Guide
Small craft advisories, Mobile Bay and the Gulf
Checked September 29, 2026 against the National Weather Service office in Mobile 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 may be the Gulf, Mobile Bay or Mississippi Sound, and each is forecast separately. This page explains which zone each town reads, how a bay forecast differs from a Gulf one, and what the advisories and warnings mean in knots and feet.
One office, four zones
The weather service office in Mobile writes the coastal waters forecast for this coast, twice a day. Mobile Bay is split into a northern and a southern zone, the Gulf is a strip out 20 nautical miles from shore with a second zone from 20 to 60 beyond it, and the Sound behind Dauphin Island is a zone of its own. The names below are the weather service’s own.
| Zone | Name | Office | Towns on this board |
|---|---|---|---|
| GMZ650 | Coastal waters from Pensacola FL to Pascagoula MS out 20 NM | Mobile | Dauphin Island, Fort Morgan, Gulf Shores, Orange Beach |
| GMZ632 | Mississippi Sound | Mobile | Bayou La Batre, Coden |
| GMZ630 | North Mobile Bay | Mobile | Mobile, Daphne, Fairhope, Point Clear |
| GMZ631 | South Mobile Bay | Mobile | Magnolia Springs, Bon Secour |
Beyond the 20-mile line is GMZ670, the waters from Pensacola FL to Pascagoula MS from 20 to 60 NM. Perdido Bay, behind Orange Beach, is GMZ633; no town on this board reads it.
Neighbours on different water
A few miles can put two towns in different forecasts. Dauphin Island reads the Gulf; Bayou La Batre and Coden, across the water behind it, read the Sound. Fort Morgan reads the Gulf; Bon Secour, on the bay side of the same peninsula, reads South Mobile Bay. Fairhope and Point Clear read North Mobile Bay; Magnolia Springs, a few miles south, reads South Mobile Bay. If you are going out through the mouth of the bay or Perdido Pass, read both: the one you leave from and the one you are heading into.
A 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 bay and Sound zones give the wind, the waves in feet, and then the state of the water in words (“Waves 1 foot or less. Light chop.”). On a bay the wind line is the one to read first. When two zones are expected to see the same weather, the office writes one paragraph headed with both names — in the forecast read for this page South Mobile Bay and Mississippi Sound shared one — and when they differ, each gets its own.
What the words mean, in knots
These are the National Weather Service’s thresholds. The advisory threshold depends on the region; 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. 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.
- 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
The office prints the same note at the top of every forecast: “Seas are given as significant wave height...which is the average height of the highest 1/3 of the waves. Individual waves may be more than twice the significant wave height.” So “seas 4 to 6 feet” is not the size of every wave; most are smaller, and some are bigger.
The weather service’s caution: wave forecasts are an average, and some individual waves “can be expected to be as much as two times greater.”
Read a forecast of six-foot seas as one that can include twelve-foot waves.
Sources
- National Weather Service Mobile, marine zone map — the zone numbers and names
- Mobile coastal waters forecast — the bay and Sound wording, Wave Detail and the note on seas
- 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