As most people are aware, a major snowstorm is bearing down on northern Illinois. The National Weather Service has issued a Winter Storm Warning for all of northern Illinois. Models have centered on the heaviest snow band falling between the I-80 and I-88 corridors. This is a major storm that will have a dramatic impact on the evening rush hour.
The NAM model has come south, while the GFS and European models have shifted slightly north. The end result is a heavy snow for the area. An usually strong upper trough will push in from the northwest on Tuesday. A smaller disturbance riding in from the southwest will combine with this, creating a mammoth storm. This southern low will allow for Gulf of Mexico moisture to be drawn in. A strong upper level jet stream associated with the main upper level low will vigorously lift the moist air and lead to heavy snow.
The system has been moving a little slower than expected. Light to moderate snow should be ongoing across many areas by morning. This will affect the morning rush hour, but not in a major way. Leaving early would still be a good bet. The heaviest snow will occur from late morning through the early evening. This means that evening rush hour will be an absolute mess. Major highways should be avoided if possible.
It looks as though 6-10 inches of snowfall is a good bet within the I-80 to I-88 corridor. Southwest of this, 4-8 inches of snow will be likely, as precipitation may start off as rain or a mix. Strong upper level cooling will take place as the upper low moves in and all locations will go over to snow. Snow to the southwest will likely have a higher water content and pack down more. Locations north of I-90 may taper down to 6-8 inches.
East winds will begin to gust to 30mph as the day goes on. This will further complicate travel with some blowing and drifting. Easterly winds may cause lake enhancement in Cook, DuPage, and Lake counties. This is the wild card that could really ramp things up. If this occurs, a few locations in these counties may see 12 inches of snow.
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