A freak “homegrown” tropical rainstorm has put millions of people on alert for ‘life-threatening’ floods. The storm, of the type that usually starts further out in the tropical Atlantic, has sprung up off the US south coast. It threatens “significant and alarming” rainfall, “spin-off” tornadoes and a possible coastal storm surge. AccuWeather meteorologist Jon Porter said: “This type of homegrown development is much more common in the late spring to early summer. “Most tropical systems that form during the heart of the hurricane season develop thousands of miles to the south over the central At…