As cold winds blow across the relatively warm waters of the Great Lakes, they are picking up moisture. That moisture, in turn, is being wrung out on the lee side of the lakes in the form of locally intense bands of snow.
For Wednesday and Thursday, very cold air – with temperatures near zero 5,000 feet above the ground – will be crossing the open lakes, where water temperatures are in the mid 30s to low 40s. The temperature difference between the air and water has created instability in the atmosphere, allowing moist air near the lake surface to rise, forming clouds and snow squalls.
(INTERACTIVE: Great Lakes Radar)
"Fetch" is the technical term for the distance over which wind crosses open water. When the wind blows down the long axis of one of the Great Lakes, snowfall potential is maximized on the lee side of that lake. In this case, it looks like three areas will have a favorable fetch:
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