Overcast days with a light or no breeze and temperatures in the upper 30's to low 40's is what I prefer. There is a huge difference between 40 degrees and 35 degrees. That small temperature difference will mean a great deal in terms of fish being active. Carry a thermometer and check the water temperatures. If they are unreasonably cold, the fish will not be active and it could turn into a wasted trip in terms of actually catching fish.
Persistence is the key.
Work a pool with your nymph for a while and keep a low profile as you stick out like a sore thumb on the bank if there is too much movement. Snow doesn't provide an angler much of a back drop to camouflage him or herself. Stick with a few basic patterns and work the runs below the pools where the trout have a tendency to sit in the winter. They will move up closer to the head of the pool as temperatures increase during the day to feed, so this means that you don't have to be on the stream at the crack of dawn to catch trout in the winter. The last thing I want to do is sit on a snowy bank with cold hands and my guides freezing with ice every other cast not catching any fish.