Hi Jim, we are a located even further north than you and generally work right up until frost is in the ground or snow cover is too deep to make work efficient. If you only have up to 2" of frost in the ground you can use a propane 'hot spot' torch to melt it out where needed. It takes a bit longer than you might think, but does work.
We have, upon customer request, installed a system in the winter and then returned in the spring to put all the wire in the ground and finalize things. I really don't recommend this approach as doing the work in the dead of winter is just no fun at all and it always takes more time than anticipated to complete the job in the spring.