I can't comment on the accuracy of weather forecasts in Anaheim in general, but the forecast for last week while we were there was wildly off the week before and even days ahead. One week before our trip the forecast was for rain on Friday. The next day the forecast was for rain on Wednesday but not Friday, the day after that no rain at all all week, the day after that rain on Thursday and Friday. So many forecasts and none of them were right!
During our trip it rained once, on Monday.
Wednesday, Thursday and Friday were beautiful. Tuesday too.
That said, better to think "rain" and get "sun" than the other way around. The forecasts did get me to bring my umbrella, which came in handy.