Lake-effect snow appears to be the cause of the blizzard that completely covered most parts of northwestern US states from November 18.

Lake-effect snow has been formed by cold air from the arctic sweeping over the Great Lakes – on the borders of the US and Canada – and picking up water vapor particles that are relatively warm in comparison.
The same effect also occurs over bodies of salt water, when it is termed ocean-effect or bay-effect snow.
The bands of freezing air then cool down the droplets until they are transformed into solid snowflakes, and this process can continue for as long as the two contrasting temperatures merge together.
The areas affected by lake-effect snow are called snowbelts.
The potential longevity of the snow can mean that New York, New Hampshire, Michigan, North Dakota, Minnesota, Massachusetts, Wisconsin, Ohio and Pennsylvania are likely to be covered in even more intense flurries before the rest of it gets a chance to be cleared or melt away.
A very mild, warm and wet autumn coupled with arctic air hitting the Great Lakes is not unusual and only a small proportion of the US is affected by the lake-effect snow.
A lake-effect blizzard is the blizzard-like conditions resulting from lake-effect snow. Under certain conditions, strong winds can accompany lake-effect snows creating blizzard-like conditions; however the duration of the event is often slightly less than that required for a blizzard warning in both the US and Canada.

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