Guide
Small craft advisories and the bay and Gulf zones
Checked September 28, 2026 against the National Weather Service offices in Brownsville, Corpus Christi, Houston/Galveston and Lake Charles, 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 bay, not the Gulf, and the two are 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.
Four offices
Four weather service offices forecast these waters. Brownsville writes the lower coast, from Baffin Bay to the mouth of the Rio Grande: the Laguna Madre and the Gulf off South Padre Island and Port Mansfield. Corpus Christi writes the middle coast, from Baffin Bay to the Matagorda Ship Channel, with Corpus Christi Bay, the Aransas and Copano bays and San Antonio Bay. Houston/Galveston writes the upper coast, from the Matagorda Ship Channel to High Island, including Matagorda Bay and Galveston Bay. Lake Charles writes Sabine Lake and the Gulf east of High Island, which is why Port Arthur and Sabine Pass read a different office from Galveston. Each office issues a new coastal waters forecast at least twice a day.
The zones, and which town reads which
Every 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.
| Zone | Name | Office | Towns on these boards |
|---|---|---|---|
| GMZ150 | Coastal waters from Port Mansfield TX to the Rio Grande River out 20 NM | Brownsville | South Padre Island |
| GMZ130 | Laguna Madre From the Port Of Brownsville to the Arroyo Colorado | Brownsville | Port Isabel, Laguna Vista |
| GMZ132 | Laguna Madre From The Arroyo Colorado To 5 NM North Of Port Mansfield TX | Brownsville | Port Mansfield |
| GMZ231 | Baffin Bay and Upper Laguna Madre | Corpus Christi | Loyola Beach |
| GMZ250 | Coastal waters from Baffin Bay to Port Aransas out 20 NM | Corpus Christi | Port Aransas, Padre Island |
| GMZ232 | Corpus Christi and Nueces Bays | Corpus Christi | Corpus Christi, North Beach, Portland, Ingleside |
| GMZ236 | Copano, Aransas, and Redfish Bays | Corpus Christi | Aransas Pass, Rockport, Fulton, Bayside |
| GMZ237 | San Antonio, Mesquite, and Espiritu Santo Bays | Corpus Christi | Port O’Connor, Seadrift |
| GMZ330 | Matagorda Bay | Houston/Galveston | Port Lavaca, Magnolia Beach, Lolita, Palacios |
| GMZ350 | Coastal waters from Freeport to Matagorda Ship Channel TX out 20 NM | Houston/Galveston | Matagorda, Sargent, Freeport, Quintana |
| GMZ355 | Coastal waters from High Island to Freeport TX out 20 NM | Houston/Galveston | Surfside Beach, Galveston, Jamaica Beach, Crystal Beach, High Island |
| GMZ335 | Galveston Bay | Houston/Galveston | Texas City, San Leon, Kemah, Seabrook, La Porte, Baytown, Anahuac |
| GMZ450 | Coastal waters from Cameron LA to High Island TX out 20 NM | Lake Charles | Sabine Pass |
| GMZ430 | Sabine Lake | Lake Charles | Port Arthur |
Three more near-shore zones have no town on these boards: GMZ135, the Laguna Madre from 5 nm north of Port Mansfield to Baffin Bay; GMZ155, the Gulf from Baffin Bay to Port Mansfield; and GMZ255, the Gulf from Port Aransas to the Matagorda Ship Channel, off San José and Matagorda islands. The 20-to-60-mile zones are GMZ170 and GMZ175 off the lower coast, GMZ270 and GMZ275 off the middle coast, GMZ370 and GMZ375 off the upper coast, and GMZ470 off Sabine Pass.
Neighbours on different water
A few miles can put two towns in different forecasts. South Padre Island reads the Gulf; Port Isabel, across the causeway, reads the Laguna Madre. Port Aransas reads the Gulf; Aransas Pass, across Redfish Bay, reads the bays. Galveston reads the Gulf; Texas City reads Galveston Bay. Sabine Pass reads the Gulf; Port Arthur reads Sabine Lake. If you are going out through a 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 zones — the Laguna Madre, the bays and Sabine Lake — give the wind and then the state of the water in words, not feet: smooth, a light chop, a moderate chop, choppy, rough. The offices write it their own ways (“Bay and inland waters choppy”, “A moderate chop on the bay”, “Lake waters light chop”), but it is the same idea: how rough the wind has made the water. The wind line is the one to read first in a bay forecast.
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 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; the warning thresholds do not.
- 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.
- Small Craft Should Exercise Caution. These offices also use this phrase in their forecast text, often paired with the advisory (“Small Craft Should Exercise Caution or Small Craft Advisory”). It is not on the weather service’s national list of marine warnings and advisories, and that list gives it no 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
Each of these offices prints the same note at the top of its forecast: 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.
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 marine zone maps: Brownsville, Corpus Christi, Houston/Galveston, Lake Charles — the zone numbers and names
- Coastal waters forecasts: Brownsville, Corpus Christi, Houston/Galveston, Lake Charles — each office’s area, the bay 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