In spite of the threat of Hurricane Irene the Farmers Market is still open. Not to sure if all the Farmers and vendors will show, but some already have. The Farmers markets in Nyc will be open from 8am until 11am an hour before the metra trains are scheduled to stop running.
If you intend to visit the market today, plan on doing so early (if you do not live in the area) the trains are sure to be crowded. Be safe, and enjoy the day..at The moment it is beautiful.