I wouldn't ride in this area without fenders; their effect on performance is negligible, and it really sucks to be caught on a long commute in steady, day-long rain without them. Rain gear and waterproof bags are really good ideas, too.
Knobbies, though, aren't so great, with or without fenders, unless you're riding on ice & snow a lot.
BTW, does anyone know how fast you have to be riding to notice drag from fenders? I've been riding a few years with fenders, year-round, usually at a pretty brisk pace for commuting, and I've never noticed any drag from fenders at all.