Saturday snowfall tally could hit 4-8 inches

The manner in which winter storms produce their snow is often more complex than it
appears to a casual observer. Take the storm in progress as Saturday dawns. The snowfall it
has generated overnight has been warm air advection snow. That's the snow which develops
as the storm's southerly winds force mild, moist air into cold air. Next comes the
storm's dynamically produced snow. That's the snow generated as upper winds exiting a powerful
jet stream wind pocket slow -- a process which also encourages snow formation. To anyone
watching from below, the snowflakes which result look exactly the same. Yet the storm
is producing its snow in very different ways. The third phase of many winter storms --
this one included -- is the lake-enhanced phase. This occurs in the final hours of a winter
storm as lake moisture is swept into its cold backside. Here, especially large flakes
form and, because of their enhanced size, are noticeably different than earlier snowflakes.
Snowfall may decrease or even ebb temporarily Saturday morning as a slot of dry air
common in winter storms shifts across the area. Snowfall resumes toward midday and through
the afternoon amid strengthening winds.
--Tom Skilling, Chief Meteorologist, WGN-TV/Chicago Tribune