This YMT mod will add Snipers on rooftops, its simple really.

Spawns:
Rooftop Snipers currently and only spawn in Mission Row and some surrounding areas. Additinally, scripted swat spawns also happen in some specific locations.

New Dispatch:
Mod also comes with improved dispatch, which makes the police bit more threatening. Requires Cpast's Combat Tweaks (link for that is in the readme.txt)


UPDATE:
Snipers will only spawn when wanted level is 4 stars or higher! And will also use vanilla noose models, meaning no addon peds! (Tho you can still replace the sniper model with something else in ambientpedmodelsets.meta)
Mod also uses scripts to enable scenarios, which means snipers will spawn on specific condition (having a 4-5 star wanted level)

I won't update this anymore, but i do made a script version from this, which you can also edit to add more locations for sniper spawns:
https://github.com/CoolDude034/PoliceSnipers




Installation is in readme.
Drop the .dll and .pdb in scripts folder