As confidence continues to grow that a winter storm will impacts parts of the Stateline, a Winter Storm Watch has been issued for most of northern Illinois beginning Saturday morning and lasting through late Saturday evening. This watch includes all our counties except for southern Wisconsin, as well as Jo Daviess and Stephenson. That’s because confidence isn’t quite as high yet on the impacts that snow could bring to areas that far north or northwest. Within the next 24 hours the Winter Storm Watch will most likely be changed to either a Winter Weather Advisory or Winter Storm Warning, based upon where some of the higher snow totals and impacts will occur.
A trough of low pressure currently along the West Coast will move inland later tonight and Friday. Ahead of it an area of low pressure will move across the desert Southwest and into the Midwest Saturday afternoon. Low pressure developing at the surface will begin to form over eastern Colorado, moving through Oklahoma, southern Missouri, Illinois and Indiana by Saturday evening. Severe weather is expected to develop to the south of the low, with wintry weather to the north. High pressure over the Upper Midwest will keep a cold North/Northeast wind in place for much of the day.
Snow is expected to develop and lift north Saturday morning, reaching most of northern Illinois by Noon. A steady, and at times moderate, snow can be expected for much of the afternoon and evening before tapering off both in intensity and amount by Saturday night. While the track of the low has been somewhat consistent for the last couple of days, there are still some issues that could cause the overall track – and therefore snow amounts/types – to change before Saturday.
The incoming upper portion of the storm system is what meteorologists refer to as a ‘positively tilted trough’. These types of upper level typically don’t produce deep and very strong surface low pressure systems. This means that the incoming low will be more elongated and not rapidly intensifying. If the upper level trough were to turn more ‘negatively tilted’ it *could* then push the surface low slightly further north, introducing a little more warm air into central and northern Illinois. There is also plenty of moisture for this low to ingest and produce snow and thunderstorms, but thunderstorms developing south of the low Saturday could actually hurt our snow amounts here in the Stateline. That’s because the storms to the south would use up and block the Gulf moisture from reaching this far north. Something we’ll need to keep an eye on as we head into Saturday. The potential is definitely there for parts of the area to receive snowfall amounts of 5″ or even 6″. I think that will occur more within a narrow area, or band, versus being widespread. As it looks Thursday night, the heaviest axis of snow appears to occur from roughly southern Iowa/northern Missouri, up through west-central Illinois and then towards Chicago – possibly reaching parts of Whiteside, Lee, Ogle and DeKalb counties. But again, a changing storm track will end up shifting that around. That’s why we’re showing, right now, where the heaviest axis of snow *may* occur, but not giving out specific totals just yet. By tomorrow (Friday) evening, we should have a better idea of how much snow will fall and who will end up seeing the heaviest amounts.
Regardless of the amount of snow that falls, whether it be three inches or six inches, the impacts will still remain high as North/Northeast winds gust to 30 mph throughout much of the afternoon and evening. This will cause whatever snow that does fall to blow around not only as it falls, but also while on the ground. Travel will become difficult and hazardous through the afternoon and evening Saturday. So if you don’t have to be out, it will be best just to remain in.
Following the snow Saturday temperatures are expected to tumble, enhanced by the freshly fallen snow. Overnight lows Saturday will fall into the low teens and upper single digits, only rising into the teens Sunday. Calming winds and clear skies Sunday night will bring overnight lows down below zero degrees Monday morning! A very cold start as kids head back to school.