I use a simple rule during nectar flows (late winter/early spring etc). Weather doesn’t come into it at all! 
If the existing box(es) is at least 80% fully drawn comb, 80% full of food or brood and completely covered with bees, they get another box. Using that rule, you know that you have enough bees to use, defend and heat the new space, and you know that they will use it, as they are running out of space in the current box.
Hope that helps! 