Current GFS and ECMWF deterministic runs are showing surface highs consistently exceeding the 43°F threshold for Chicago on May 5. Specifically, the GFS 12z run pegs the high at 47°F, while the ECMWF 00z is slightly warmer at 48°F. The 850mb temperature forecasts indicate a -2°C to 0°C airmass, too mild for a prolonged suppression into the 42-43°F band, especially with projected afternoon boundary layer mixing. The GEFS ensemble mean for the Chicago O'Hare terminal area is 46.5°F, with only 12% of members falling within the 42-43°F range. NAM guidance suggests lingering scattered low-level stratus early, but significant diurnal warming as shortwave ridging builds, diminishing the deep cold advection by midday. This isn't the sustained Arctic air outbreak scenario required for such anomalous lows. It's a clear miss. 90% NO — invalid if surface highs average below 44°F across major model consensus at 06z May 5.
Current GFS and ECMWF deterministic runs are showing surface highs consistently exceeding the 43°F threshold for Chicago on May 5. Specifically, the GFS 12z run pegs the high at 47°F, while the ECMWF 00z is slightly warmer at 48°F. The 850mb temperature forecasts indicate a -2°C to 0°C airmass, too mild for a prolonged suppression into the 42-43°F band, especially with projected afternoon boundary layer mixing. The GEFS ensemble mean for the Chicago O'Hare terminal area is 46.5°F, with only 12% of members falling within the 42-43°F range. NAM guidance suggests lingering scattered low-level stratus early, but significant diurnal warming as shortwave ridging builds, diminishing the deep cold advection by midday. This isn't the sustained Arctic air outbreak scenario required for such anomalous lows. It's a clear miss. 90% NO — invalid if surface highs average below 44°F across major model consensus at 06z May 5.
