'Take cover now' warnings issued as tornadoes close in on millions in the Northeast

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Nearly 50 million Americans are in the danger zone for a severe thunderstorm that could bring tornadoes to multiple states throughout the Northeast in just hours. 

Some 17 states are in the path of the storm, including New York, New Jersey, Pennsylvania, Maryland, Delaware, Virginia, West Virginia and Ohio, according to the National Weather Service (NWS).

A tornado watch has been issued across a large portion of the Northeast until 9pm ET Tuesday as powerful storms threaten New York, New Jersey, Pennsylvania, Connecticut and Delaware.

A tornado watch means conditions are favorable for tornadoes to develop, but it does not indicate that one has been confirmed. Residents should remain alert and be prepared to take shelter if a tornado warning is issued. 

The alert covers all 21 counties in New Jersey, all five boroughs of New York City and more than two dozen additional New York counties, including Nassau, Westchester, Rockland, Orange and Putnam.

'Take cover now! Move to a basement or an interior room on the lowest floor of a sturdy building. Avoid windows. If you are outdoors, in a mobile home, or in a vehicle, move to the closest substantial shelter and protect yourself from flying debris,' the agency said in an alert.

The NWS Storm Prediction Center projected a 5 percent chance of tornadoes touching down Tuesday night within a corridor from Kentucky to New York where 57 million people live.

Flash flooding, hail and wind gusts up to 75mph are also expected on Tuesday in major cities including New York City, Philadelphia, Baltimore, Pittsburgh and Washington, DC.

AccuWeather expert meteorologist Tyler Roys said: 'While tornadoes can't be ruled out, the bigger threat today is damaging straight-line winds as lines of storms sweep through the region.'

'In a densely populated corridor like this, that kind of wind can down trees and power lines fast and it's timing up with the evening commute, which is exactly when we don't want people caught off guard.'

Meteorologist Max Velocity added: 'The best chance of tornadoes today will stretch from the Ohio Valley into the Northeast and back into the I-95 corridor of the Mid-Atlantic.' 

Officials said there is a small chance this weather could develop into a supercell, a rare and extreme thunderstorm capable of producing tornadoes that topple trees or tear apart roofs.

The NWS has issued tornado warnings in isolated parts of New York, New Jersey and Pennsylvania, stating that downpours of quarter-size hail, flying debris and possible tornadoes were imminent Tuesday afternoon.

North Carolina, Tennessee, Kentucky and parts of Arkansas, Missouri, Illinois, Indiana, Connecticut and Massachusetts also face a chance of seeing extreme weather Tuesday night.

Potentially deadly flash flooding is expected to cause major travel disruptions along the entire East Coast on Tuesday and Wednesday, with the heaviest flooding expected to span from the New York City area to North Carolina.

New York City's Department of Emergency Management has warned residents to expect the worst of the storm between 6pm and 11pm ET. 

New York's John F Kennedy and LaGuardia airports have both issued ground stops due to impending thunderstorms, delaying travelers by more than two hours.

AccuWeather meteorologists expect the storm to have its biggest impact on the evening commute for millions of Americans on Tuesday.

Heavy showers and thunderstorms will increase the risk of localized flash flooding, especially in urban areas where drains are unable to handle that amount of water pouring down, sometimes in just a matter of minutes. 

This often catches people by surprise with little or no warning to get to safety. Forecasters have warned that this makes flash flooding one of the most dangerous types of flooding.

NWS officials warned: 'Turn around, don't drown when encountering flooded roads. Most flood deaths occur in vehicles. Be aware of your surroundings and do not drive on flooded roads.'

AccuWeather revealed that the major storm event formed after a large cold front pushed eastward across the US and crashed into very warm and humid air lingering after a massive heat dome blanketed the region earlier in the month.

When cooler air clashes with this hot, moist air, it forces the warm air to rise quickly - creating powerful thunderstorms.

Cities with large public transportation systems, such as New York and New Jersey, have already warned that subways could be flooded during the heavy downpours Tuesday evening.

The Office of Emergency Management in Newark, New Jersey has begun placing storm barricades in areas where flooding is expected.

Residents in Connecticut and Pennsylvania have already begun reporting afternoon storms sparking flash floods.

Specifically, the National Weather Service's New York office issued flash flood alerts in Bridgeport, Stamford and Norwalk, Connecticut through 3.30pm ET.