Transmission slipping on 2001 Honda Accord. Did not start this until we got 30 inches of snow and it had to sit in it for several days. Could it be from water getting into something?

Replace the Transmission Fluid from Honda Dealer.. simply drain from drain pitcock and refill. The damage may be to much at this point pending how many miles you have. There is no transmission filter to deal with. Worst case it may give you a little while longer on the transmission. ...this is assuming you have an automatic trans, that was not stated in the ? Honda automatic transmission are a weak link in their automobiles.