NEW YORK — Thousands of flights were cancelled and schools were ordered to close as the northeastern United States braced for the arrival of a blizzard on Tuesday.

The National Weather Service issued blizzard warnings for parts of Pennsylvania, New Jersey, New York and Connecticut, Reuters reported, with forecasts predicting 6 to 12 inches of snow and high winds for New York City.

Boston is also under a blizzard watch and could experience at least a foot of snow.

Meanwhile, Washington D.C. and Philadelphia may not be hit by a blizzard but can still expect 4 to 8 inches of snow.

Added to the travel chaos, financial commentators predict the inclement weather could cost the economy more than $1 billion, including lost wages, according to financial website 24/7 Wall Street.

One condition required for a blizzard to form is a mass of warm air rising over cold air. This causes strong, cold winds and precipitation to fall.

A blizzard will then occur if temperatures are below freezing at ground level and in the clouds, and there is enough moisture in the air to allow clouds and snow to form.

Blizzards are defined by the National Weather Service as severe snowstorms with wind speeds hitting at least 35 mph, and falling or blowing snow that reduces visibility to under a quarter of a mile, which last for at least three hours.