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Chicago area headed for major snow spread over 30-36 hours and driven by strong winds

by Tom Skilling Today at 11AM

An impressive 8 to 12″ of snow is on the way–in some locations there may be more–and it’s to be spread over an impressive 30-36 hour period starting this evening and continuing Sunday and Sunday night during which “NE” winds are to strengthen. The 35 mph gusts due by Sunday afternoon and evening are to send 12 ft. waves into the Wisconsin, Illinois and far northwest Indiana shorelines.

The morning Saturday model suite is now in and produces storm guidance which is a virtual carbon copy of preceding forecast cycles. Continuity reinforces confidence in any storm forecast. The packed isobars in the surface forecast panel at the top of this post, portray the driving force between the wind field projected to develop formidably this storm system. The tight, wind-generating pressure gradient predicted is to occur between a sprawling 1040 mb (30.71″) Canadian high pressure in the Dakotas and the storm system’s 1008 mb center predicted to march across downstate Illinois and Indiana.

Here are some of this storm’s predicted features which support the notion it is to be a significant one. This meteogram offers a graphic portrayal of this storm’s extensive moisture field over time. It depicts an atmosphere which is to be fully saturated and begin snowing Saturday evening–snowfall which may vary in intensity but is expected to continue until  just before 6am Monday morning. (A lake-effect moisture plume may be flirting with the Chicago or northeast Illinois are into Monday morning and that could keep some snow going in shoreline areas in the early Monday morning period. That’s quite a period of snowfall!

This meteogram depicts as shades of red and purple, the region of the atmosphere saturated with moisture.  The bottom of the graphic corresponds to ground level while the top is up around 40,000 ft. The times are displayed across the bottom--12Z is 6am, 18Z is noon and 00Z is 6pm Chicago time.

This meteogram depicts as shades of red and purple, the region of the atmosphere saturated with moisture. The bottom of the graphic corresponds to ground level while the top is up around 40,000 ft. The times are displayed across the bottom–12Z is 6am, 18Z is noon and 00Z is 6pm Chicago time.

You can have all the moisture you want, but to induce snowflake formation, the development of dendrites—i.e. tiny ice crystals which seed the cloud and encourage snowflakes to develop—must occur within the saturated air mass. And, that’s just what the predicted vertical temp profile show here over Chicago leading to a “dendritic formation zone” in the areas shaded in green. That’s where temps are between occurs 12 and 18-below zero celsius.   This graphic depicts this zone and represents it with green shading. These zones are of varying depths in different storms. The deeper the dendritic zone, the better the prospect for snow.Screen Shot 2015-01-31 at 8.58.35 AM    Moisture, and plenty of it, is required top fuel a storm like the one approaching.  The precipitable water depiction here verifies at  shows 0.50″ to 1″ of more of northbound moisture feeding into this system.

This is the precipitable water forecast which depicts the moisture flowing into our approaching winter storm.

This is the precipitable water forecast which depicts the moisture flowing into our approaching winter storm.

Already, the storm’s satellite presentation–certainly the scope of the system—is growing more impressive on this morning GOES satellite image.

Saturday morning's GOES satellite view of the developing winter storm.

Saturday morning’s GOES satellite view of the developing winter storm.

The means of lifting the incoming moist air is critical.  By lifting air, it’s cooled to condensation which leads to cloud and precipitation formation.  Dr. Louis Uccellini and Paul Kocin have written extensively of the role of speed maxima in the jet streams—what meteorologists refer to as “jet streaks”–in producing lift and their critical role in the development of our most powerful winter storms. The “coupling” or combining of areas of lift which occurs when two jet stream wind maxima are in place in relatively close proximity is a key component driving winter storm development and we see it with this weekend’s storm in this 500 mb. (roughly 18,000 ft.) depiction of winds as predicted Sunday by the Weather Service’s WRF model. This panel comes from the excellent College of DuPage model site. You can see one jet stream wind max over the upper Great Lakes and another south of us. The rear southwest entrance region of the northern jet max and the front left exit region of the speed max south of us each produce the lift which is to aid in the development of the widespread area of cloudiness and snow with our latest winter storm.

Upper winds at the 18,000 ft. level as predicted Sunday by the Weather Service's WRF model. This panel depicts an Uccellini-Kocin  coupling of jet stream wind maxima.

Upper winds at the 18,000 ft. level as predicted Sunday by the Weather Service’s WRF model. This panel depicts an Uccellini-Kocin coupling of jet stream wind maxima.

Here’s the latest high resolution WRF model forecast of total snow accumulation with the incoming winter storm.

High resolution 4km WRF model snowfall forecasts for the incoming winter storm of Saturday mornings 6am run.

High resolution 4km WRF model snowfall forecasts for the incoming winter storm of Saturday mornings 6am run.

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Tom Skilling

Tom Skilling

Tom Skilling, WGN-TV chief meteorologist, appears weekdays on WGN Midday News, WGN Evening News and WGN News at Nine. He celebrated his 30th anniversary with WGN-TV in August 2008. Since 1997, Skilling has masterminded the weather page in the Chicago Tribune. In the city of Chicago and beyond, he has become the superman of meteorology: when in doubt, it is Tom that the city turns to when figuring out the mysteries of nature. WGN-TV received the 2005 Environmental Reporting Award from the Audubon Society, an accomplishment that is due to Tom's performance.

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