I'm planning to fish all winter. Last year I made 16 fishing trips between December and February, and had some excellent days. I'm figuring to make more trips this year. I live in Centre county, so all of the streams I fish have some level of limestone influence. Water temperatures on some of our true limestone streams don't drop below 45 degrees even during extended periods of cold weather.
The one thing that does drop water temps on the limestone streams is snowmelt. A lot of people want to go fishing when they see a 40-degree day after three weeks of sub-freezing temps. When snow is melting, fishing during the warmest part of the day can correspond with the lowest water temperatures as well as the worst fishing of the day. Fishing can be good on those warmer days, but you have to hit the stream before the snowmelt does.
I prefer to fish when the air temperatures are in the low- 30s with as little wind as possible, which ensures minimal snowmelt and it is warm enough that you can keep your hands from turning into icicles. I don't like to fish when the air temperatures are below the mid-20s, simply because de-icing your guides every 10 minutes is a pain in the ***. Most weeks provide at least a few windows of worthwhile fishing.
Also, don't fall in. I seem to do it several times per winter, and its never fun. Always keep a change of clothes in the car.