Lake-effect snow, a relatively rare occurrence in Chicago, is providing the city with its biggest snow event of the season to date. Lake-effect snow develops when sufficiently cold air blows across the relatively warm waters of the Great Lakes, but this usually happens when winds are from the northwest. Lake Michigan’s lake-effect snows therefore usually fall in northwestern Indiana and western Lower Michigan. In the present situation however, northeast winds are directing snow across the Chicago area, with snow arriving in the northern suburbs Monday evening and gradually working southward into Tuesday morning, then finally arriving in northwest Indiana. Meanwhile, an intensifying storm system just off the East Coast of New Jersey is forecast to produce blizzard conditions and heavy snow, in excess of 20 inches in some places, for much of the Northeast.