Originally Posted by FBinNY

As LarDasse says the grease within the levers stiffens up when cold. In my experience it shouldn't be enough to bind the lever, except maybe a bit at first.

I suspect that it's equally likely that the problem isn't in the shifter mechanism as much as in the cables. Lube within the housings also stiffens when cold, and is enough to overcome the RD return spring tension. Before thinning the lube in the levers, possibly giving you cause for regret next summer, slacken the cables by shifting to low gear, then with the bike stationary s bring the lever to the high gear position. Wick some very light oil up into the housings from both ends and see if that solves your problem.