Colder air, cooler waters, and higher shear contributed to a weakening trend, and Cecil was downgraded to a tropical storm on August 11.

On August 14, Cecil turned east and struck South Korea as a minimal tropical storm before dissipating over the Sea of Japan.

Over the next four days, the disturbance tracked westward and remained embedded in the monsoon trough, maintaining a closed surface circulation and an area of enhanced convective activity on satellite imagery.

Initially, a Tropical Cyclone Formation Alert (TCFA) was not issued because strong easterly wind shear was expected to inhibit development.

On August 4, following an increase in convective activity, a decrease in wind shear, and ship reports in the area measuring central pressures of 1,000–1,003 mbar (29.5–29.6 inHg), the Joint Typhoon Warning Center (JTWC) issued a TCFA.

[3][nb 2] The JTWC upgraded the system into a tropical depression at 06:00 UTC on August 5 after a Hurricane Hunter aircraft mission observed sustained winds of 50 km/h (30 mph).

Over the ensuing 24 hours, Cecil entered a period of rapid deepening, and by 00:00 UTC on August 8, the JTWC had raised the intensity to 185 km/h (115 mph).

[5] Midday on August 8, the JMA reported that Cecil attained its peak intensity of 200 km/h (125 mph) and a minimum barometric pressure of 920 mbar (27 inHg).

[3] The JTWC estimated that Cecil attained a peak intensity of 235 km/h (145 mph) at 18:00 UTC while located 220 km (135 mi) east of Taiwan.

[5] By August 14, Cecil had passed to the east of Japan was beyond the northward influence of the subtropical ridge and began to track eastward, striking South Korea as a minimal tropical storm.

Cecil's circulation was unable to reorganize after crossing the Korean peninsula and dissipated in the Sea of Japan on August 15,[2] with the JMA ceasing to track the system at 06:00 UTC.