11:45 am Update: There have been no significant changes to the forecast from earlier this morning. Winter Storm Watches have been extended further east to cover the Chicago Metro area. Precipitation in southern Wisconsin and northwest Illinois will likely already be starting off as snow Thursday morning with rain or a rain/snow mix further east and southeast. Low pressure is expected to move very close to Chicago by Noon Thursay with cold air wrapping south of the low in central Illinois. Heavy snow will continue from Monroe to west of Freeport with a transition to snow between Noon and 3pm elsewhere. Once the snow begins the speed at which the air is rising in the atmosphere behind the low will increase likely causing a period of heavy snow to fall at rates of 1" to 2" per hour for a time. This combined with wind gusts nearing 45mph or 50mph will make afternoon and evening travel very difficult.
Earlier Post:
Winter Storm Warnings continue for southern Wisconsin and northern Illinois beginning this evening and lasting through Thursday evening.
Blizzard Warnings have been issued for Green and Rock counties in Wisconsin and will go into effect Thursday afternoon. A Blizzard Warning has been issued for Jo Daviess County and will go into effect later tonight and last through Thursday evening.
Our first winter storm will make its presence known late tonight and definitely by Thursday morning as warm, moist air drawn in from the Gulf will mix with cold Canadian air from the north. Low pressure will track from Oklahoma tonight to St. Louis, MO by Thursday morning and near South Bend, IN by Thursday night. Initially precipitation across north-central and northeast Illinois will begin as a light rain/snow mix after 4pm/5pm this evening and continue overnight. West and northwest of Rockford may experience a little more snow at the onset as temperatures will remain closer to or slightly below freezing from central Wisconsin to northeast Iowa. As the low moves northeast overnight and early Thursday morning I wouldn't be surprised if there were a few rumbles of thunder; both where the rain is falling and where the snow if falling. By mid-morning Thursday low pressure will strengthen very close to the Stateline which will cause the wind to increase and possible thundersnow to develop in southern Wisconsin and northwest Illinois. Where thundersnow is observed it's possible totals may exceed 10", but this is expected to remain north and west of the immediate area. There will be light rain mixed with snow early Thursday morning but as the low moves south of Chicago by the afternoon the cold air will move in from west to east and change any leftover rain to snow by Thursday afternoon. Snowfall totals will be less the further east and southeast of Rockford and higher the further north and northwest of Rockford. In fact, locations from Madison to Monroe to just west of Freeport to Mt. Carroll and Galena could see totals nearing 12" while locations closer to Rochelle, DeKalb and Sycamore could have totals near 4".
It's important to not necessarily focus on the snow totals but the wind that will accompany this storm. The low will strengthen significantly once it moves into Illinois by late morning Thursday causing northwest winds to gust as high as 50 mph. Near blizzard-like conditions will be found from southern Wisconsin to northwest Illinois and expand east through the day and road conditions are expected to deteriorate through Thursday evening. With temperatures nearing 32° as the switch from rain to snow occurs we'll experience a heavier snow which may cut back on drifting some but where higher totals occur it could weigh down trees and power lines.
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