With nearly all flights out of Boston Logan (BOS) cancelled today, and more than 90 percent of flights at New York's La Guardia Airport (LGA) canceled, the Northeast U.S. will likely be the hardest-hit region.
Weather experts predict it will become a "bomb cyclone" because the system's pressure is expected to fall very rapidly and create one of the most intense winter storms the United States has seen in decades.
New York City was under a winter storm watch, with between three and six inches (8-15 centimeters) of snow expected from Wednesday night through Thursday.
"If you're in the area or due to travel, you should monitor local media, follow the advice of the local authorities and check with your airline or transport provider before travelling to your point of departure".
More than a foot of snow was forecast for Boston and coastal areas of northern New England, with as much as 3 inches per hour forecast, a pace that made it hard for plow crews to keep roads clear.
State officials said three of the deaths occurred in North Carolina and a fourth happened in SC, as the weather system barreled up the eastern coast. Tallahassee, Florida and South Georgia even got a thin blanket of snow Wednesday morning. "It has one of the greatest rapidly deepening rates we've ever seen", Bob Oravec, a lead forecaster with the National Weather Service, tells The Verge.
In addition to the snow, coastal areas are at risk for flooding, the weather service said. Two people died of cardiac arrest during the storm Thursday on New York's Long Island, officials said.
ABC News meteorologists spotted lightning on the radar this morning in Long Island, New York, as well as parts of CT, which means residents there could be experiencing a "thundersnow".
They frequently occur in North America, when cold air collides with warm air over the Atlantic Ocean - though they've also been reported in eastern Asia and South America.
Meteorologists define a bomb when an extratropical cyclone intensifies 1 mb (measurement of pressure) every hour for 24 hours. In New York and the rest of the Northeast, the low temperatures and high winds will make the air feel as cold as 30 degrees below zero.
"If the temperature in your home begins to fall, we recommend taking shelter elsewhere until service can be restored", National Grid power company, which serves MA, said on Twitter.
The governors of Florida, Virginia, North Carolina and Georgia have declared states of emergency and warned residents to brace for freezing temperatures.
In New England, the powerful winds brought coastal flooding that reached historic levels in some communities with icy water overflowing piers, streets and restaurants and stranding some people who had to be rescued.
"If the temperature in your home begins to fall, we recommend taking shelter elsewhere until service can be restored".
Snow will end for much of the Northeast by the evening on January 4, 2018.
Bomb cyclones draw air from polar regions after they leave.
The National Weather Service has warned that there could be blizzard warnings and travelling could be "very unsafe to impossible".