I suspect crabgrass. Look for a three or four-fingered seedhead. Look for a dark maroon color to the seed heads in the fall as the temps come down below 70. Perhaps the long dry spell acted like a winter dormancy. I have seen clover, oxalis and other weeds blooming in the fall when they would normally bloom in spring.
http://www.msuturfweeds.net/details/..._crabgrass_48/