Point Forecast:

O’Hare

Tomorrow:  82/56, Mostly Sunny

Wednesday:  76/58, Thunderstorms in the morning, chance of thunderstorms in the afternoon, could be severe.

Thursday:  71/50, Partly Sunny.

Friday:  66/50, Chance of Thunderstorms.

 

Rockford

Tomorrow:  84/54, Mostly Sunny

Wednesday:  74/58, Thunderstorms in the morning, chance of thunderstorms in the afternoon, could be severe.

Thursday:  74/48, Partly Sunny.

Friday:  66/48, Chance of Thunderstorms.

 

Quad Cities

Tomorrow:  83/54, Mostly Sunny

Wednesday:  75/58, Thunderstorms in the morning, chance of thunderstorms in the afternoon, could be severe.

Thursday:  75/50, Partly Sunny.

Friday:  70/52, Chance of Thunderstorms.

Discussion:

We have really been lucky with respect to the abundance of nice weather for the past couple weeks.  Today and tomorrow are no exception either.  Tomorrow I would expect to see temperatures overachieve versus guidance.  Of late, the NAM and NAM Nest have been noticeably cooler than short-range guidance like the HRRR and RAP.  The HRRR-X has again hinted at a scenario like that, so I would not be surprised to see some mid-80’s pop up around northern IL.  Weak southerly flow should persist for most of the day as high pressure departs from our area.  By later evening there should be an uptick in surface winds as a sfc-low moves into Iowa during early Wednesday.  This will result in better alignment of the low-level jet, and resulting surface winds will also respond accordingly.

Wednesday should feature southerly winds of 15-25mph with gusts of 30mph+ possible.  Among the strong southerly winds, a strong shortwave will move into the upper Midwest.  With the return of better moisture thanks to our southerly flow, and synoptic forcing, we should see some ongoing thunderstorms for the morning of Wednesday.  The more interesting part of Wednesday falls towards the afternoon.  The NAM and NAM Nest poses an interesting scenario of a little bit of clearing behind the initial morning thunderstorms.  With strong southerly flow, additional sunshine, and an inversion left behind from morning convection, it could pose a severe weather scenario for later in the afternoon.  Both are rather consistent on the clearing ahead of the attendant cold front.  As most of us have seen, it does not always take clear days for the atmosphere to destabilize enough to cause concern for severe weather, especially in IL.  At this time, I would hedge my bets more towards a scenario of a hail and wind threat.   I have some reservations with the surface wind field as well as upper-level flow.  Regardless, there is potential for severe weather later Wednesday for northern IL.

NAM NEST’s depiction of the potential environment over northern IL during the later afternoon on Wednesday.

The NAM’s depiction of the shortwave at 500mb overlayed with areas of Positive Vorticity Advection (indicating possible areas of vertical motion).

Thursday appears that it will be rather pleasant.  There is some disparity between guidance (GFS and NAM) on the warm front position and related temperature gradient across the Midwest, but for the majority of the area, we should see temperatures in the mid 70’s.  Easterly flow near the lake would temper high temps some for areas closer to Lake Michigan, and as daytime heating weakens, we could see a lake breeze push inland.

Friday is where guidance really differs.  The GFS shows a warmer scenario, with a sfc-low in southern Minnesota.  The ECMWF shows a scenario of a sfc-low in the Central Plains.  The GEM displays a known cold-bias with the warm front hanging south of I-80 and northern IL stuck in the 40’s and 50’s for highs.  Regardless of the setup in mid-range guidance, there is a potential for wet weather to return.  This would be especially true at this time for the northern most counties in northern IL.  Another thing to watch out for are rainfall amounts.  If the ECMWF scenario plays out with a 24hr period that the warm front is draped across northern IL, any subtle perturbations in the upper atmosphere could lead to elevated thunderstorms training along that front.  At this time I will keep temperatures near 70 degrees for areas away from Lake Michigan and south of I-90, but this will have to be monitored throughout the week.