While a larger parent storm system is typically required to produce heavy rain or snow events, lake effect snow is unique. All you need to produce some of the largest snow events on record is cold wind blowing over warm water.

Lake effect snow cannot develop on just any lake. The lake must be at least 62 miles across, and exist at a latitude far enough north to experience very cold temperatures. The size requirement for the lake comes from something called the "fetch" -- the distance that wind blows over the water. The fetch must be great enough for the cold wind to extract sufficient heat and moisture from the water to produce clouds and snow.

Lake effect snow events are most common and most severe in the Great Lakes region due to their enormous size and northern latitude. In the late autumn into early winter, lakes are still relatively warm and ice-free. When frigid, polar cold fronts blow from Canada across the Great Lakes, the lakes turn into massive steaming pots of water — expelling huge amounts of heat and moisture into the passing winds.

To produce lake effect snow, the air temperature 5,000 feet high needs to be at least 23 degrees Fahrenheit colder than the water temperature, and the air temperature 10,000 feet up needs to be at least 36 degrees colder than the water. This is easily achievable with warm early-winter lake temperatures and strong Canadian cold fronts.

Wind speeds near the surface must be 11 mph or greater, with most lake effect snow events featuring surface winds between 11-22 mph. If winds are too strong, heat and moisture transfer are limited.

There should also be very little wind shear present or change in wind direction with height. This keeps the lake's heat and moisture in a focused channel in the atmosphere instead of diffusing it over a larger area. Specifically, you want wind direction from the surface to 10,000 feet high to vary less than 60 degrees on a compass.

As the frigid, dry air picks up heat and moisture on its journey across the lake, the now warmer, moisture-laden air rises on the downwind side of the lake. This creates clouds and snow.

As the unrestricted wind blowing freely across the lake encounters a rougher shoreline with buildings and trees, friction causes air to pile up along the shoreline, forcing the air upward. This creates deeper clouds and enhances snowfall rates. Orographic lift from the wind rising up nearby hills and mountains has the same effect.

Wind speeds determine how far inland lake effect snow bands can travel. Light winds lead to heavier lake effect snow accumulations near the shoreline, while stronger winds will push the heaviest snowfall totals more than 10 miles inland.

In our area, lake effect snow is more common in northwest Indiana than it is in Chicago, since the cold winds necessary typically blow in from the northwest. Heavy lake effect snow events in Chicago require a northeast wind, tapping into a greater fetch of Lake Michigan.

Early autumn can bring less-often discussed lake effect rain, where the same factors are at play, but surface temperatures are above freezing.