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Bands of lake-effect snow are moving inland into the Chicago area this evening covering areas from the Wisconsin state line south to Kankakee. Some of the heavier lake-effect snows are occurring north of the city. In Deerfield, visibilities dropped to about 1/2 mile as the snow quickly accumulated to about 1/4 inch. Farther north in Beach Park 1.7 inches of snow fell in just 45 minutes.

A Severe Thunderstorm Watch is in effect for much of west central and southwest Wisconsin, a small portion of northeast Iowa and the extreme northwest corner (Carroll, Jo Daviess, and Stephenson Counties) of Illinois until 5AM CDT. At 1:30AM a line of thunderstorms from Central Wisconsin into northeast and north central Iowa was moving east at 35 to 40 mph. Heavy rainfall downpours of an inch or so in a half hour with localized flooding and winds 50 to 65 mph uprooting and breaking branches of some trees has been reported with some of the stronger thunderstorms.

Current indications, subject to change of course, are that the band of thunderstorms will continue moving east and sagging slightly south, meaning they could be tracking into northeast Illinois between 6AM and 9AM Sunday morning and while still packing a punch could be in weakening stages.The outflow winds from these storms could set the stage for thunderstorm redevelopment in the afternoon ahead of an approaching cold front. The cold front is forecast to move through the Chicago area between 6 and 8PM Sunday evening with showers and storms moving east through northern Indiana at that time.

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Tonight's snow to wind down in a few hours...another storm to follow Thursday night-Friday

The snow will be gradually winding down across the Chicago area as the evening wears on with only flurries expected in the area by about 10 p.m. Through 6 p.m. storm snow totals were approaching 4 inches at O'hare and 3 inches at Midway. By the time the snow ends area most area totals should be in the 4-6 inch range.

Before this storm's snow is even cleared, forecasters are already taking note of another storm that threatens the Chicago area during the late-Thursday-Friday period. This storm will have considerably more moisture to work with than tonight's storm and temperatures will be warmer so the area could be facing a messy mixture of heavy snow, ice and even rain before it is over. Winter storm watches are already posted across southern portions of Wisconsin and Lower Michigan and may be issued for northern Illinois and the Chicago area by Wednesday morning.

Beyond that another snow threat looms late Saturday and Sunday followed by an arctic blast that could bring the season's first widespread subzero temperatures.

Stay tuned!!

Steve Kahn WGN-TV/Chicago Tribune Meteorologist

Lake effect snow has developed Tuesday morning ahead of the main "system snow" due to overspread the area this afternoon and continue Tuesday night

Lake effect snow has developed in areas along Lake Michigan Tuesday morning in advance of the main snow expected to be generated by the latest wintry weather system to affect the area this afternoon and tonight. Light easterly winds and the intense cold situated over and adjacent to Lake Michgian are to blame for the early onset of lake-effect snow in areas closest to Lake Michgian including the city of Chicago.

The amount of snow weather systems produce is linked in part to the temperature of the atmosphere through which their snowflakes fall. While most weather systems produce snow in a ratio of about 10 inches for every inch of moisture available to them, temperatures with the Tuesday/Tuesday night system are at levels which will approximately double the volume of snow which falls from the roughly 0.30" water equivalent moisture the current disturbance is to generate. It's likely this system will produce 15 or 20-to-1 ratio snow. In other words, snowflakes will be fluffier and have one and a half to two times the volume of most systems' snow here. It if for that reason, our accumulation predictions have been set at 3 to 6"--where in a warmer environment, a system with the same level of moisture available to it might be expected to produce only 3" of snow. Since the city and southern suburbs have yet to receive a 3" or greater snow to date this season (our northern and northwest suburbs already HAVE seen 3"+ snows)---the current system is to produce the city and southern suburb's heaviest snow to date this season. Weather updates will be issued as necessary.

Tom Skilling
Chief Meteorologist-WGN-TV/Chicago Tribune

Wind damage, rain, and even some hail in certain areas ... this is your Severe Weather Update!

Tom Skilling is tracking the powerful thunderstorms that have been sweeping across the Chicago area, with wind gusts of up to 62 miles per hour. The WGN Weather Center has received recent reports of damage from the storm in the Western suburbs, including wind damage, rain, and even some hail!

Tune in tomorrow for another edition of Before the Forecast, and as always continue
to watch WGN for more coverage: Morning, Noon & Nine.

Tracey Surface provides us this photo of Tuesday afternoon’s snow as it swept into Montrose Harbor earlier this afternoon. Many thanks Tracey! By late evening (5:30 p.m.), Chicago area snow tallies have reached the 2-5” range—including 2” at O’Hare, 3” at Downers Grove and our meteorological colleague Steve Kahn reports 4” is down at Arlington Heights. Chicago’s Midway Airport—where the mid-afternoon 2007-08 seasonal tally has risen to 49.7” (14.7” of it having falling in February alone)—is likely to have played host to a total of 50+” before the evening ends, says veteran Midway and Chicago weather observer Frank Wachowski. Frank also reports this snow, because of the cold temperatures in which it has formed and fallen, has a 22 to 1 snow to water ratio—indicating this snow has more than twice the volume of conventional snow here.

Radar (at 5:45 p.m.) is indicating a band of snow is approaching from the west—snow likely to be reinforced by lake moisture riding NE winds. So the snow’s not done as of this posting—but will end later tonight. It’s not the last snow likely to occur here in the coming week. Though glorious (and welcome) sunshine is due Wednesday—more snow is predicted Thursday (Valentine’s Day) night. The area’s lack of February sun is running at record levels. Only 12% of Chicago’s possible sun has occurred—February’s “normal” tally is 46%.

More on WGN-TV’s Nine O’Clock News Tuesday evening, on our wgntv.com “weather blog” and in Wednesday’s Chicago Tribune.

Tom Skilling
WGN-TV Chief Meteorologist

Photo courtesy: Tracey Surface, Chicago

We’re boosting our Chicago accumulation forecasts further, a foot or more of snow likely in parts of the city—much lighter amounts south

The early onset of snow suggests even higher storm snow totals are likely in the city. Expectations precipitation would start as a rain/sleet mixture in the northern suburbs due to a layer of warm air aloft have NOT materialized suggesting northeast winds have injected a layer of drier air in the lower atmosphere producing evaporative cooling there. That’s why snow has begun as soon as it has. This means assumptions that the first hours of this storm’s precipitation would come down as liquid or a mix must be updated and that moisture must be added to our predicted snow tally. As a result, it now appears likely 8-14” will fall from Chicago north and west. The presence of potential thunderstorms could lead to locally higher totals. Our latest in-house RPM (Rapid Precision Mesocale) model forecast now puts totals at 13” at O’Hare and 10” at Midway. The University of Wisconsin NMS model is even suggesting 12-18” totals in portions of northern Illinois.

Thus a major winter storm, by far this winter’s biggest to date, looks likely to produce serious travel problems well into Wednesday. We’ll update as new information becomes available.

Tom Skilling

Chief Meteorologist, WGN-TV/Chicago Tribune

As of 6:21 p.m. parts of our western suburbs have had as much as 4.5 inches of snow. The snow will continue for many more hours with the height of the storm expected during the morning rush hours. This could be the biggest snowstorm that we've seen this winter.

Tune in early to WGN-TV. Our morning news will have the latest weather and school closings as early as 4:30 a.m.