The weatherflow IFTTT service has a trigger for when a station determines that it is raining. I’d like to reproduce that functionality based on the REST API. Given the different rain properties in the returned json I’m just not how to leverage them properly for this.
You have got two options. If you want a near-instant notification that it is raining you can use the Websocket and listen for evt_precip messages. These will be sent when it starts reading. WeatherFlow Tempest API Websocket Reference
If you want to use REST API requests, your best bet is to pay attention to the rain accumulation field in Sky (index 3 of the obs JSON) and Tempest (index 12 of the obs JSON) responses. This gives the amount of rain accumulated over the previous 1 minute. You’ll want to look for when that value becomes non-zero (i.e. it has gone from not raining to raining).