How is it possible that prediction and Radar are so much off. Forecast gives a full sun for the next hours but radar shows full clouds for the next hours. I don’t trust any forecast anymore.

I encounter more and more discrepancies between forecast and realtime radar. The hour by hour forecast shows full Sun but when I look at the realtime radar it shows overcast clouds which is, when I look outside, the reality. How can it be that the hour to hour forecast is totally wrong and not updated when the radar tells me another story.