It may be the first week of spring, but someone forgot to tell the weather that… A potent low-pressure system is set to bring some heavy snow to the Stateline tonight and early tomorrow. A Winter Storm Warning is in effect from 4AM tomorrow morning through 1PM in the afternoon. This Warning is out for Winnebago, Boone, McHenry, Ogle, Lee, and DeKalb Counties in Illinois, along with Rock and Walworth Counties in Wisconsin. The rest of the Stateline remains under a Winter Weather Advisory, also in effect from 4AM to 1PM tomorrow.
This system is rapidly strengthening to our South already, bringing heavy rain and even some severe weather to the Southeast. As this system lifts to the North, it will continue to intensify, leading to what is called dynamic cooling. This is when there is strong upward motion in the atmosphere to produce heavy precipitation. As this precipitation falls, it transfers latent heat from the warmer column of air to the process of melting the surge of precipitation. This transfer of heat then cools the column of air, allowing heavy snow to fall in place of heavy rain.
Many in the Stateline may initially see rain to start out as the precipitation shield inches closer. But as the aforementioned dynamic cooling takes place, we will see a rapid transition from rain to snow. Most model guidance has all snow for most of the area by 4AM, with only a few spots to the SE holding into rain a little longer. Heavy bands of snow stick around through the early morning hours before gradually tapering off into the late morning and early afternoon.
The worst of the snow does appear to be within the timing of the winter weather headlines, or between 4AM and Noon. This is when the heaviest bands of snow will be falling, with rates near 1″-2″ per hour at times. It is during this time that visibility will drop quickly, and travel may become hazardous with untreaded roads gaining some slushy accumulation very quickly. The heavy rate of snowfall will be more than enough to overcome the warm surface temperatures from Friday afternoon, when some spots warmed into the upper 40s.
Total accumulations look to be highest to the Northeastern portions of the viewing area within the counties under the Winter Storm Warning. There will lie the greatest chances for the heaviest bands of snow to set up, leading to total accumulations of around 4-8″ likely with even some locally higher amounts within there. Otherwise, a general 2-4″ of heavy, wet snow is expected where the advisory is in effect. There will be a very sharp cutoff of snow totals to the South, likely between I-88 and I-80, where the transition from rain to snow is much slower. Some counties closer to I-80 may see no snow at all! There is still a bit of uncertainty as to exactly where the heaviest bands set up, along with how much snow will accumulate on some of the warmer surfaces. However, the main takeaway is that heavy snow is expected tonight with some hazardous travel through the morning tomorrow.