AccuWeather.com is another one I like to check although it seems you can get quite different forecasts between the sites this far out.
We'll be there the 1st week of May and will be packing shorts for all of us and sweatshirts for layering. Ill probably throw a couple of pants in the suitcase on the off chance it happens to be cooler some of the days. I'll also put a couple of small blankets in the stroller for my little ones incase it gets chilly in the evening.
You should check the weather reports right before your trip. It was nice and cool here in Orange County (next to the park) then bam - this week brought warmer temps and a little humidity. Weather will be in the 90s for a few days then will slide back down to the 80s then 70s. Weather can be crazy, this past year more than most!!
We went a few times in May and in June. We were a little surprised at the weather. We expected the California sunshine and warmer weather than what we have in the midwest but found a strange phenomena called "May Gray and June Gloom." Ever day we woke to clouds, and we could just tell the sunshine was right behind the clouds trying to break through but it just wouldn't. So it was cloudy the whole day, every day, for every trip we took during May and June. Finally, we just decided to skip it and went in July and the weather was picture perfect. It was definitely NOT a showstopper, but I guess I wish I knew about it before I went. I am not a weather fanatic, I do not know if this happens every year or what, but you can google it, it's definitely a phenomenon for that area.