A succession of slow-moving thunderstorms late Monday into early Tuesday dumped nearly six inches of rain on parts of Chicago in less than two hours and as much as seven inches on communities in northwest Indiana, according to the National Weather Service. The deluge followed weekend storms that had already saturated the ground, turning Monday night’s system into what Chicago’s water chief called a “100-year” downpour. Flooding spread across the region. In Chicago, basements on the Southwest Side filled with water, while the DuPage River overflowed its banks and submerged the popular Naperville Riverwalk. DuPage County officials said flood-control facilities absorbed more than 82 million gallons of water. In northwest Indiana, at least 200 homes in Hammond experienced sewer backups after 160 million gallons surged through a system designed for 68 million. BP’s Whiting Refinery, the company’s largest U.S. plant, reported operational disruptions after residents detected unusual odors, and scattered power outages were recorded across Lake County. Municipal leaders spent Tuesday surveying damage and promising relief. Mayor Brandon Johnson walked Gage Park streets with emergency-management officials, distributing cleaning supplies and urging residents to complete flood-damage surveys that could unlock state and federal assistance. Johnson said the city must raise “progressive revenue” to overhaul aging sewers, a stance some aldermen challenged as they pressed for immediate rebates to help homeowners install backflow valves. In Hammond, the sanitary district offered residents up to $2,000 toward similar devices. Forecasters expect a cold front to bring drier, cooler weather by Wednesday, providing crews a window to pump out water and assess structural damage. Even so, officials warned that climate-driven extreme rain events are likely to become more frequent, intensifying pressure on the region’s drainage infrastructure.