With iOS 15, you can now receive a weather notification when rain, snow, hail, or sleet is about to start or stop in your current location. In addition, you can also receive precipitation notifications for locations in your weather list.
To receive notifications, turn on Location Access in Settings. Go to Settings > Privacy > Location Services > Weather, then tap Always. Turn on Precise Location to receive the most accurate notifications for your current location.
To enable notifications:
Open the stock Weather app on your iPhone.
Tap the bullet list icon in the bottom-right corner of the screen.
Tap Turn on Notifications at the top of the location list. You'll see a "Stay Dry" box at the top, if you don't, tap the circular ellipsis icon in the top-right corner of the screen, then tap Notifications.
On the "Allow Notifications" screen, tap Continue.
In the prompt that appears, tap Allow.
Toggle the switches next to the locations that you'd like to receive notifications for, then tap Done.