Hi, I'm in the same general area that you are, and just today I was prepping for the cold (on monday, high of 23, low of 1... Fahrenheit.
What I've done so far:
Put up their heated roost
Plugged the electric waterer base that keep the water from freezing
Changed the bedding so it will (hopefully) be dry in the coop
Am going to give them extra corn (which helps retain body heat)
Covered the windows (which are their main source of ventilation) with a towel, so it will still have ventilation, but hopefully the wind won't blow as strong.
I have a hen with a really large comb and wattle, so I'm a little bit worried. I'm trying to keep everything as dry as possible, but since Washington is so humid, I'm worried that it will be pretty moist, no matter what I do.
Fingers crossed all the chickens will be ok. I have a ton of different breeds, some I think will be ok, but others I'm more worried about. Especially since most of them are molting...
(I have a brahma, sapphire gem, welsummer, australorp, egger, barred rock, silkies, etc.)
Hoping everything will work out for you and your flock!
Photos of are the hen with the large comb, and the forecast for the coldest day.