Hurricane-strength winds and driving rain are nearing the coast of the United States as tropical storm Isaac continues its progress across the Gulf of Mexico. After lashing southern Florida, Isaac is heading towards the northern Gulf Coast, threatening New Orleans seven years after Hurricane Katrina hit. Louisiana, Florida, Mississippi and Alabama have declared emergencies. This combination of National Oceanographic and Atmospheric Administration (NOAA) satellite images shows Tropical Storm Isaac (top) on 27 August 27 and Hurricane Katrina (bottom) on 28 August 2005. Click through for more.