I am definitely not an expert but in my experience, with a temp drop like that, I would expect AF today or tomorrow, as you said. As you chart more, you'll start to get a sense of your average LP which will also help in guesstimating when to expect AF.
Re: Newbie with a charting Q
Team Purple!!!!
Just had a nice temp drop like that myself the other day. AF showed up a few days later.
Your chart looks great, a nice clear temp shift.
I usually get AF on the day of a big drop like that.