Tropical Storm Isaac is gaining strength as it gets closer to predicted landfall along the northern Gulf Coast.

At 5 p.m. Monday, the National Hurricane Center reported that Isaac's top sustained winds had reached 70 mph (113 kph). A tropical system becomes a Category 1 hurricane once winds reach 74 mph (119 kph).

A hurricane warning is in effect from Morgan City, La., to the Alabama-Florida line. East of the state line to Destin, Fla., hurricane forecasters have now posted a tropical storm warning.

The storm's center was located about 255 miles (415 km) southeast of the mouth of the Mississippi River and is moving northwest at 12 mph (19kph). Isaac was expected to make landfall as early as Tuesday, possibly in Louisiana south of New Orleans.

Storm surge is considered a major threat.