10] Sepat continued to slowly gain more organisation and intensify further,[11] and started to move faster under the influence of a subtropical steering ridge.

[15] Late that night, Sepat underwent rapid intensification as expected, and was upgraded by the JTWC to a Category 4-equivalent super typhoon the next morning.

Continuing to intensify,[22] Sepat reached a peak minimum central pressure of 910 hPa, as a Category 5-equivalent super typhoon, on the morning of August 16.

[22] High ocean heat content and good equatorward outflow allowed Sepat to maintain its intensity, but an eyewall replacement cycle began later that night.

[32] After Sepat made landfall in China, the regional meteorological office in Hunan Province sent over 2.25 million text messages to residents to warn them about the storm's remnants.

[31] Heavy rainfall in Taiwan, which reached 984 mm (38.7 in) at Tianhsiang in Hualien,[37] resulted in numerous mudslides, especially in mountain regions and in the south[31] while the combination of the precipitation and winds caused an estimated $1.14 billion (2007 NT$, $34.5 million 2007 USD) in agricultural damage.