Low pressure will move in from the southwest late tonight increasing cloud cover this evening, but also pulling in a little more warmth and a lot more moisture. This means that a mix of accumulating snow, sleet, rain and freezing rain will occur within a 24 hour period.Here's the way it looks as of Wednesday morning:
Light snow will be possible late this evening with a mix of snow and sleet overnight tonight. By early Thursday morning freezing rain will mix with the snow and sleet at times across portions of northern Illinois with heavy, wet snow falling just north of Rockford. Minor ice accumulations will be possible Thursday morning which will make travel difficult. By Thursday afternoon enough cold air may wrap back into the system for a switch over to all snow. Temperatures will be right around 32° both at the surface and above so the snow that does fall won't be fluffy like what we had this weekend. It'll likely be a little more heavier and wet. Preliminary indications suggest a band of 3"-6" snow across northern Illinois from Highway 20 to the Wisconsin/Illinois state line. Lesser amounts to the south and higher amounts to the north.
Here's the atmospheric setup that will lead to the wintry mess Thursday:
To get a better understanding of what will happen with temperature, dew point, wind speed, relative humidity, etc throughout the atmosphere meteorologists look at something known as a Skew-T diagram. This is basically a snap shot of all the different layers of the atmosphere at a given time. During winter, this is key to help us determine what type of precipitation we'll experience as a storm moves through. The graphic on the left is a Skew-T diagram for Rockford at midnight tonight. The solid blue line represents the temperature and the dashed blue line represents the dew point. Both lines start at the surface and go up to over 30,000 ft; height is found on the left side of the graphic (Y-axis) and temperature is on the bottom (X-axis) and is in Celsius. What's important to note during this time period are where the temperature and dew point are from the surface to roughly 3,000 ft above. The temperature at the surface is slightly under 0°C (32°) indicating a sub-freezing surface. As you travel further up through the atmosphere the temperature line moves above freezing which is an indication of warm air advection aloft. The dew point line does the opposite indicating a drier wedge of air will be present as the temperature warms. Now, you may be asking yourself why this is important. Drier air aloft can sometimes lead to a period of freezing rain or drizzle because as precipitation falls through that dry layer it evaporates which cools the surrounding air.
As a southwest wind continues overnight it'll help transport more moisture into the Stateline and by 6am Thursday morning the atmosphere should be almost completely saturated. This is the Skew-T chart for Rockford Thursday morning; and notice how the temperature and dew point line are almost on top of each other. The temperature aloft continues to show a shallow warm layer but at the surface remaining right at or slightly below freezing. This will continue the threat for an accumulating wet and heavy snow, sleet and freezing rain mix during the morning hours but for locations north of Rockford closer to the Wisconsin/Illinois state line the warm air may not occur meaning an all snow event with locations receiving over 6" of snow!
By Thursday afternoon colder air returns with a likely transition back over to all snow; which could accumulate by Thursday evening for all in northern Illinois and southern Wisconsin. Details on the forecast continue Wednesday morning with updates provided throughout the day. Travel Thursday will become difficult beginning in the morning and may worsen through the afternoon.
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