I remember that when I used Geometric Weather (not developed anymore, its more known fork is called Breeze Weather) it had a option to notify me when it was going to rain, but know that functionality was removed from Breeze Weather, it’s there any other app with this functionality?
It’s not open source but Rain Alarm draws a circle around you on the map and alerts you if weather radar detects any precipitation entering that circle. I don’t know if that’s what you want but it’s what I want in a rain notification and it’s the only app I’m aware of that does it.