@Gilles , good question, my post wasn’t clear enough. I firstly wanted to show that OWM is a reasonable proxy for local weather conditions by providing a side-by-side comparison of what my PWS says at any point in time compared with what OWM think. In my case the two sources are pretty correlated. So that is good (but your mileage may vary). Secondly, I wanted to point out that the logic was very different i.e. using forecast rather than historical data and the effects of that on water level. The results can be counter-intuitive unless you understand what is going on. Not saying that one method is better than the other just pointing out the differences.
What I didn’t show was the stability of the forecast data used in the calculation. So the OWM logic uses an average of the next 30 hours worth of forecasts (i.e averaging over the next 10 x 3-hour forecasts) not just the next three hours. To show this more explicitly, the weather in London is getting better with temps going up over the last 4 days. Attached is the PWS temp at any point in time alongside the average temp for the next 30 hours. You can see that the later is fairly stable but trending upward so water level “shouldn’t” fluctuate too much between day and night but rather gradually increase in anticipation of warmer/dryer conditions. It is the 30 hour forecast (along with the humidity and rainfall equivalents) that is used in the new OWM based zimmerman calculation.
So Weather Underground changing their model has impacted a bunch of products/projects. Unfortunately, there is no de-facto source of historical weather data anymore so the OS guys have chosen to go with OWM but that only provides cheap access to forecast data. So historical data is not an option in the current setup. Realities of the whole internet cloud business model shaking out.