Northeast snowstorm: Here's what you need to know

(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.
    -- New York City could get up to 20 inches of snow, but with warmer air filtering into the area, weather models predict that snowfall could be closer 10 to 14 inches.
    -- Washington DC is expected to get 5-10 inches of snow. The federal government is opening three hours late and employees have the option to take unscheduled leave or work from home, according to a press release from the US Office of Personnel Management.
    -- Parts of Massachusetts could see 24 inches or more and similarly powerful winds, said Gov. Charlie Baker. Boston could see as much as 12 inches of snow for the metro region.
    -- Snowfall of this magnitude is not uncommon in March, but this storm has the chance to be historic. The greatest recorded snowfall New York City has seen in March was 16.5 inches, on March 12, 1888.
    -- Schools are closed in Philadelphia, New York City, Boston.
    -- New York state is under a state of emergency, and 2,000 members of the National Guard have been called up. Virginia called a state of emergency as well.
    -- New Jersey Gov. Chris Christie declared a state of emergency, authorizing a response to the storm with all county and municipal emergency operations and agencies.
    -- Connecticut Gov. Dannel Malloy signed an order activating a statewide travel ban that will begin at 5:00 a.m. Tuesday and remain in effect until further notice.