On December 26, both the JTWC and PAGASA started to monitor a tropical disturbance well east of Mindanao, Philippines just where Sinlaku formed.

[3][4] Over the next day the depression gradually developed further and was named Seniang by PAGASA as it moved northwesterly along a ridge of high pressure, while atmospheric convection wrapped into the system's low level circulation centre.

The storm traversed the Caraga region, and exited the landmass of Mindanao during the afternoon hours of the same day, as it had accelerated to the northwest.

Late on December 30, Jangmi made landfall over Cebu and southern Negros, right before the JTWC downgraded it to a tropical depression.

On December 31, the JTWC issued the final warning to Jangmi, and the JMA downgraded it to a tropical depression at noon the same day.