The system moved west-northwest, developing into a tropical depression and tropical storm on December 4. Lex remained a tropical storm for one day, before weakening back into a tropical disturbance on December 5 due to vertical wind shear caused by Typhoon Kim and an upper trough nearby from the northwest.
After interacting with an upper trough, the disturbance strengthened, and by the following day, it developed into the fourth tropical depression of the season at 0600 UTC.
By June 24, it began interacting with the extension of a monsoon trough in the region, generating broad cyclonic flow and scattered convection in conjunction with an upper trough to its northwest.
The hurricane held peak intensity for several hours, however, an upper trough northwest of the Hawaiian Islands was gradually approaching Ignacio.
An upper trough over the Western United States forced Marty to moved toward the northwest and north for a while, bringing the hurricane to waters previously cooled by hurricanes Kevin and Linda.
Pulled by an upper level trough, Percy swung to the east, passing Tropical Depression 24S in the process, before being caught up by another trough.
The hurricane was curved north by an upper-level trough.
However, by August 5, vertical wind shear began increasing, which was generated by an upper-level trough.
Tropical Depression Twenty-Three moved west-northwestward, lured poleward by an upper-level trough near northern Mexico.
The wave's associated convection, or thunderstorms, were amplified by an upper-level trough to its west.
