UPDATE | Storm threatens the Northeast with historic snow

NEW YORK (AP) — The Philadelphia-to-Boston corridor of more than 35 million people began shutting down and bundling up today against a potentially history-making storm that could unload a paralyzing 1 to 3 feet of snow.

More than 5,000 flights in and out of the Northeast were canceled, and many of them may not take off again until Wednesday. Schools and businesses let out early. And cities mobilized snowplows and salt spreaders to deal with a dangerously windy blast that could instantly make up for what has been a largely snow-free winter in the urban Northeast.

Snow was already falling during the morning commute in several cities, including Philadelphia and New York, with Boston up next in the afternoon. Forecasters said the brunt of the storm would hit this evening and into Tuesday.

All too aware that big snowstorms can make or break politicians, governors and mayors moved quickly to declare emergencies and order the shutdown of highways, streets and mass transit systems to prevent travelers from getting stranded and to enable plows and emergency vehicles to get through.

"You cannot underestimate this storm. It is not a regular storm," New York Mayor Bill de Blasio warned in ordering city streets closed to all but emergency vehicles beginning at 11 p.m. "What you are going to see in a few hours is something that hits very hard and very fast."