Weather apps are the most downloaded category of apps in any app store. People seem to be obsessed with them.
Apple delivered a ton of "hidden" improvements in iOS15 that you probably won't be able to find on your own. One such improvement is the ability to receive real-time precipitation alerts through Apple's iOS weather app.
In order to enable this new functionality:
Open the weather app.
Tap the icon with three dots on the lower right corner of the screen
Click on the 3 dots in the upper right corner of the screen
Select Notification
Turn on the toggle next to the locations you want alerts for.