Chicago O'Hare Weather Area Forecast Discussion Sunday June 25, 2023

Area Forecast Discussion 
National Weather Service Chicago/Romeoville, IL 
626 AM CDT Sun Jun 25 2023

.SHORT TERM... Issued at 310 AM CDT Sun Jun 25 2023

Through Monday night...

Regional water vapor imagery depicts a wound-up low pressure system wobbling through the northern Plains beneath broad upper-level cyclonic flow approaching the Great Lakes. A notable upper-level shortwave is evident diving into northwestern Iowa around the periphery of the center of the system near Aberdeen, South Dakota. A cold front stretches from near the Twin Cities through central Iowa, and is racing east toward Illinois. Meanwhile, a mesoscale convective system continues to move through northern Illinois and northwestern Indiana this morning along the northern edge of a 40- 45kt LLJ as sampled by the KILX VWP. Much of our area is hence seeing beneficial rainfall with amounts as high as 1.25-1.5" in a few locations. Over the next few hours, the MCS will continue pushing southeastward as the low-level jet veers. Widely scattered showers and storms may continue south of I-80 through daybreak.

Later this morning, the aforementioned upper-level shortwave will scream across northern Illinois and catch-up with the cold front and a narrow warm sector advecting eastward into far eastern Illinois and western Indiana. Increasing DCVA and cooling temperatures along the leading edge of the shortwave and approaching cold front will kick off rapid thunderstorm development around or just after noon most likely over Lake Michigan. However, forecast soundings from the RAP and HRRR suggests the first attempts of convective initiation may occur as far west as I-55 between 11 AM and 12 PM. At this point, opted to maintain 20% chances for thunderstorms across far northeastern Illinois and northwestern Indiana from late morning to early afternoon, keeping in mind most of the area may end up dry. Otherwise, westerly winds will quickly pick u Pbehind the cold front with gusts of 30-40 mph. Aggressive mixing may yield a few gusts as high as 45 mph during the heart of the afternoon. As the core of the system moves into the Great Lakes this evening, steepening low-level lapse rates by virtue of the advection of a pocket of seasonably cool mid-level temperatures will afford the development of widely scattered showers. On-and- off showers will then continue through the overnight period.

The deep and expansive upper-level cyclonic flow and pocket of seasonably cool mid-level temperatures around the system will allow for partly to mostly cloudy skies, blustery northwest winds, and scattered showers to continue through Monday. Highs should range from the low to mid 70s, which may provide just enough low-level instability to support a thunderstorm or two within otherwise scattered showers during the afternoon. Showers should taper after sunset as the cyclonic shear axis of the upper-level low pivots overhead.


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