Current Research on Geography, Earth Science and Environment Vol. 10

On 2 July 2023, a destructive, two-stage extreme precipitation event produced approximately 225 mm (9 in) of rainfall in less than 12 hours within a narrow corridor between O’Hare and Midway Airports in Chicago. Understanding how synoptic, mesoscale, lake-breeze, and convective processes combined to focus this rainfall is important for improving forecasts and flood preparedness in densely populated urban regions. This study aims to identify the multiscale dynamical and thermodynamic processes that organized the event before considering any urban enhancement. It also examines how three quasi-linear convective systems (QLCSs) interacted and evolved into a larger mesoscale convective system with mesoscale convective vortex (MCV) characteristics. The analysis uses ERA5 reanalysis, 13-km Rapid Refresh (RAP) analyses, Storm Prediction Center diagnostic fields, upper-air soundings, Chicago-area surface and Lake Michigan buoy observations, and NOAA NEXRAD radar data. These datasets are examined across meso-alpha to meso-gamma scales to diagnose the evolving jet-streak circulation, potential-vorticity structure, moisture transport, instability, surface convergence, and convective organization. The precursor environment featured an upstream deep cold trough, a mid-to-upper-tropospheric jet streak with coupled potential-vorticity maxima, a stationary warm boundary and deformation zone, and a west-southwesterly low-level jet that transported abundant moisture towards northern Illinois. The first rainfall stage developed as three QLCSs interacted over Chicago and formed a T-bone convective structure near Midway Airport, whereas the second stage followed upscale growth into a long-lived MCV-like circulation with persistent wraparound stratiform precipitation. These linked processes concentrated extreme rainfall over a small urban corridor even before urban heat-island and drag effects were isolated. The results show that extreme urban flooding can arise from close coupling among larger-scale atmospheric forcing, Lake Michigan-related convergence, and local convective evolution. Operational forecasters should therefore monitor transitions between jet-streak forcing regions, low-level moisture and convergence corridors, interacting QLCSs, and emerging T-bone or vortical radar structures. Incorporating these signals into warning operations and urban flood-preparedness planning may improve early recognition of localized, high-impact rainfall events.