The GFS’ depiction of a sounding on Friday evening. Notice the EML (dry layer) at 700mb and warmer surface temps.
The 3KM NAM’s sounding with a noticeably colder surface depiction with still a rather stout dry layer above 850mb.

Point Forecast:

O’Hare

Today:  68/43, Partly cloudy, breezy.  Slight chance of showers overnight. 

Tomorrow:  65/45, Cloudy, chance of showers towards noon.

Saturday:  52/36, Cloudy and rain likely.

Sunday:  41/29, Rain with wintry precip mixed in.

Monday:  40/28, Cloudy, breezy.  Sun breaking through late.

Rockford

Today:  66/44, Partly cloudy, breezy.  Slight chance of showers overnight. 

Tomorrow:  61/43, Cloudy, chance of showers towards noon.

Saturday:  48/34, Cloudy and rain likely.

Sunday:  39/27, Rain with wintry precip mixed in.

Monday:  40/25, Cloudy, breezy.  Sun breaking through late.

Quad Cities

Today:  71/49, Partly cloudy, breezy.  Slight chance of showers overnight. 

Tomorrow:  70/45, Cloudy, chance of showers.

Saturday:  53/30, Cloudy and rain likely.

Sunday:  39/23, Rain with wintry precip mixed in.

Monday:  45/25, Partly sunny, breezy.

Discussion:

Who’s enjoying this weather? 

Today should be the warmest day of 2018, and areas aside from the NE portion of the state, should see temperatures climb into the mid-70’s.  As afternoon warmth begins to lessen, the typical lake breeze will kick further inland similar to yesterday evening.  Some guidance is more aggressive on the pace at which it moves inland, but I would say it could reach Kendall County by 8-9pm.  The further inland it moves, the more diffuse it will become. 

The caveat for tomorrow’s temperatures will be the location of the warm front.  For instance, the RGEM has mid-70’s up to the I-90 corridor, with temperatures rapidly dropping north of there.  Combined with the warm front placement, and possible precipitation north of the warm front, that will aid some evaporational cooling.  For areas south of this boundary, the primary issue is rain.  Sure, there could be some rumbles of thunder mixed in, but in terms of severe it looks unlikely at this point.  With this warm air convection, an EML will be the primary inhibitor to convection.  This is where guidance differs the most:  both the NAM and NAM 3km lend a hand to being more bias to a colder solution.  If I were to take them for verbatim, we would see the warm front sink south more quickly, and the lake breeze move inland quicker as well.  Whereas the RGEM and GFS display a warmer solution, with cooling more based off of the loss of daytime heating.  This also acts as a point for possible rain as the daytime heating decreases.  The biggest issue with all of this, is not whether we will see severe, but if we will see any instability left.  I think rain is a given, and maybe some rumbles of thunder, but the pace at which we cool/see the warm front sink south, will play a huge role in precipitation, and also our surface conditions.  The further north you are (lets say I-90 and north), the more likely you are to see temperatures struggle to moderate nicely like areas south of I-88.

Saturday is again a troubling forecast.  It stems a lot from how guidance is handling it.  The NAM would depict a scenario of a cold rain with gusty NE winds for areas north of I-80.  And by 1pm, the NAM would have the warm front almost as far south as Champaign.  On the other hand, the GEM, GFS, and Euro all have a more northerly solution.  The GFS keeps temperatures for the counties near I-80 in the mid-60’s and low-70’s till at least 1pm and does not decrease temperatures even after that timeframe a whole lot.  It also has the warm front almost directly over I-80, vs. a solution further downstate.  I am more apt in riding with a warmer solution at this point and will keep temperatures for areas south of I-88 in the mid-60’s for longer.  The other factor which could help temperatures, is the approaching dry slot.  We should see some clearing during that timeframe.  During the early morning we should see some rain, but by and far, the heaviest of it should be in areas north of I-88. 

I wish Sunday looked better, but unfortunately it looks rather raw.  By late Saturday, the low-pressure system should move to our east, ushering in cold air advection with gusty northerly winds.  We should see precip as rain, but as the column cools through evaporation, and cold air advection entrenches into northern IL, we could see some snow or other wintry precip mix in.  Undoubtedly, the high for the day will likely occur in the morning hours, and by evening we could see temps in the low-30’s. 

On Monday we will likely see some cloud cover in the morning, but as the day wears on, we should begin to see some breaks in the clouds.  Don’t let that fool you, though.  It will feature a blustery NW wind, and temperatures will struggle to reach the low-40’s. 

The RGEM’s depiction of the lake breeze moving inland this evening.
The NAM’s solution for around 1pm on Saturday.
The GFS’ solution for around 1pm on Saturday.