Winter time is likely your best bet for a fish over 13 pounds. If you look at the Texas top 50 bass, 7 of the top 10 fish were caught from the last week of November through the first week of March. In the winter, I have caught more 10+ bass than any other time of the year (excluding the spawn). Considering what little time I spend on the water in the winter, that is an impressive statistic. Winter is usually when I spend the most time preparing for future trips. This is when I clean my reels and sort through my tackle and work on my boat. This is also the best time for promoting my fishing business for the rest of the year and doing website work. I don't seem to get a lot of clients that want to fish in the extreme cold of winter, so I find myself with a lot of free time.
During the winter, the fish on Lake Fork get very big, but when the water is cold, it is tough to consistently get good numbers. This is because bass don't feed as often in the winter. If you can put in your hours, though, it will often pay off. We catch most of our bass on spinnerbaits, lipless crankbaits, suspending jerkbaits, and weightless worms. The grass that grows in the warm months will be dying off in the cooler months. But those areas will still hold some fish. If you can find some green hydrilla, you will be around the fish. If not, look for heavy cover and/or steep drops around deep water. Fish a drop-shot or jig around the steep drops and you're likely to get bit.
If you are out for big fish, Fork is most always the best bet any time of the year. Those days when you aren't getting any bites could be one of the days you get the bite of your life. However, I like to guarantee fish to my clients and I don't know of anybody who would guarantee fish with water temps in the 40s. When the water is below 60 degrees, I recommend the warm water lakes to my clients who want guaranteed fish. I guide on Lake Monticello and Lake Welsh from late November through early March and recommend those lakes for those of you who prefer quantity over quality. These lakes have warm water in the winter, so you can usually catch fish even in the worst conditions.