Dormant winter of 2012-13 appears to be awakening; far more active pattern taking shape in December’s back half

Most Chicagoans figured it couldn’t last forever! A weather pattern that maintained an 11+-degree temperature surplus while setting new records for the longest string of measurable-snow-free days and blowing past the record for the latest date for measurable snow this past Sunday (Dec 16), had an air of unreality to it. So it’s not entirely surprising the party may be ending.
The season’s first sticking snow appear likely to be delivered by a series of weather systems in coming weeks—the first to hit Thursday. And, the prospect that establishing a cover of snow is to clear the way for colder temperatures to move in makes it clear Chicago’s long delayed winter of 2012-13 is about to put in an appearance!
Sticking snow should come as no surprise to Chicagoans. Winter begins officially Friday at 5:12 a.m. and history tells us the city’s first 2 inches or more of snow has typically occurred on or about Dec. 11. An accumulation in that range here Thursday would by that accounting be more than a week late!
The latest it’s ever taken for 2 inches or more of snow to fall was in the 1936-37 season when it didn’t occur until March 24-25, 1937.


Computer models suggesting Thursday’s winter storm could take more southerly track increasing city’s snow accumulation risk

The risk of a significant accumulation of snow with Thursday’s storm system may end up concentrated over a six to 8 hour late-morning, afternoon and early evening period when it’s likely to be accompanied by howling 50 mph wind gusts and plunging temps. The snow would come after a period of rain expected to begin late Wednesday night and continue into the first hours of Thursday morning.
Computer model storm track forecasts—a critical factor in determining where a given system’s heaviest snow is likely to fall—are now predicting the storm center will follow a path to the south of Chicago—rather than to the north as some early model runs hinted Sunday into Monday. If that’s true—and if this forecast trend were to continue—a far more formidable period of snow than first forecast could be headed for sections of the Chicago area—not just southern Wisconsin or northwest Illinois as first thought.
Clearly, it’s still early in this storm’s evolution and only now is the storm moving into this country’s dense Lower 48 radiosonde (weather balloon) network where its structure and prospects for further development can be most accurately assessed.
Our current read on this system is that the greatest risk for significant accumulating snow my be centered on the northwest suburbs north into southern Wisconsin—but a wind-driven period of accumulating snow in the city and surrounding metro area is also likely to occur and could produce a nightmarish Thursday evening rush hour. After all, area motorists have had no experience with accumulating snow this season and municipalities and other entities have yet to spread snow-melting chemicals on area thoroughfares. This could increase the level of icing which occurs as snow hits comparatively warm pavements then melts and refreezes.
A further southward shift of the storm track could engulf even more of the metro area in significant snowfall.


Storm’s first precipitation isn’t to occur until Wednesday night

This storm’s first precipitation isn’t to hit until Wednesday night then continue Thursday into Thursday night. Current indications are the precipitations may fall as rain Wednesday night then switch over the snow mid or late morning Thursday. Given our predicted 20-degree drop in thermometer readings from just before noon to just after nightfall Thursday and the onset of howling winds, wind chills are likely to plunge to single digits before the sun sets Thursday.


26% of Chicago’s biggest snows occur in December

Over a quarter of Chicago’s biggest snows have historically fallen in December. There have been 171 snows of 6 inches or more since the city’s snow record began in 1884-85—and 45 of them have occurred in December.
Precise figures on how much snow may fall in Chicago Thursday are likely to become possible in the coming 24 hours as model storm track forecasts become firmer.


Second storm hinted next week; early indications: another potential snow-maker

Thursday’s system, likely to exit expeditiously allowing the sun to return Friday and into Saturday, doesn’t look like the last, in terms of snow production. Models have suggested since last week still another snow of some substance could sweep into the Chicago area next week.

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