12 degrees, huh!

I've found a lot of powders to be temperature sensitive, WW231 included.
The colder the ammo gets, the lower the pressure; conversely the hotter the weather, the higher the pressure. Most reloading manuals take this into consideration and woe to the poor chap who goes beyond a manual's upper limit (THIS IS ESPECIALLY TRUE OF RIFLE ROUNDS) in the winter and then takes the same ammo out into the sun during August.

What I'm trying to warn you about is that if you take your handloads out into the cold and they sit out for awhile, you might think that the loads are way under.

I love to watch shotgunners in zero weather or below freezing weather; even their factory loads sound more like POOF than BANG.

Just a quick story from years ago: Our Olympic team was wondering why their .22s were erratic until some kindly Europeans issued them the .22 stuff that is especially made for use in the winter Alpine mountains.