I agree with the others. Yours should be OK outside. Just try to set it up so the wind does not hit them on the roosts. Think “wind chill”. I don’t know how your coop is laid out or what your ventilation looks like, but you might need to block off more than one window.
How do northerners handle it? It’s all in what you get used to and how you set up your coop. I used to work in -20 Fahrenheit weather. As long as you dressed for it and the wind was not blowing, it wasn’t bad once you got used to it. If you don’t have the clothes and are not used to it, even what you are seeing can be vicious.
Where you are, heat in summer is more a danger than cold in winter, even this cold snap. It sounds like you are counting on windows at the level they are sleeping for ventilation. If so, I’d suggest sometime before summer you put in some permanent ventilation above their roosts to let the hot air out. Maybe a roof vent, a gable vent, or cut out some area under an overhang and frame that in with hardware cloth to stop predators. Ammonia and hot air rises. Ventilation over their heads really helps get rid of both and any breezes from that is over their heads. You can block off the windows in cold weather and you are covered. You can open those windows in the summer and you are covered.
Something else to consider. Some people a lot further north than you have coops with one wall totally wire. As long as the chickens are roosting back in that protected alcove formed by the three solid walls, they are OK. I’ve seen chickens sleep outside in trees in zero degree weather. These chickens were not on a dead tree limb overlooking a ridge top squawking defiantly in the teeth of a blizzard. That would be a Disney cartoon. They were in a protected valley in a thicket out of the worst of the wind. It’s things like this that make me think your chickens will be fine.
