Futures on the Dow Jones Industrial Average indicated an opening drop of more than 1,300 points. The S&P 500 futures indicated a 5% drop at Monday’s open. The S&P futures trading was briefly halted overnight. The sharp declines in the futures market signaled more turbulence ahead after a roller-coaster week that saw the S&P 500 swing up or down more than 2.5% for four days straight.

The massive sell-off could trigger key market circuit breakers during regular trading hours. If the S&P 500 drops 7%, trading will pause for 15 minutes. The SPDR S&P 500 Trust ETF (SPY), the exchanged-traded fund tracking the S&P 500, pointed to a nearly 6% decline at the open.

While Monday’s drop is poised to be significant, it still wouldn’t crack the 20 worst days in the S&P 500.