Louisiana

Louisiana fronts the northern Gulf of Mexico from the Pearl River at the Mississippi line west to the Sabine at the Texas border, with the bird-foot delta of the Mississippi pushing out into deep water in between and the Atchafalaya building new land south of Morgan City. The tide signature here is one of the smallest and most diurnally-skewed in the lower 48 — mean range at the Grand Isle gauge sitting on the offshore barrier-island chain south of New Orleans is about 0.4 metres, with most of the lunar month producing a single high and a single low per day rather than the two-and-two semidiurnal pattern more common further east. The astronomical forcing is small because the Gulf basin is broad, shallow, and partially enclosed; what dominates day-to-day water levels is wind and pressure rather than the moon. A sustained south wind across the Gulf can pile water against the marsh edge and lift levels 30 cm or more above predicted; a north wind can drop them by the same. Hurricane season runs June through November and tropical-cyclone surge can stack two to five metres on top of the tiny astronomical signal — the Hurricane Katrina (2005) peak at Bay St Louis was about 8.5 metres, and Hurricane Ida (2021) drove similar surges across the lower delta. NOAA CO-OPS runs Grand Isle and a dense interior gauge network including Lake Pontchartrain, the Mississippi River outlets, and the working ports at Cocodrie and Calcasieu Pass. The harmonic predictions on this site assume normal weather; for surge during a tropical landfall, the National Hurricane Center is the authoritative source.