Sarika was first noted by the United States-based Joint Typhoon Warning Center (JTWC) as a disorganized tropical disturbance on October 11, while it was about  southeast of Manila. The system quickly consolidated within a favorable environment of low wind shear and high sea surface temperatures of about , leading the JTWC to issue a Tropical Cyclone Formation Alert. By 00:00 UTC on October 13, the system's low-level circulation center became increasingly symmetric as it tracked northwestwards and the Japan Meteorological Agency (JMA) upgraded the disturbance to a tropical depression. The JTWC followed suit six hours later. Intensification slowed thereafter as an upper-level anticyclone suppressed outflow on the eastern side of the system. However, the system managed to intensify into a tropical storm by 18:00 UTC on October 13 as it traveled west-northwestwards along a subtropical ridge. At this point, the system was given the name Sarika by the JMA. The Philippine Atmospheric, Geophysical and Astronomical Services Administration (PAGASA), which had been tracking the storm since October 12, assigned it the local name Karen.