A highly impactful and potentially historic nor’easter is expected to quickly strengthen as it collects itself offshore near Delaware, Maryland and Virginia on Sunday, leading to major and potentially extreme impacts for millions along the I-95 corridor.

More than 50 million Americans were on alert on Sunday morning for winter storm conditions beginning later Sunday and continuing into Monday.

Blizzard warnings are in effect for more than 35 million Americans from Cape Charles, Virginia, to Dover, Delaware, up to the I-95 corridor from Philadelphia to Boston for increased confidence in snowfall of more than a foot and gusty winds that will likely cause blizzard conditions. The entire states of Delaware, New Jersey and Rhode Island were included. 

Winter storm warnings were in effect for parts of central Virginia and Maryland, east-central Pennsylvania, southern New York, northern Connecticut, west-central Massachusetts, southern New Hampshire and Vermont, and southern Maine for increased confidence of snowfall.

Some areas are expecting about 6 inches, while some areas may potentially see more than a foot, as well as gusty winds that will likely cause blowing snow and whiteout conditions.

Those conditions could hit major cities, including Baltimore; Harrisburg and Scranton, Pennsylvania; Albany, New York; Hartford, Connecticut; Concord, New Hampshire; and Portland, Maine. 

New York City and Philadelphia were under a blizzard warning for total snowfall reaching between 16 and more than 20 inches, with potential winds gusting up to 55+ mph, causing whiteout conditions and difficult-to-impossible travel conditions later Sunday through Monday.

New York City hasn’t been under a blizzard warning since March 2017, close to a decade ago. The last such warning for Philadelphia was in January 2016, more than a decade ago.

“The snow is back,” New York Mayor Zohran Mamdani said on social media early on Sunday. “But New York is ready.”

More than 6,700 flights have been canceled for Sunday and Monday, according to flight tracker FlightAware. Over half of all flights at John F. Kennedy and Laguardia airports have already been canceled ahead of the storm.

Airports in Newark, Boston, Philadelphia , Reagan and Baltimore have also seen significant cancellations. Between 70% and 80% of flights scheduled for Monday at New York airports and in Boston have already been canceled.

Coastal Flood alerts were also up from coastal Delaware, Maryland and Virginia to the Jersey Shore, as well as from Long Island to the coast of southern and eastern New England for minor to moderate coastal flooding during high tide.

-ABC News’ Gio Benitez contributed to this report.