Snow increasing west and south of the city

At 5 pm snow was falling west and south of Chicago and was increasing in intensity as visibilities continued to drop. At Peoria, Moline and Freeport visibilities were down to 3/4 of a mile while reduced to 1 mile at Peru, Decatur, Lincoln and Springfield.

The leading edge of the snow was approaching the Fox Valley and will continue to slowly overspread the  Chicago area this evening and tonight.
A winter storm  warning remains in effect for the entire Chicago metropolitan area through noon Wednesday. Total snowfall should be in the 8-14 inch range with heaviest amounts in areas near Lake Michigan where lake-enhanced snowfall is expected to up accumulations.
Strong winds will also develop late Tuesday and Tuesday night causing much blowing and drifting snow along with very poor visibility in open area as near blizzard conditions develop,

Leading edge of snow around I-39 and spreading to the east.

At 4pm  snow had begun in the Rockford area and snow was falling south from there through Rochelle to Peoria In the Chicago area skies have clouded over after a partly sunny afternoon. A layer of dry air in the lower levels of the atmosphere is currently  in place over the Chicago area at this time and it will take a few hours for this dry layer to saturate and the snow to begin. The snow will be moving in from the west reaching the Fox Valley between now and 6 pm and overspreading the rest of the area during the evening.

Entire Chicago metropolitan area upgraded to winter storm warning

The entire Chicago metropolitan area is being placed under a winter storm warning effective from this evening through noon on Wednesday. Previous the winter storm warning had been in effect only for counties close to Lake Michigan where lake-enhanced snowfall was expected to boost snowfall total and surrounding areas were under a winter weather advisory. The warning area was expanded because strong winds are expected to develop gusting to 35-40 mph. The high winds will cause much blowing and drifting of the newly fallen snow creating very hazardous conditions that could result in near blizzard conditions in open areas.

Chicago area counties now included in the Winter Storm Warning include..
In Illinois
Cook, DuPage, Lake, IL, McHenry, Kane, Will, Boone, Winnebago, Ogle, Lee, DeKalb, Kendall, La Salle, Grundy, Kankakee, Livingston, Ford and Iroquois
In Indiana
Lake, Porter, Jasper, Newton 

Snow picking up in areas to the west of Chicago

Shortly after 3 pm skies were clouding over in the Chicago area as the advancing snow area moves toward the city. Snow is falling in areas to our west and visibilities are dropping, a sign that the snowfall is increasing in intensity. The visibility had dropped to 1 mile and both Moline and Peoria and light snow had begun at Rochelle. Some light snow should be moving into the Fox Valley within the hour and overspreading the city after dark. The snow should be light at first but gradually increase in intensity through the evening hours.

All appears go for Winter 2009-10's heaviest and lengthiest snow

The sunshine with which Monday afternoon has opened belies the area’s meteorological fate in coming days. The Chicago area sits at the precipice of a major snow event which may prove the heaviest and lengthiest of the 2009-10 season. To date, the 7.5″ which fell Jan 6-8 tops this season’s snowfall charts. The system which looms appears capable of producing 8 to 14″ accumulations, heaviest in counties adjoining Lake Michigan from northeast Illinois into southeast Wisconsin and northwest Indiana, in what could be a 35 to 40 hour snowfall. A snowfall of nearly a foot would be the heaviest here in over a year, according to our climate guru Frank Wachowski.  A 35-40 hour stretch is a VERY LENGTHY period of snowfall by Chicago standards which could impact 2 and possibly 3 rush hours Tuesday into Wednesday morning!
    We understand a great deal more about snowflake formation than we once did–and this understanding is crucial when it comes to making accumulation forecasts. Here’s why. Because the condensation nucleii–the tiny particles which float in the air around which snowflakes form are primarily composed of clay in this region of the world–this makes it critical for there to be to layer  of air with a temperature of-12 to -14-degree C or colder in order to initiate snow crystal formation around the clay nucleii which predominate in the Midwest. (Were these particles composed of substances other than clay, that would change the temps required to form crystals to something other -12 to -14 deg C). The deeper and colder that cold layer is in a storm, the more efficient snowflake formation is and the more “fluff” which occurs in the snowflakes which form.  This storm will have a comparatively DEEP cold layer–so fluffier snowflakes seem a good bet.
    One of the truly useful tools forecasters today have available are computer model estimates, based on analysis of the depth and temperature of the layer of cold air in snow situations as well as the predicted winds (strong winds can limit snowflake size by bumping flakes together and crushing the crystals at the edge of flakes), of what’s called the “snow-water” ratio–in other words, a calculation of how much snow is likely to develop from the water a storm is expected to take out of the atmosphere.
     The incoming storm is predicted to produce a 16.3 to 1 snow/water ratio. (The typical storm here produces ratios closer to 10 to 12 to 1.) That means that given the size of snowflakes predicted in this storm, 16.3″ of snow would result from an inch of water. A suite of computer models storm suggest the storm which hits in coming days will generate 0.45″ to 0.80″ of water. Using the 16.3 to 1 snow/water ratio, this suggests 8 to 14″ of snow may occur by Wednesday morning. The winds forecast with this storm will be modest Monday night but will increase markedly Tuesday and Tuesday night. Gusts to 25 to 35 mph are not out of the question later Tuesday night and Wednesday morning–more than enough to encourage blowing and drifting.
    We expect snow to overspread the area this evening and to be falling steadily areawide by midnight–then to continue heavy at times Tuesday and much of Tuesday night. Lake effect snow could spill over into Wednesday morning and presents one of the wildcards of this storm situation-namely, how much lake moisture is to be entrained in this storm. The presence of lake moisture in the snowfall equation with the new storm makes it most likely the heaviest snow totals by the time the storm winds down Wednesday morning are to occur in lakeside counties from Kenosha County WI south to Lake and Cook Counties Illinois into Lake, Porter and La Porte Counties IN.
    We plan to keep the updates coming here at on our Severe Weather Blog as well as on our reports over WGN TV and radio.  Stay safe and thanks for checking out our blog!

Tom Skilling