The index price does not follow a mean reverting Ornstein-Uhlenbeck process. Contrary to Naive experience. SPX, NDX are not stationary, nor Markovian. However the index log return are approximately stationary and Gaussian. If we assume the index log return follows Ornstein-Uhlenbeck process. The return will catch up to historical means and likely overshoot. Given the low return 2014-2016, 2000-2016. This is a volatility breakout and we must catch the train.