There is really no reason to offer complicated reasons for said rally. After the premarket mark up tingled the microchips of HAL9000, the dollar continues to get hammered and therefore any risk asset on Earth can be bought with impunity. Thousands of PhDs at quant funds across the land deem it so in their collective wisdowm, so first grade logic it must be.
The NASDAQ and Russell 2000, coming into the week ahead of the S&P 500 now sit at or just above resistance. The S&P 500 is still 7 points away from reaching the same point but nothing the next premarket rally cannot take care of. Or if they can hammer the dollar another 0.3% or so, perhaps this afternoon.