A classic ice storm coming together Wednesday in Chicago’s northwest and north suburbs away from Lake Michigan.

At greatest risk for significant ice accumulations: Is the northern Fox Valley area and Lake County, Illinoi (away from the Lake Michigan shoreline) as well as McHenry, Boone and Winnebago counties. Counties across southern Wisconsin north of the Illinois line and away from the Lake Michigan–that would include Kenosha, Walworth and Rock counties – are also at risk.

Ice accumulations in the affected areas may reach 0.25″ to 0.50″.

Wind and rain in Chicago

Heavy, cold wind-driven rains are due in Chicago but the city to be “protected” from ice by Lake Michigan’s mid-30-deg waters.

Travel a concern all day Wednesday throughout Illinois, Midwest

Also of concern are travel conditions in ice-impacted areas. While de-icing chemicals work well in keeping treated thoroughfares wet rather than icy, less traveled roads can become icy—-as can sidewalks and driveways–thus CARE should be taken in taking on and venturing across icy surfaces in affected areas as ice accumulates.

Be ware of power issues

Studies of ice storms indicate the impact of that level of ice buildup can lead to downed branches and, potentially, some power issues if power lines come done. These lines can fall prey to falling branches and their own ice buildup. You can be sure electricity providers are aware of the situation and monitoring developments closely.

It’s actually two storms in one

This winter storm–which actually an amalgam of two storms–is huge. Modeling has been on to its development for a week. Here are two links to high resolution weather satellite animations of the storm:

• From Penn State
• From the College of DuPage

A handful of degrees can profoundly impact the form precipitation takes. Lake Michigan water temps are important because this storm’s powerful east/northeast winds are likely to gust to 30-35 mph at times Wednesday into Wednesday night. Winds will then shift west and gusting to near 40 mph Thursday — after a brief pause in Thursday’s pre-dawn hours as the storm center passes nearby. Horizontal air movement—i.e. winds–pause there as most air movement shifts into the vertical there because air is rising vertically through the atmosphere there. But once that pressure center passes, winds shift and begin to roar from a direction opposite that observed on the storm’s front side.

It shows the layer of above freezing air above the colder air in a shallow layer hugging the surface. This is how freezing rain and sleet develops. In this sort of weather set-up, the clouds are comparatively “warm”–in the case of Wednesday’s storm, temps around 5,000 ft. are to warm to 46-deg while our north and northwest suburbs hover around freezing at ground level. The cloud “warmth” defeat any attempt of the atmosphere to generate snowflakes. Instead, raindrops come together at cloud level and fall into the colder layer near the ground. Some of these raindrops can freeze which is how sleet (i.e. ice pellets) form and mix with the rain at times.