Traditionally the week leading up to Easter and the week of Easter are two of the three most crowded weeks of the year; the other is Christmas Week. For popular rides you can get sustained lines of 90 minutes to 2 1/2 hours for the entire day and night. Even the dud, Swiss Family Treehouse, can get to an hour or more. Fast passes for the popular attractions can run out for the day before noon and the waterparks often reach capacity before 11:30. It is doable but expect a larger crowd than you have ever experienced unless you have done it before.
As to weather, it tends to be ideal that time of year, 80's, not severe humidity, occassionally it gets into the 90's and sometimes you can even get a cool day in the low 70's. Thunderstorms often are limited to night hours when you are asleep.