It is working with WU. You need to setup the backend, activate sensors you’re interested in, in my case it was today’s precipitation chance %, and today’s precipitation mm, which I use for establishing if it has rained and if it is going to rain.
It’s good to have a look for a local weather station, just to improve the accuracy of local predictions and observations.
Just a word of warning, there are times when a proper soil moisture sensor beats any external data. You could find that the day before it rained so hard that the soil is still waterlogged, yet “today’s” predictions and observations, might let the sprinkler water your garden.
You could improve that further by capturing and storing a variable from the day before, i.e. capture it today and use it tomorrow… someone correct me, but I’m not sure if WU provides historical data through the API used. If so, then just look a day back, set a slider for sensitivity of the wetness, so you can dynamically change it later (similar to current observations).