Will using portable heaters in the attic during heavy snow help melt it?
Very unlikely.

what would happen is that the snow or ice on the roof would melt somewhat, flow down to the edge of the roof where the attic ends, and refreeze, forming large ice dams. The ice and snow would continue to melt, backing up liquid water against the roof and evenutually finding its way under the shingles and through the sheathing into your home where it would drip down the ceilings and walls onto your dog, your kids and your computer. You would then curse and try to find a solution, but by then its too late. Your kids would hate you, your computer would be ruined.