Snow’s coming: starting toward midday; 1-3" possible

The Chicago area appears headed for its first 1"+ snow accumulation Tuesday since the 1.6" which fell Jan. 15. Snow is to commence toward midday and may fall 6-8 hours before ending Tuesday night. A small but fast moving disturbance racing southeastward is encouraging milder air to blow north into the formidable dome of cold air which has dominated here since Friday, a process often referred to as “overrunning”. Snowfall in air as cold as Tuesday’s tends to “fluff up” more than usual. That’s why the modest 0.13" (water equivalent) predicted to fall by a suite of computer models may lead to 2-3 times the normal volume of snow. Rather than a 1" snow, Tuesday’s system could put down as much as 1-3" across the city—less to the north but as much as 4-6" totals south to Kankakee and Morris. Monday morning’s low reached -10° at O’Hare, the coldest temp since Jan. 5, 1999 when the low hit -16°. --By Tom Skilling, WGN-TV Chief Meteorologist