If your parents are flying on Saturday from BUF direct to CLT, they souldn't have any problems in terms of the weather in BUF or CLT. It is pissing rain in CLT today, but that will stop sometime tonight. So as long as there is equipment in BUF and equipment in CLT, I don't think your parents will run into any trouble. The only thing I can think of is if there are crazy equipment mis-alocations due to the weather in DCA and PHL hubs, but with US and their early declaration of trouble weather and their pre-cancelation of flights today and tommorow, they might have the metal where it needs to be for flights that don't touch the bad weather blizzard zone. Eitehr way, the best of luck to your parents and may they have a safe and happy trip.