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@Peter Perfect analysis, explaining the differences between OS historical and forecasted weather data usage.

In my opinion only historical weather data is acceptable as OS Zimmerman Method input. Temperature forecasts may be acceptable and not be the problem. However, in regards to rain and especially the amount of rain, weather forecasts are often far away from reality. I can’t speak for all countries, but at least for Germany rain prediction is not always trustworthy.

I started last year with an OSBee and a selfmade IFTTT, weather algorithm based on weather forecasts. My program often failed for garden watering as there was rain announced, which never happened during the ongoing day. This causes me to change to an OpenSprinkler 3.0 and the accompanied Zimmerman method. Based on historical WU data, forwarded by my own weather station, this worked and still works perfect for me.

I also believe that it is much preferable to have an own rain gauge. There is sometimes rain forecasted for the ongoing day, but there will be no remarkable rainfall at all at my backyard. With an own rain gauge and the use of historical weather data it is possible to avoid these uncertainties.

In regards to free access to historical weather data, there is a possibility to provide and collect weather data by an own weather station. This still works for me in conjunction with WU. However, I’m also depending on cloud services like Meteoware to forward the data from my Netatmo weather station to WU.
There are already some solutions like private weather servers or my ESP32 Node MCU program that controls the OS Watering via API commands discussed at this forum. Unfortunately, these solutions need some programming knowledge and additional HW efforts.

The majority of OS users will still depend on the OWM weather adjustment feature provided at no extra costs by OpenSprinkler. I’m not sure if this will work out at the end for weather controlled garden watering. Forecasted weather data will always have uncertainties. With the current implementation it changes during night and day, which makes it difficult to perform sufficient early morning garden watering.