I am farther south than you, and generally my girls don't start going broody until March at the soonest (no one has yet), but most commonly they usually wait until April and May (and then my nest boxes get "clogged" with hens wanting to set, with lengthy waiting lines of ladies wanting to lay in the 1 or 2 available nests, LOL). If it's still getting down into the 30s at night, I doubt any of your hens are going to go broody. Their instinct usually tells them it's too cold, and so wait a little longer. I've heard Silkies will go broody in January, but most breeds will wait to set until late spring and summer.
And yes, leaving a nest of 10-12 eggs in a dark private cozy nest is the best way I know of to encourage a hen to go broody.
Eggs kept in perfect conditions can give very good hatch rates 7-10+ days post laying. Outside in your nest boxes, probably factor up to 5-7 days (for very good hatch rates). Make sure to turn them every day, twice a day is even better, so that the yolk doesn't get stuck to the shell from sitting in one place too long.
I would probably (A) Look into possibly buying or borrowing an incubator, if you want to hatch your hen's eggs, (B) Buy some chicks, if you're wanting chicks, or (C) Plan to possibly get some fertile eggs later this year, if and when one of your hens goes broody; or you can also buy some chicks to slip under her, when she's been broody for at least a week, if you're wanting hen-raised chicks (I've had great success with getting hens to adopt hatchery chicks!). The chances of one of your hens going broody in the next several days - with temps that low - is not very good, so I'd plan on a Plan B.