Hi Sukumar,
Thanks for writing! Don't worry about the possibility of rain...you are going to
Disney World! In most cases, the vast majority of attractions remain open during passing rain storms, except in the event of lightning, when they may be closed for safety concerns. (This is a tiny percentage of attractions, and they'll reopen as soon as the storm passes in most cases.) Most Disney guests have saved and anticipated for a long time for their trips, and won't let rain put a damper on their fun. In fact, when it starts to drizzle, you'll see the parks erupt in a sea of Mickey ponchos that are adorable (albeit not particularly fashionable!)
If it's truly a "wash out" kind of day, I like Epcot, because most of the attractions (and lines) are indoors. Test Track is really the only ride that tends to stop because the track can get slick, but I've even been on that in the rain. Hollywood Studios is another good choice, as, again, many queues and rides are indoors. Finally, on a "super soaker" we also like to head to DisneyQuest at Downtown Disney. This is an indoor, "virtual" amusement park.
Admission is included with the Waterpark Fun and More Option, or you can purchase a one-day ticket.
Have a wonderful trip, rain or shine!
Tanya