Isaac was on the verge of becoming a full-blown hurricane Tuesday as it rolled over the Gulf of Mexico toward Louisiana, where residents of the low-lying coast left boarded-up homes for inland shelter while people in New Orleans waited behind levees fortified after Katrina.  Forecasters predicted the tropical storm would power up to hurricane strength, which starts at winds of 74 mph, later in the day and be at least a Category 1 hurricane by the time it's expected to reach the swampy coast of southeast Louisiana early Wednesday. The forecast track has the storm aimed at New Orleans, but hurricane warnings extended across 280 miles from Morgan City, La., to the Florida-Alabama state line. It could become the first hurricane to hit the Gulf Coast since 2008.  In the Big Easy on Monday, Mayor Mitch Landrieu did not activate a mandatory evacuation.   Federal officials said the updated levees around New Orleans are equipped to handle storms stronger than Isaac but with the landfall expected Wednesday, Katrina's seventh anniversary, anxiety is high, especially in the Lower 9th Ward that was wiped out by Katrina after floodwalls burst and let the waters rush in.