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

Snow to arrive this evening- Biggest snow of season on way with 8-14 inches possible

Despite sunshine in parts of the Chicago area this afternoon, a major snowstorm is approaching and should begin in western areas by evening and overspread the city tonight. The snow will be one of long duration, not ending until Wednesday morning once it begins this evening. It appears that this evening’s rush hour in the city will not begreatly impacted by the snow, but it will be a different story for both the Tuesday morning and evening commutes as well as the one Wednesday morning. Snowfall totals are expected to be in the 8-14 inch range with the highest totals in areas near Lake Michigan, where lake enhanced snowfall will add several inches to the storm totals. Light snow is already falling in western and central Illinois as well as in Iowa and will slowly advance toward the Chicago area as the afternoon wears on. At 1 pm light snow was being reported at Moline, Peoria, Galesburg, Macomb and as close as the La Salle-Peru area.

The storm has already produced substantial snowfall overnight and this morning in Iowa with 4-6 inches in areas from Des Moines south to the Missouri border and 1-3 inches from Des Moines east to the Cedar Rapids- Iowa City region.

Tim's Weather World: Here We Go Again!

We already are more than 10″ above average on snowfall to date this winter.  We only need about six more to bring our total to 38″.  That would match the average amount of snow we get in an entire winter.  That shouldn’t be hard to do with the prolonged, steady snow that will begin late tonight and last through Tuesday night.  More than two dozen states have advisories, watches or warnings.  Washington DC is bracing for more snow but not as much as the most recent storm. 

This could end up being one of the biggest snow storms of the year as we are expecting  anywhere from 6 inches to a foot of new snow. 

In addition to the updates on CLTV and WGN Radio, you can follow the storm through my facebook updates at


or check out my twitter updates on



First substantial snow to arrive this evening

While some western and northern suburbs have already experienced a little light snow and flurry activity this morning, the first wave of accumulating snow is not expected to arrive until this evening.  Some far western suburbs may see the first meaningful snow as early 4PM. 

The heaviest snow is predicted to occur on Tuesday afternoon and Tuesday evening, when snowfall rates will be enhanced by moisture drawn into the system from Lake Michigan on strong northeast winds.   Communities in northeast Illinois (including the city of Chicago) within 30 miles of Lake Michigan are in line to see the majority of the “lake enhanced snow”. 

Winter Storm Watch upgraded to a Warning for much of the Chicago area

The Winter Storm Watch issued on Sunday has been upgraded to a Winter Storm Warning for most, but not all of the Chicago area.  This warning does not go into effect until 9PM tonight, although some snow is possible before that. 

Here is a complete breakdown of the region’s warnings, watches and advisories:

For Cook (including the City of Chicago), Kane, McHenry, Du Page, Lake(IL) and Will Counties:    A Winter Storm Warning is in effect from 9PM tonight (Monday) until 4AM Wednesday.

For Lake(IN), Porter(IN) and Jasper(IN) Counties:   A Winter Storm Storm Warning is in effect from 9PM tonight until 10AM Wednesday.

For De Kalb, La Salle, Lee, Ogle, Boone and Winnebago Counties: A Winter Weather Advisory is in effect from 6PM this evening until 4AM Wednesday. 

For Kendall, Grundy, Kankakee, Linvingston, Ford, Iroquios, Newton(IN) and Benton(IN) Counties:  A Winter Weather Advisory is in effect from 9PM tonight until 10AM Wednesday.

For Kenosha(WI), Racine(WI), Walworth(WI) and Milwaukee(WI) Counties:  A Winter Storm Warning is in effect from 12AM Tuesday until 6AM Wednesday

For LaPorte(IN), St. Joseph(IN) and Berrian(MI) Counties:  A Winter Storm Watch is in effect for late Monday night through Wednesday.    


Counties shaded in RED= Winter Storm Warning  (6 to 13 inches)

Counties shaded in WHITE= Winter Weather Advisory  (5 to 9 inches)

Counties shaded in YELLOW= Winter Storm Watch

WGN Weather Center 7 Day Forecast





Current winter storm forecast appears on track: 6-10 inches of snow expected here Monday night into Wednesday morning

The evening suite of computer guidance is in and at this time the current forecast for heavy snow in the Chicago area appears rock-solid. Current timing places the onset of significant snow here on Monday evening  at the tail end or just after the evening rush hour and continuing into Wednesday morning. Snow totals should be in the 6-10 inch range with the highest amounts near Lake Michigan where some areas could reach 12 inches. In addition to the system snow, a period of lake-enhanced snow is expected late Tuesday night into early Wednesday in the storm’s waning hours. The lake-effect should shift into northern Indiana during Wednesday as winds become more northwesterly. 

Unlike most Chicago snowstorms where the rain-snow line bisects the area, this system is expected to produce all snow here. The storm is a product of the phasing of two storms, one dropping south from Canada and the northern Plains and the second moving east out of the southern Rockies.  As these systems consolidate into one low center significant snowfall is expected to blossom over this area.
The entire Chicago area along with much of the adjacent Midwest areas remains under a winter storm watch. If current trends continue, it is expected that the National Weather service will upgrade the watch to a winter storm warning on Monday.
Periodic updates will be carried on the WGN Weather Center Severe Weather Blog as the storm approaches and be upgraded to near-continuous coverage once the storm begins. Additional storm information will be available Monday morning on the WGN- Channel 9 Morning News, on CLTV and on WGN radio.