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.
When I retire I’ll learn to program JavaScript to stave off dementia, work out how to parse the BoM data predictions and observations. I’d like to add another page to the weather app that shows a chart something like this, but not necessarily as fancy.
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.