Barry is no ordinary storm. May become hurricane. The levees may overflow. [thinkprogress.org]
Things not mentioned in the article:
Barry is moving at only 5-10 mph over thr Gulf. At that speed it may stall out once it makes landfall. That means that an overly excesdive amount of rainfall on the state.
The Mississippi River is still at partial flood stage. This means that new discharge into the river due to Barry will return it to critical flood stage.
The maximum flood level ever reported for New Orleans is 21 ft. Barry is expected to crest at a minimum of 19 ft.