I can say with great certainty that it is going to be cold for Christmas and for a few days after. But just how cold remains to be seen. I'm concerned because the global upper level wind pattern is one that can allow VERY cold arctic air to plunge south into Texas. If we do get this 'cross polar-omega block' pattern in place, it brings sub freezing temps into play.
For now, we'll go with a strong cold front crossing Houston on Christmas, making for a cold, cloudy, windy afternoon with afternoon temps in the upper 30's and low 40's. Since the last two Christmas Day's in 2015 & '16 saw highs up in the 80's... I'm down for a little Christmas cold!