The increasing cloud cover Tuesday night means a few changes are in the forecast later this week. Sunshine from Monday and Tuesday will fade away to mostly cloudy skies as high pressure slides to the east Tuesday night. The start of Wednesday, and most of Wednesday afternoon and evening, will remain dry. Winds will increase from the south, gusting at times to 25 mph. This will bring temperatures into the low 30s, and above freezing, during the afternoon.
Incoming low pressure from the Pacific Northwest will become ‘cut-off’ from the main flow of the jet stream Wednesday night and Thursday. This will cause the low to slowly sink south through Iowa and Missouri and eventually into central Illinois Thursday afternoon and evening. Temperatures should initially remain cold enough to support mostly snow through Thursday night, but could warm just enough to get a little sleet or even rain showers to mix in from time to time Thursday afternoon. Friday looks to have the better chance for warm air to make it far enough north into northern Illinois for a little rain, but I am watching Friday evening and night for the possibility for a few bursts of heavier snow on the backside of the low as it moves into Indiana and Ohio.
Most of the snowfall Wednesday night and Thursday should remain light, but it is Friday evening when the majority of the snow accumulations could take place – following a chance for rain during the afternoon. Temperatures at the surface will remain very close to freezing – 32 degrees – so the snow to liquid ratios will remain low, and possibly lower than a 10:1 ratio – meaning a heavier snow. It’s hard to say at this point, and this early, how much snow will fall over the several day event but at least a few inches is possible by the end of Friday night.