The term "normal" when referring to a days "average high temperature" or "average" anything... is taken over 30 years.
It's an official National Weather Service term.
Climate changes over time, but over a 30 year period you can get a pretty good idea of what is "average" for the region and period in history.
On October 17th, 2013 we made it to 68 degrees at Billard Airport. That's considered "normal". On October 18th, 2013... we are looking at afternoon temperatures in the 40s! Not "normal".
Of course, earlier in the month we had 4 days in the 80s and we even hit 90 on October 4th!
As of October 17th, we have seen an "average" this first 17 days of just over 74 degrees each afternoon.
By the middle of next week, our October 2013 number is going to come down quickly. When you add 48 degrees to the equation on Friday... the number will begin to drop.
I hope you enjoyed the warm start to October. The cold air is going to stick around a while.