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 wgntv.com 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!