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A car rocked like flotsam in thigh-deep water in Brooklyn. A cascade ran down subway stairs in Harlem while a nonchalant commuter checked her phone. The rain knocked out power for some 200,000 New Jersey customers, a day after a heat wave caused tens of thousands to suffer blackouts in New York City.

The chaos came not from a “superstorm,” but an intense, short thundershower.

A day later, on Tuesday, officials and residents across the region sounded an alarm: If summer weather swings create this level of havoc, they said, the New York area is not ready for the sharper extremes that climate change will bring — let alone the next hurricane.

“Every data point suggests that climate change is moving a lot quicker than city government,” Scott M. Stringer, the city’s comptroller, said in an interview. “We did not have a superstorm last night. We had rainfall, and people were literally swimming on Carroll Street,” he added, referring to flooding in Brooklyn. “If that is not a clarion call for focus, then I don’t know what is.”

Some New Yorkers on Monday took matters into their own hands. On the flooded Long Island Expressway, Daphnee Youree, 50, waded out of her car in Crocs and pulled debris from a clogged grate with a traffic cone, draining the pool.