Weather will remain cool and tranquil through the end of the week ahead of the next major weather system Friday night into Saturday. A surface low pressure system will develop over the western plains later this week and drop southeast during the day on Friday. The system will be in a weakening phase, but strong Warm Air Advection (WAA) and impressive frongtogenical forcing coupled with instability will promote a shield of overrunning precipitation to develop across the area Friday evening. Central IL should see mainly rain with the exception likely being along and north of I-74. A band of heavy snow is looking more likely for portions of Northern IL, likely setting up somewhere near or just south of I-80. Precipitation may mix with and eventually change over to snow as far south as I-74 by Saturday afternoon.
Given the seasonably moist airmass associated with this system, precip totals may be significant with QPF likely exceeding and inch across the northern half of Central IL. It is important to note, that a subtle shift in the track of the surface low will likely have a big impact on precip type. If the low track shifts a little further south than currently anticipated, accumulating snows may push into northern portions of Central IL. If the low tracks further north than currently expected, a cold rain is likely in store for all of Central IL. The zones of uncertainty regarding the track of the system and associated band of snowfall can be seen in the graphic below. No need to focus on the specifics at this point as the eventual tracks will be ironed out in future updates. The main message to take way is that a strong storm system will impact much of IL, Friday evening into Saturday.