While I believe you can catch large brown trout at any time of the year, I am convinced that the biggest and meanest fish in the river are looking to fatten up in late fall through December. There is a window of opportunity given the right river conditions that typically lasts from some time in October and into late December where the streamer angler will have their best shot at brown trout in the 20 plus inch range. In a good year when the rivers don't freeze the fishing can be productive all winter.
In order to find these larger brown trout you need to break down your water and find out where the fish will have the best access to food over the winter months. Surprisingly many of the more quality brown trout caught every year are pulled out of what most anglers would consider marginal waterways. Typically water ways that a have a wide range of water temps from low to high will have more abundant bug life and more forage fish.
What do you need to target these fish? Since we at Wild East Outfitters focus on mainly bigger waterways, we prefer rods in the 7-8 weight range. We are using leaders in the 3-6 foot range and no lighter than 12 pound test...nothing fancy. We go heavy because this isn't finesse fishing. If you get stuck in a tree you want to be able to tear your fly out. If you hook into a big brown trout you want to have something strong enough to have control over the fish that won't snap on the hookset. More often than not the fish violently eat the fly and you don't want to learn the hard way that you need to beef up your leader.
It will also pay off to have a few different lines to help you put your fly where you want it. I rarely fish floating lines with streamers...almost never. My go to is an intermediate line that will pull my streamers down just a few feet. A lot of people comment that we aren't getting down deep enough. I explain to them that we are trying to mimic a dying baitfish struggling up high in the water column. A favorite streamer to do this with is the Drunk and Disorderly or the Double Deceiver fished on an erratic but slow retrieve. During elevated flows fish will push up on the banks and that is when this strategy is very effective for us.
Another strategy we employ consists of what we call "dredging." We dredge by throwing weighted flies on sinking lines and we typically move off the banks. We seek out the deeper water looking for fish that are living in prime safety lies. These are the fish that are rarely caught, rarely see a fly and have enough wild instincts that they know how to avoid encounters with predators. Running a weighted streamer through these lies such as a bunny leech or an oversized bugger can produce some impressive fish.
Instead of hanging around the house and complaining about the cold go out and hunt for these bigger brown trout. It's a low numbers game but it's so rewarding to find fish where people will tell you trout don't live. Just because nobody is catching them it does not mean that they don't exist. These are next level trout..."super trout."