BY kennEth enriquez

stocks are facing major CHALLENGES at the s&p 500's 200-day moving average

Stocks have roared back to lows since mid-June but have encountered stiff resistance at the S&P 500's key moving average. 

A FAILURE TO OVERCOME COULD SET OFF CHART WATCHERS, THEY SAY. 

The S&P 500 SPX -0.72% closed at its high on Tuesday, April 21, and is up 17% from its low on June 16. 

THE 200-DAY MOVING AVERAGE IS WIDELY TRACKED

Given the market's rapid ascent from the June lows and the breadth of that ascent, the 200-day moving average is a natural resistance level

“The index hit its slightly falling 200-day average yesterday to the penny and the machines turned off their buying and hit the sell button.” Mark Arbeter said  

WEDNESDAY'S AVERAGE STAND IS 4,324.51. 

The 200-day test is bringing some poignant memories for veteran investor

MEANWHILE,