Netro support two weather sources: darksky and wunderground. Darksky is the default one, if it doesn't work, you can try with wunderground.
Netro periodically update weather data and re-calculate schedules if weather changes. If forecast data is not accurate, for example, not enough rain on 20th, Netro knows the accurate data later (e.g. 21th) and water more later.
Currently, Netro adds a system-level constraint: "do not water if the effective rain (rain *probability) is greater than 0.1inch (2.5mm)". The next release of Netro app will allows customers to personalize the threshold.