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@Sturdley, so I have 48-hours of data (except for an annoying 6 hour internet outage) in the attached image. In each graph the thick yellow line is what OWM thinks the conditions are at that point in time. I don’t have easy access to your national provider so I have used two local Weather Underground PWS as a proxy for the actual conditions on the ground. I have also included comparable data from DarkSky as they are a leading alternative.

For temp, so far I think OWM is doing an OK job. It tracks the PWS references and does a better job than DarkSky that has some divergence in some circumstances. There is perhaps a few degrees Celsius error (this would be +/-15% on zimmerman)

For humidity, the OWM data is “choppier”, my guess is that they don’t update humidity as often as temp and that leads to some timing bias. But again OWM tracks the PWS conditions better than DarkSky. Perhaps with ~15% error in certain areas (this would be +/-15% on zimmerman).

For precipitation, the values just look wrong. They are reporting rainfall, and heavy rainfall at that, when all other services are reporting dry. I have raised this with OWM so will see what happens. I suspect that OWM are leveraging PWS data and that something has gone wrong in the mapping of PWS locations (i.e. they are pulling in some data from non-local PWS – pure speculation on my part). Not exactly confidence building.

On each chart (temp, humidity, precip), I have also shown a grey, dashed line. This is the information used by the OWM Zimmerman calculation. The grey line is the average of the next 24-hour forecast data. So you can see that over the course of the two days, OWM was predicting temp to increase over the period and humidity to decrease i.e. weather will get hotter and dryer. What is interesting is the grey line for precipitation is zero for the duration. I think this is further evidence that OWM has a “problem” in the model. I would have expected the grey line to show forecast precipitation before the yellow line showed what they think is actual rain. This is somewhat curious ! Again, I have raised this with them.

The final graph shows the Zimmerman water level calculated for the period. I forgot to update my logging software to use a Cupertino-friendly baseline so the graph values are based on London setting. So the absolute values are not representative but the shape of the curve is relevant. In essence the values look reasonable given the grey lines for the temp/humidity/precip graphs i.e. increasing the watering level as the forecast predicts hotter/dryer conditions.

This is only a couple of days worth of data and hardly conclusive. I just wanted to get something out quick. I’ll keep it running for the week and we can see how it goes.