Because the Weather App doesn’t yet have cloud support to save your local weather data predictions and observations and how much water is applied, it’s not possible.
As far as I can tell, the Weather App computes an instantaneous fudge factor (Zimmerman or ET0) to apply to each hard wired watering sequence just before the OS Controller runs a schedule. In Queanbeyan you often only get rain if you are lucky enough to be under a cloud if it drops, one side of town can get 20mm and the other side none, so it’s hard to predict. The BoM rainfall prediction for 3rd Nov is 95% chance of 15-20mm which is 95% chance of any rain, 50% chance of 15mm and 25% chance of 20mm, I’m not sure how the Weather App handles that. My experience locally is I’d rather it water anyway but reduce it to 10% min so if it doesn’t actually rain the plants won’t suffer too much.
I’m wondering if I could knock up a prototype in Google Sheets that does this, there must be a way to get GS to send API calls to the controller.