“The Russell 3000 tracks the performance of the 3000 largest US traded stocks which represents about 98% of all US incorporated equity securities” Source – Google.
Essentially that means the Russell 3000 is a fair representation of the US stock market, so if the advance is close to ending 5 waves to complete an impulse wave from its 2016 low, then there is a strong chance that a decline on the Russell 3000 will coincide with a decline on other US stock markets.
Overlaying the Russell 3000 with the SPX gives us a near perfect match, so in a nutshell; the Russell 3000 and SPX are one and the same market. The current advance appears to be in the late stages of wave  of an impulse wave from the 2016 low. When wave  finally does complete, a significant decline would be expected, until we see a big move under 1650 as well as a strong move under 2775 on the SPX, then further upside could still be seen before wave  finally completes.