Sudden frame drop

Using the Medium setting so far and been doing okay with the FPS. But recently encounter some really bad FPS drop, especially in the city area during the winter season.
My build is
Processor: i7-7700
Installed RAM : 16GB
Graphic :GTX 1060 3GB
Did run the benchmark and it set me to Medium setting automatically. Don’t know if this is games’ issue or my PC’s problem.
Any help is appreciated.