Check the following:
Boston.com weather info.
WaPo Capital Weather Gang blog.
Right now it looks as though the rain/snow line runs right along 95 through New Hampshire, but southern ME is supposed to be above freezing starting in a few hours and running through the early evening.
I agree 100% that the drive is the dangerous part of the trip. Do you trust Grandma to have good sense and stop for the night if.conditions are bad?