We were there the third week of June last year. The weather is hot and humid with daily rains. For the most part the rain does not last very long, but we did have several hours of rainfall. Luckily, the rain had no impact on us since we were fortunate enough to be either ahead of or behind the storms. On our first day it poured after breakfast and during our bus ride to MK. Then as we approched the MK the rain stopped. Actually, with all the clouds the temperatures were kept down.
The crowds are large, but by planning and using Fast Passes we really had no problems. Also, get to the parks early and then take a break back at the hotel. We found it really refreshing. E-night was very enjoyable. We were in the MK until 12:45am. Had a great time.
Keep in mind that Test Track is VERY popular. Get there early or you might not get to ride.
We are returning this June and expect to have a great time. Don't get overly concerned about crowds and weather. There are never any guarantees at any time of the year (especially with the weather). Go and ENJOY!!!!!