Rain happens. (It rained all or part of 6 of the 10 days we were at WDW.) Prepare for it. Everyone talks about wearing ponchos, but think about what you and the others in your party like to wear in the rain. A raincoat? A poncho with a hood? Umbrellas? I prefered a ball cap to the hood of my poncho. If you're going the poncho route, buy the thicker ones...not the ones for under $1 that are like a dry cleaner bag.
Next, think about shoes...everyone MUST have at leat two pairs. One night we got caught in a storm at MGM that left all of our tennis shoes totally soaked.
Everyone thinks about summer showers feeling good on a hot day...but you're wet and you go on a ride or in a store or show or restaurant and the AC makes you freeze. So consider bringing a few outfits that give you more coverage than a tank top and shorts would.
If your clothes get totally soaked, remember that the resorts have laundries that have dryers. After the MGM soaking, I just put all of our clothes in the dryer for one cycle...better than trying to dry out soaking clothes all week.
Be flexible in your park choice (have a "plan B" for rainy days). IMHO, AK is a terrible park if it is raining much at all. Only Dinosoar, ITTBAB and the Lion King Show are inside. Everything else is outside. Also, Epcot World Showcase is pretty miserable in the rain. In contrast, Mk is great...there are so many rides close together. Epcot Futureworld would be pretty good in the rain, too. Some attractions are long, and there are multiple attractions in the same building.
Don't worry. Prepare. Have fun!!! (We got to be the first family on Dumbo one drizzly morning at MK...of course, our feet were in about 3 inches of water

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