Portland should be fine with RWD. Winter tires will do more for you than AWD alone but I'm not sure I'd bother given the infrequent snow days in Portland. We live in Bend and get a decent if inconsistent amount of snow but still a lot more than Portland. Winter tires are absolutely awesome on snow and ice. AWD is still nice for moving from a stop or up a hill on the worst days. PNW has more hills than most of the cold climate cities mentioned so if your home or commute is on one of the steeper hills in Portland, AWD would be nice.