Apple Weather forecasts for my area have shown a 30-50% chance of precipitation for eternity, even when there isn’t a cloud in sight. My meteorologist has told his readers to ignore Apple’s forecasts.
This started around the time the Apple Weather outages a few months ago and haven’t improved. YSK that you can report inaccurate forecasts from within the app, but it hasn’t improved for me.
I’m currently using Foreca on iPhone and the free version of Carrot on Apple Watch.
Per apple it looks like for the US they use the weather channel.