I don't know how these things work, but the most recent advisory, 027, seems to imply that United Airlines has a full ground stop until 15:00 UTC / 10:00 Eastern.

Edit: Of course as I was typing that, Advisory 028 pops up with a nationwide ground stop until 9:30 Eastern: "GROUND STOP ALL FLIGHTS / ALL DESTINATIONS EXCLUDES MILITARY AC AND MEDEVAC FLIGHTS"

https://www.fly.faa.gov/adv/adv_otherdis.jsp?advn=28&adv_dat...