We were there last week and forecasts called for low 60s our arrival day and then warming as the week went on. We packed accordingly, as we were coming from Boston in a blizzard 60's was "nothing" to us! Well, it poured buckets the 3rd day of our trip late in the afternoon and continued ALL night. Dipped into the high 40's- low 50's that night. MK cleared out!! 7DMT was less than 20 minutes as was Anna and Elsa M & G!! We embraced it while others flocked out of the parks. So, if it is lousy weather, you will have that as a positive. Oh, after the rain it was picture perfect clear blue skies the next 2 days, but....in the 30's and 40's in the mornings and only warmed to barely 50 during the day!
My point is, do NOT rely on the forecasts. And pack for a variety of conditions. Oh, also keep your ponchos with you, don't leave them in the hotel (yes we inadvertently did this!) I refused to purchase the $9 ones so after DHS before heading to MK we went back to get them.
It is Disney, have a magical time!!