Winter Storm Watches have been issued for all of northern Illinois and southern Wisconsin beginning Friday evening, lasting through late Saturday night. It’s hard to believe that winter weather is on the horizon with temperatures currently in the upper 40s and low 50s, but an incoming cold front Thursday night will allow cooler air to move in for Friday. Temperatures during the day Friday will remain above freezing, so any precipitation during that time will be liquid. By Friday evening northerly winds will pull down cooler air, bringing surface temperatures down closer to freezing, if not slightly below, late Friday evening. This is when we could begin to see some freezing rain and sleet develop. The big question is just when the temperatures will begin to fall.
While there is a chance for wintry weather Friday evening, I do think a better chance will be after 8pm/9pm Friday night, lasting through Saturday morning. During that time, sleet and ice accumulations could add up to a tenth of an inch, or higher, across parts of north-central Illinois. It’s important to note that the amount of freezing rain overnight Friday will depend on just how heavy rainfall to our south is. Very heavy rain could cut back on some of the freezing rain potential locally. However, if the full ice potential is realized (combination of sleet and freezing rain) there could be an area of ice accumulation up to a quarter of an inch over parts of DeKalb, Ogle and Lee counties.
By Saturday morning temperatures will have cooled below freezing, taking out the freezing rain potential, but keeping a chance for sleet. As colder air arrives Saturday afternoon a mix of sleet and snow will be possible for some before switching over to all snow Saturday evening. Total snow accumulations *could add up to over six inches, especially for areas that see snow for a longer duration of time Friday and Saturday.
As low pressure winds up into northwest Indiana Saturday afternoon, gusty Northeast winds will begin to develop. This will likely reduce visibility with blowing snow/sleet during the day Saturday. The majority of wintry weather will be exiting to the east early Sunday morning. I do want to stress that there is still some room for this storm system to shift some before arriving, or even as it is moving overhead. This could have an impact on a number of things from precipitation type and how much sleet, freezing rain and snow could fall locally. It’s likely numbers will change a little between now and Friday, but the overall impact is expected to remain the same. Very dangerous travel from Friday night through Saturday night is expected. Roads may still be a little icy on Sunday, despite the precipitation coming to an end, as temperatures are expected to remain below freezing.