Hurricane Irma, the most powerful Atlantic storm ever recorded, laid waste to parts of the Caribbean overnight, pummeling Turks and Caicos, hammering the Bahamas and taking aim at South Florida, home to millions of residents. The storm’s death toll rose to 18, but officials warned that figure will increase as rescue workers search through the rubble of islands that have seen over 90 percent of all buildings destroyed. Irma weakened slightly as it pushed west, and was downgraded Friday from a Category 5 to 4 storm, but it remains extremely dangerous, with sustained winds of 155 miles per hour. The storm lashed the northern Dominican Republic Thursday and flooded parts of Haiti overnight, but Irma’s eye passed well north of Hispaniola, sparing the two countries from the worst of the hurricane’s wrath.