Winter fly-fishing in Northern Colorado is all about going small.
Ouzel Midge 
Old Town Midge
For the Cache La Poudre River and the Big Thompson River our go to flies are all of the midge variety. During the winter months midges are usually about the only insect that is active. If you want to consistently catch fish during the winter having a well stocked fly box of various midge patterns is the ticket. Nymph fishing is always a good bet in the winter, however the Poudre and the Big T will give it up to dry fly action when the weather is right. Below is a list of flies that the guides at Liarflies tie and use during the winter.