Icing Locations
In my experience, I've flown in three main locations for icing: (1) the Lake Erie region where NASA has their icing lab; the North Atlantic; and Punta Arenas in Chile. I haven't done icing work in the Seattle area, but it should be very suitable.
Of the three I've operated at, Punta Arenas is probably the best for icing and certainly the worst for logistics. I'd probably only consider Punta Arenas if you were constrained to July-August. Otherwise, I'd choose Seattle or Cleveland.
Goldfish