Portland is the nearest city to Mount St. Hilary, the site of the crashed Ark. Human friend of the Autobots Sparkplug Witwicky lived there, and was implied to have done so for years before first meeting Transformers in 1984.
If you're reading a Marvel Transformers comic and the characters are in an urban area on Earth without their location being specified otherwise, they're most likely in Portland. Specific locations in and around the city include the suburb of St. Petersburg, Jake Lomax's estate, and Mister "O"'s Diner.
Portland may or may not have been the home of Sparkplug and his family here as well. What is known about the city is that when Ultra Magnus arrived on Earth to enforce Shockwave's edict that the Earth-bound Autobots surrender themselves to the Cybertronian alliance, Optimus Prime ordered a portion of his troops into hiding there.
Transformers/G.I. Joe: Divided Front
G.I. Joe and Cobra independently detected the crash of an Autobot shuttlecraft near Portland. Flint was sent to retrieve it with a team of rookies, and managed to repel a Cobra team sent to do the same thing and recover an unconscious Blaster.
It rains a lot in the Portland area, yet the inhabitants are almost always seen to be enjoying clear skies in Transformers. Perhaps rain is just a nuisance to draw.
Portland is widely considered to be the greenest city in the United States, and possibly the second-greenest in the world. Sun-bleached rock formations and dusty desert roads are in short supply in lush, heavily-forested river valleys.