With a jacket. You are best off with a monopod or tripod and I would consider a "lenscoat" for your long lens. You will need a long lens for sure 300mm+. To be honest though, we need more information. Pack some snacks and extra batteries.
With lots of snow in the scene the camera's built-in reflected light meter is thrown off because it's calibrated to an average color quite close to a neutral gray, instead of mostly white.
The solution is to add some exposure compensation. How much compensation will be needed will vary somewhat and the only way to decide how much is to use the camera histogram.