Heat is escaping from the living space into the cavity of the overhang, and melting the snow or causing a condensation problem. Most likely, in any case, you'll have to get 'inside' the cavity to insulate. I'm not sure if this is the time of year to go 'busting' into that space due to the cold. If you can keep the over-hang free of snow and ice, or perhaps you can lay a tarp / plastic over it until Spring, you may be better off. Still, the problem may be ABOVE the window, and not necessarily with the window. You may want to do some 'exploratory' work to see what's behind all the 'walls' of the window inside the house. Usually, all you need to do is drill a 1/2" hole in the drywall and use a coat hanger, or something you can make a 'hook' out of, that you cans stick in the hole to see if you can 'catch/hook' any insulation. If so, that's good! If not, you're on your way to finding the cause of the problem.