Guide

Small craft advisories and the lake, bay and Gulf zones

Checked September 28, 2026 against the National Weather Service offices in Lake Charles and New Orleans/Baton Rouge, and the National Weather Service’s marine definitions.

The Marine tile repeats the National Weather Service’s coastal waters forecast for the water your town opens onto. On this coast that is often a lake or a bay, not the Gulf, and each is forecast separately. This page explains which zone each town reads, how a lake or bay forecast differs from a Gulf one, and what the advisories and warnings mean in knots and feet.

Two offices

Two weather service offices forecast these waters, and the Atchafalaya River is the line between them. Lake Charles writes the western coast, from the Lower Atchafalaya River to High Island: Calcasieu Lake, the Vermilion and Cote Blanche bays, Atchafalaya Bay and the Gulf off Cameron Parish and Vermilion Parish. New Orleans/Baton Rouge writes everything east of the Atchafalaya: Terrebonne and Barataria bays, the Gulf off Port Fourchon, Grand Isle and the mouth of the Mississippi River, Breton and Chandeleur sounds, and Lakes Pontchartrain, Maurepas and Borgne. That is why Morgan City and Berwick read Lake Charles’s forecast while Houma, next parish east, reads New Orleans/Baton Rouge’s. Each office issues a new coastal waters forecast at least twice a day.

The zones, and which town reads which

Every lake and bay system has its own zone, and the Gulf is split into strips out 20 nautical miles from shore, with a second zone from 20 to 60 beyond each. The names below are the weather service’s own, with the state letters left off. Barataria Bay appears twice because towns on two boards read it.

More near-shore zones have no town on these boards: GMZ430, Sabine Lake, on the western line; GMZ455, the Gulf from the Lower Atchafalaya River to Intracoastal City, off Marsh Island; GMZ551, the Gulf from Port Fourchon to the Lower Atchafalaya River; GMZ529, Lake Maurepas; GMZ533, Lake Salvador and Lake Cataouatche; GMZ535, Breton Sound; and GMZ536, Chandeleur Sound. The 20-to-60-mile zones are GMZ470, GMZ472 and GMZ475 off the western coast, from Lake Charles, and GMZ570, GMZ572, GMZ575 and GMZ577 off the eastern coast, from New Orleans/Baton Rouge.

Neighbours on different water

A few miles can put two towns in different forecasts. Cameron reads the Gulf; Hackberry, on the shore of Calcasieu Lake, reads the lake. Holly Beach and Cameron are both on the Gulf but either side of a zone line, so they read different strips. Pecan Island reads the Gulf; Intracoastal City reads Vermilion Bay. Grand Isle and Port Fourchon read the Gulf; Golden Meadow and Lafitte read Barataria Bay. Venice reads the Gulf off the mouth of the river; Buras and Port Sulphur, up the road, read Barataria Bay. New Orleans reads Lake Pontchartrain; Chalmette, next door, reads Lake Borgne. If you are going out through a pass, read both: the one you leave from and the one you are heading into.

A lake or bay forecast and a Gulf forecast read differently

The Gulf zones give the wind and the seas in feet, and often a Wave Detail line after them: each wave train by direction, height and period in seconds. A short period means close-set waves, steeper for their height.

The lake and bay zones are written two ways, one for each office. Lake Charles gives the wind and then the state of the water in words, not feet: smooth, a light chop, choppy, rough (“Lake waters light chop”, “Bay waters choppy”). New Orleans/Baton Rouge gives its lakes and bays a height in feet, like the Gulf: “Seas 1 foot or less” on Lake Pontchartrain, “Waves 1 foot or less” on Lake Borgne, and on Terrebonne Bay often a Wave Detail line as well. On a lake or a bay the wind line is the one to read first.

When two zones are expected to see the same weather, an office writes one paragraph headed with both zone names; when they differ, each gets its own. The lake or bay and the Gulf off the same town do not always match, which is why the board shows the town’s own water.

What the words mean, in knots

These are the National Weather Service’s thresholds. The advisory threshold depends on the region, and New Orleans/Baton Rouge publishes its own; the warning thresholds do not change.

• Small Craft Advisory. In the weather service’s Southern Region, which includes this coast: sustained winds of 20 to 33 knots, or forecast seas of 7 feet or more expected for more than two hours. New Orleans/Baton Rouge states its own criteria for its waters within 60 miles of the coast, “including Lake Pontchartrain and Lake Maurepas”: sustained winds or frequent gusts of 21 to 33 knots, or waves of 7 feet or higher, expected to begin within 36 hours. The service says the thresholds are “specific to geographic areas”, and “There is no precise definition of a small craft”: whether your boat and your experience are up to it is your call.
• Small Craft Exercise Caution. Both offices also use this phrase in their forecast text, sometimes as a headline over a zone (“Small Craft Exercise Caution in effect”) and sometimes in a sentence (“Small craft should exercise caution with elevated winds. Advisory criteria is not expected.”). It is not on the weather service’s national list of marine warnings and advisories, and neither that list nor the New Orleans/Baton Rouge criteria give it a threshold, so this page gives none either.
• Gale Warning. Sustained winds or frequent gusts of 34 to 47 knots, not directly tied to a tropical cyclone.
• Storm Warning. 48 to 63 knots, likewise.
• Special Marine Warning. A short warning, usually for up to two hours, for thunderstorm winds or gusts of 34 knots or more, hail of three-quarters of an inch or more, or waterspouts.
• Tropical Storm and Hurricane Warnings. Gale and storm warnings are for wind not tied to a tropical cyclone; for a tropical storm or hurricane these are the warnings. A tropical storm warning means sustained winds of 39 to 73 mph are expected within 36 hours. A hurricane warning is issued 36 hours before tropical-storm-force winds are expected, because preparing becomes difficult once they arrive.

The line most often misread

Both offices print the same note at the top of their forecasts: seas are given as a range of the average height of the highest third of the waves, with the occasional height of the highest tenth. So “seas 4 to 6 feet” is not the size of every wave; most are smaller, and some are bigger.

Read a forecast of six-foot seas as one that can include twelve-foot waves.

Sources

• National Weather Service marine zone maps: Lake Charles, New Orleans/Baton Rouge — the zone numbers and names
• Coastal waters forecasts: Lake Charles, New Orleans/Baton Rouge — each office’s area, the lake and bay wording, Wave Detail, the caution wording and the note on seas
• National Weather Service New Orleans/Baton Rouge, watch, warning and advisory criteria — the office’s own small craft advisory criteria
• National Weather Service, marine warning and advisory definitions — the Southern Region thresholds and the warnings
• National Weather Service, marine forecast FAQ — significant wave height and individual waves