The Now button is a learning AI setup to see routines, what apps are used and what is used for gathering daily info, then it will begin to to form a more operational Now Bar.
Anyway, the weather app can be added to the lock screen. But as far as I know, only the factory weather app an be added. Anyway if that is OK, then do this. While on the lock screen press and hold until you get the security input used to unlock the phone. Then you will get a smaller screen of the AOD/Lockscreen. Click on widgets, scroll until you find the weather app and add it.
