Self-described "weather nut" Richard Lelonek of Baltimore watches local rainstorms on weather radar. He says they often seem to split as they reach Baltimore, "almost as if a wall were present," then merge again beyond the city. If so, "global warming surely would be the cause," he says. I don't see how. Local "heat island" warming of urban streets and roofs in summer can fuel convection - rising hot air - over cities, increasing downwind rainfall. But I'd bet what you're seeing is just some quirk of local radar echoes.