I actually went last thursday, and the lines were not bad at all. The longest wait was for Firehawk of course, and I waited for around 52 minutes during the 7-8 PM hour. The Beast wait was fairly short (roughly 5 minutes) and Vortex, Flight of Fear and Topgun were walk ons. Of course, that was just my luck, as the day for you could be fairly different. It all depends on the crowds at the waterpark and the leveling out at the main park.
As for the weather, I believe Cedar Fair adopted their policy at Kings Island when mainly all rides close. Someone else on this board may know the actual information.