I'm not sure I'd have to look at it a bit - probably look at APIXU's attributes and tie it in using RM - haven't looked at the forecast stuff yet so not sure how it's reported.
You can also check with the developer @bangali for advice as well.
That APIXU driver is also good for illuminance checking etc for things like automated exterior lighting instead of just sunrise/sunset.
edit: Looking at the driver code there is a "percentPrecip" attribute. So you could do a comparison like you were thinking maybe, uncertain how far out that goes.
edit2: so I think exposing that attribute you may have to use another app by @bangali called WATO
[RELEASE] WATO - When any Attribute Then this cmd Otherwise that cmd
You'd use this (I think) to set a virtual switch when the chance of rain crosses your threshold.. then in RM you'd compare the moisture sensor + chance of rain switch and act accordingly.
