When there is a flow control program in effect, national center can put a ground stop in place, preventing take offs and causing metering to take place starting from nearby stations going outwards. Usually priorities are given to birds already in the air, so the int'l and transcons from the east coast will already have arrival slots, and if the weather is really bad then there will be go arounds adding to the delays. UA dispatch then decides from there who to add to the deck. Of course they will assign priorities to mainline vs. UX.