(CNN) The Nor'easter moving through the mid-Atlantic and northeastern United States put some of the biggest metropolitan areas at a standstill late Monday into early Tuesday as thousands of flights were canceled, schools announced thousands of closures and governors issued travel bans and states of emergency. With 31 million people placed under a blizzard warning, here's a quick overview of what you need to know throughout Tuesday.

---The heaviest snow is expected to hit between 6 a.m. and 9 a.m. and could turn into sleet later, according to CNN Weather Center.

-- 31 million people are under a blizzard warning, 31 million are under a winter storm warning and 9 million are under a winter weather advisory.

-- About 7,600 flights that were scheduled for Tuesday have been canceled, according to FlightAware.

-- Amtrak is running on a modified schedule, with many trains canceled or running at different times. The Acela Express from New York City to Boston is canceled, and there will be limited Acela service from New York City to Washington, DC.

Read More