I've been on a couple of Sundays this year, the lines before noon have been VERY short. The first hour we've practically walked right onto Superman and Batman. My advice is to get there early, like 10:15, so you can get right in when it opens and go directly to Superman. It has the longest wait of any of the coasters. Then double back to Goliath and Batman. When the after-church crowd gets there, the wait will probably be at least 30-45 minutes for most of the coasters, longer for Superman.
With rain in the forecast and no big concerts (as far as I can tell) I don't think it'll be crowded. When it rained Sunday during Memorial Day weekend it thinned the crowds out a bunch. They shut the rides down when the rain got too heavy, but opened them back up pretty quick, since there was no lighting nearby. I even rode Goliath a couple of times when it was raining a little. About 2 hours from closing (8 pm) the wait was like 5 minutes. So if it looks like it won't be raining all day, go!
Hope that helps---and have fun!
Get there AS EARLY as possible. The first hour is the least crowded, and you'll get to go through quite a few rides without anyone in your way. Then, you can pick your fav throughout the day and ride those. It should be quite crowded, but not bad.