Pegged!

I predict the day's high temperature the Rube Goldberg way. I take a 9 am reading from the oldy moldy thermometer located on the north side of my house. Then I subtract 20 degrees, and there's my prediction for the day's high temperature. Well, it works, and much better than any of the local weathercaster's do - for the last 10+ years!

But today, my thermometer threw me a curve ball. It was pegged past it's printed temperatures. So what do I subtract the 20 from?