As the meteorological fall neared its end late last week, parts of the northeastern United States experienced heavy snowfall due to a phenomenon known as “lake effect snow.”

This happens when cold air flows over relatively warm lake water, causing the surface air layer to absorb heat and moisture. This layer of air is warmer than the air above and then begins to rise, cools, and condenses into clouds that deposit snow in extremely localized bands on the windward side of the lakes.

Conditions in this region were ideal for producing sleet-like snow accumulations over the weekend. A low pressure system over eastern Canada sent cold Arctic air flowing across the Great Lakes, resulting in significant snowfall in five states downwind of Lakes Ontario, Michigan and Erie.

Western New York state was hit particularly hard, accumulating nearly 4 feet (1.22 m) of snow in just four days. In response to the heavy snowfall, New York and Pennsylvania declared states of emergency, allowing National Guard troops to be deployed to rescue drivers trapped in the snow traveling after Thanksgiving.

The Buffalo Bills American football team was looking for volunteers to clear snow at its home, Highmark Stadium in upstate New York, before a game on Sunday night.

More snowfall is expected early this week, with up to 6 feet of snow forecast in northern New York state by Tuesday. Colder conditions are expected to spread further south across the eastern U.S., with temperatures 10-15F below forecast average temperatures for the central and eastern Gulf Coast states as high pressure builds over the mid-Mississippi Valley.

Snow has also caused disruption across South Korea, with the capital Seoul recording its third heaviest snowfall on record. More than 40 cm (16 inches) fell last Wednesday, the heaviest November snowfall since records began a century ago.

142 flights and 76 ferry services were canceled or suspended as temperatures persisted into Thursday, and more than 1,000 schools were closed in Gyeonggi, South Korea’s most populous province, which borders Seoul.

At least five people died in the snowfall, four of them from the collapse of structures and the fifth from a traffic accident. Since then, temperatures have risen and much of the snow has melted by Sunday.