000 AGXX40 KNHC 271810 MIMATS Marine Weather Discussion NWS National Hurricane Center Miami FL 210 PM EDT Wed Sep 27 2017 Marine Weather Discussion for the Gulf of Mexico, Caribbean Sea, and Tropical North Atlantic from 07N to 19N between 55W and 64W and the Southwest North Atlantic including the Bahamas. ...GULF OF MEXICO... A thermal trough will develop each evening in the NW Yucatan Peninsula, move W-NW across the SW Gulf during the overnight hours and dissipate in the SW Gulf waters by late each morning. Expect the trough to be accompanied by a surge of moderate to locally fresh winds that shift from NE to SE along with possible brief seas to 4 ft. Weak high pressure will continue to support mainly gentle E-SE winds and 1-3 ft seas through tonight. The flow increases slightly on Thursday in parts of the western Gulf where mainly moderate NE-E winds are expected along with seas of 3-4 ft. Gentle to moderate NE-E winds are expected elsewhere on Thursday along with seas of 2-3 ft. The first cold front of the autumn season is depicted by model guidance to move across the northern and western Gulf waters on Thursday, reach a position from northern Florida to the central and SW Gulf waters on Friday, then stall late Friday through Saturday night. Guidance indicates that this front will be rather shallow, thus on the weak side. The moderate NE-E winds are expected across parts of the western Gulf on Thursday, and then will begin to expand N and S Friday through Saturday night as a result of the tightening of the gradient between the front and strengthening high pressure that builds southward behind it. The resulting tight pressure gradient attributed to the strengthening of the high pressure is forecast to induce NE-E winds of fresh to strong intensity over the NE Gulf beginning Saturday night. These winds then will expand westward to S of southeastern Louisiana on Sunday. The southern extent of these winds should reach near 27N. Seas resulting from these winds should build to 9 ft by Sunday night. Elsewhere N of the front winds will be mainly in the gentle range from the NE-E in direction through Thursday evening, with seas of 2-3 ft. These winds will increase gradually to the moderate range through Sunday night, with seas building to 3-4 ft. NW-N winds of about 20-25 kt are expected within about 60 nm of the coast of Mexico from 19N-20N from Saturday afternoon through Sunday night. Winds S and SE of the front are expected to be gentle to moderate SE-S in direction with seas of 2-3 ft. Scattered showers and isolated thunderstorms are possible with the front. ...CARIBBEAN SEA AND TROPICAL N ATLANTIC FROM 07N TO 19N BETWEEN 55W AND 64W... Trades have increased across much of the eastern and central Caribbean Sea since yesterday, as a central Atlantic Ocean ridge builds southwestward toward the eastern Caribbean Sea in the wake of a tropical wave that is currently along 76W/77W. The latest Ascat pass shows fresh to strong E winds S of 15N between 68W and 74W. Seas of 6-8 ft accompany these winds. Seas elsewhere are in the 4-6 ft range, except for slightly lower seas of 3-5 ft in the NW part of the Caribbean Sea. Seas in the tropical N Atlantic zones have built to the range of 6-8 ft due to a long period NE swell propagating through those waters. The fresh to strong trades will diminish to mainly fresh trades this afternoon, with seas lowering to just under 8 ft. The latest Ascat pass showed that E-SE winds of 20-25 kt are in the Gulf of Honduras. Ship with ID "C6BR3" indicated winds near this magnitude. These winds are expected to pulse again tonight before diminishing to mainly the moderate to locally fresh range through the remainder of the forecast period. Seas of 6-7 ft with these pulsing winds will subside to 3-5 ft this afternoon, and then build again to around 7 ft tonight. These seas are expected to remain within the 3-5 ft range during the rest of period. A trough that is in the eastern Yucatan Peninsula along 88W will move slowly westward through early Thursday. The combination of this trough with pronounced upper level diffluence has lead to numerous showers and thunderstorms in much of the northwest Caribbean Sea and as well as over portions of the N-central Caribbean. This activity will linger through the next few days. Some of the related moisture will translate north- northeastward under upper level southerly windflow, that is found to the E of an upper level trough that presently is located in the eastern Gulf of Mexico. This activity is accompanied by gusty winds frequent lightning at times. ...SW N ATLANTIC INCLUDING THE BAHAMAS... The latest Ascat data highlighted SW to W winds in the range of 20-30 kt N of 30N between 71W and 76W, to the south of the center of T.S. Maria, that now is well N of the area. Seas are in the 10-14 ft with these winds. Swell produced by Tropical Storm Maria will continue to propagate across the forecast waters NE of the Bahamas through most of the forecast period. Wave model guidance suggests that the swell will decay slowly through the end of the week, and while the bulk of the swell group shifts to the NE portion of the basin. The aforementioned 20-30 kt winds will shift N of the area later this morning. The 10-14 ft seas will subside to 7-10 ft by this evening, then shift to the far NE forecast waters by Friday and to NE of the forecast waters Friday night. Otherwise, large swells generated by Tropical Storm Maria are affecting are affecting Bermuda, the Turks and Caicos Islands, and the Bahamas. It is likely that these swells may cause life- threatening surf and rip current conditions. A surface trough that is in the wake of Maria will extend from the N central forecast waters SW to near the Central Bahamas through Thursday night, then dissipate Friday as central Atlantic high pressure builds westward across the eastern part of the area. Scattered showers and thunderstorms are possible with the trough. On Friday, the ridge is forecast to extend E to W near 26N as a weak cold front approaches the SE United States coast, and far NW waters. Latest models suggest that it is most likely that the front may wash out in the far NW waters on Saturday as continental high pressure builds southward and strengthens across it. This will lead to increasing NE winds in the NW waters on Saturday along with building seas. Guidance indicates that these winds will increase to strong intensity on Saturday and through Sunday, with the possibility of gusts to gale force in parts of the NW waters. Will await future guidance to decide if a gale warning needs to be issued for some parts of the NW waters for the upcoming weekend. Expect seas to build to large values in the NW waters with the expected increasing winds. $$ .WARNINGS...Any changes impacting coastal NWS offices will be coordinated through AWIPS II Collaboration Chat, or by telephone: .GULF OF MEXICO... None. .CARIBBEAN SEA AND TROPICAL N ATLANTIC FROM 07N TO 19N BETWEEN 55W AND 64W... None. .SW N ATLANTIC INCLUDING THE BAHAMAS... None. $$ *For detailed zone descriptions, please visit: http://www.nhc.noaa.gov/abouttafbprod.shtml#OWF Note: gridded marine forecasts are available in the National Digital Forecast Database (NDFD) at: http://www.nhc.noaa.gov/marine/grids.php For additional information, please visit: http://www.nhc.noaa.gov/marine $$ .Forecaster MT. National Hurricane Center.