Lake effect snow is a phenomenon that is most common in the Great Lakes region of the United States. It occurs when cold air passes over a relatively warm lake. However, in order for lake effect to form, there must be a 13°C difference between the lake temperature and the air temperature about a mile above the ground. The winds must also be well-aligned out of the same direction from the surface all the way up through the atmosphere. The longer the path of the air over the lake, known as the “fetch,” the stronger the lake effect band will become because more moisture will be able to be evaporated into the clouds. Lake effect events are most common early in the winter season because the lakes are warmer, creating a larger temperature difference between the lake and air temperatures. Once the lakes freeze over, it is much harder for lake effect to form because the moisture source is lessened dramatically.
One of the most extreme lake effect events in history occurred not long ago over western New York on November 17 - 20, 2014. In this case, the air temperature a mile above the suface was around -16°C and the temperature of Lake Erie was around 7°C. That gives you a temperature difference of 23°C, which is 10°C greater than required and why the lake effect snow band turned out to be so powerful. In addition, winds were aligned out of the west-southwest which used the longest “fetch” possible over Lake Erie. With this wind trajectory remaining rather constant for 24 - 36 hours, the lake effect band was nearly stationary over areas just south of Buffalo for the entire event (radar courtesy of WeatherTap):
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In most lake effect snow events, the winds shift at some point causing the snow band to move to a different location. In these more common cases, you won’t see snow totals of 70”+ because the band doesn't sit over one location long enough, although it is very common to see storm totals of 1 to 2 + feet. The isolated nature of lake effect is what makes it so fascinating to meteorologists. One location could be dealing with white-out conditions and receive feet of snow, whereas another location just 5-10 miles to the north or south could be experiencing clear skies and never see snow at all. See the time lapse below from YouTube user Alfonzo Cutaia:
While the November 17 - 20, 2014 lake effect storm was historic and produced up to 85" in Wales Center, NY, it is amazingly not the most extreme event. Below are some of the heaviest snowfalls in NY state history for comparison:
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