Hi Stephen,
I sometimes use the OpenWeatherMap data on the OBS computer when I have it connected to the internet. I have never noticed a problem. Having said that it may not be 100% accurate because I am 20 km away from the actual WeatherMap station that I am monitoring. Have you checked online what the station you have chosen is actually transmitting for cloud cover etc. It would be easy, any time of day to check that against your actual skies.
Right now my local station is showing cloud cover 92%, but as I look up, I would say it is more like 60% max.
Brendan.