I go to Disney with a nylon poncho (more comfortable than plastic, and living here, you get a lot of use out of it
) a compact umbrella and a golf umbrella.
If the chance of rain is higher than 70 percent, I'll take the poncho and the golf umbrella.
If the chance of rain is between 40-70 percent, I'll just take the poncho.
If the chance of rain is less than 40 percent, I'll just take the compact umbrella. If it's really low, say 10-20%, I may not take anything at all, depending on the park. I think MK is a great rainy day park, because the buildings are so close together and you can walk through so many of them to get where you're going. I think Epcot and AK are the worst for staying dry because Epcot is so spread out (especially WS) and AK isn't laid out well for going from building to building.
I'll be there starting the 17th, and so far, it's forecasting rain every day. However, I'll take rain with temps in the low 80's over sun in the mid 90's any day!