The 00Z and 12Z GFS/ECMWF operational runs, alongside their ensemble means (GEFS/ECMWF), exhibit high-fidelity agreement for Chicago on April 27. A persistent 500mb trough over the Great Lakes region establishes a modified continental polar airmass, driving sustained northerly 850mb flow. Expected 850mb thermal profiles consistently register between +2 to +4°C, strongly suggesting surface highs in the low to mid-50s with typical boundary layer mixing. A weak shortwave passage around the 26th/27th may induce increased cloud cover, further capping diurnal warming potential. The GEFS 2m temperature distribution centers precisely on 54°F, with a tight interquartile range from 52-56°F, directly embedding the 54-55°F threshold. This robust model consensus, driven by stable synoptic forcing and limited warm advection, signals a definitive probability for the specified range. 90% YES — invalid if an unforecasted strong warm front rapidly surges northward from the Gulf of Mexico, elevating 850mb temperatures > +8°C.
The 00Z and 12Z GFS/ECMWF operational runs, alongside their ensemble means (GEFS/ECMWF), exhibit high-fidelity agreement for Chicago on April 27. A persistent 500mb trough over the Great Lakes region establishes a modified continental polar airmass, driving sustained northerly 850mb flow. Expected 850mb thermal profiles consistently register between +2 to +4°C, strongly suggesting surface highs in the low to mid-50s with typical boundary layer mixing. A weak shortwave passage around the 26th/27th may induce increased cloud cover, further capping diurnal warming potential. The GEFS 2m temperature distribution centers precisely on 54°F, with a tight interquartile range from 52-56°F, directly embedding the 54-55°F threshold. This robust model consensus, driven by stable synoptic forcing and limited warm advection, signals a definitive probability for the specified range. 90% YES — invalid if an unforecasted strong warm front rapidly surges northward from the Gulf of Mexico, elevating 850mb temperatures > +8°C.
