We had a big plunge or selloff in the US stock market yesterday, as can be seen in the S&P 500 index charts below. The index closed below the 2.800 level, and below Feb-March-June highs and July-August lows. It is difficult to see the next possible support area on the daily, perhaps it lies somewhere in the 2.750-2.725 zone.
On the weekly chart, I have drawn in an uptrend line from the low in the beginning of 2016. It is still a tentative line and it will be interesting to see if it gets respected or rejected. A strong close below the line on a weekly basis could lead to lower index levels. Keep in mind the weekly is set to semi-log scale.