45 Degrees isn't much of a problem. Rain is. Moisture content of the wood above 15% WILL peel, regardless of how you prep it. You need 2 or 3 consecutive days of dry weather to do this. You may be better served to let it go for the winter and deal with it in the spring.
There are cold weather paints. They can be Use an oil based primer, such as Ben Moore's "Fresh Start". That needs to dry for 2 days, then apply two coats of paint.
I personally don't like cold weather paints. they run easily, and getting an even sheen is difficult.