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You are right to be concerned both with driving the 800+ miles between Orlando and Washington in one sitting and with deriving to Niagara Falls in Winter. However, with just a little preparation and a nod to reality both drives can be very enjoyable. Between Orlando and Washington, Florence, SC would make a good overnight stop. It's about equidistant from the two cities and is at a major Interstate junction and so will offer many choices for motels and restaurants. As an extra benefit of doing two 400 mile days, you will get some time to take a few scenic and relaxing breaks along I-95.

My preferred route from Washington up to Niagara Falls is US-15 to Harrisburg and up the Susquehanna Valley to southern New York, and then I-86/I-390/I-90 to the Buffalo area. You will have to watch the weather on this leg as it is subject to frequent and heavy 'lake effect' snows. There's no way to predict more than a few days in advance what the actual weather will be like, so just check the forecasts once you get to Washington.

AZBuck