Since Rachio already monitors the weather and will skip the upcoming sequence if above a set forecasted threshold, the system should be able to take weather data (from the same station) and determine if it is raining and stop the system. All the information would seem to be there?
Last night we had scattering thundershowers so no defined precip was predicted. However, after the Rachio started, we received almost .25" of rain. It would make sense that the system would stop as it is already polling the weather.
Having a separate rain sensor with the sprinkler system seems superfluous.