000 AGXX40 KNHC 270817 MIMATS Marine Weather Discussion NWS National Hurricane Center Miami FL 417 AM 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 through early this morning. 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 over portions 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 portions of the western Gulf on Thursday 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 latest guidance from the global models reveals this scenario. 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 will then expand westward to S of southeastern Louisiana on Sunday. The southern extent of these winds should reach to near 27N. Seas resulting from these winds should be build to around 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 gradually increase 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 increases across much of the eastern and central Caribbean since yesterday as a central Atlantic ridge builds southwestward toward the eastern Caribbean Sea in the wake of a tropical wave that is currently along 75W. The 0134Z Ascat pass revealed a patch of fresh to strong E winds is S of 15N between 68W and 73W. 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 over the NW portion of the Caribbean. Seas over the tropical N Atlantic zones have build 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 0814Z Ascat pass suggested that E-SE winds of 20-25 kt are occurring over the Gulf of Honduras. In addition, 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 over the far western Caribbean Sea near 84W will move slowly westward toward the Yucatan Peninsula through early Thursday. The combination of this trough with pronounced upper level diffluence has lead to numerous showers and thunderstorms over 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 an upper level southerly flow pattern found to the E of an upper level trough presently located over the eastern Gulf of Mexico. This activity is accompanied by gusty winds frequent lightning at times. ...SW N ATLANTIC INCLUDING THE BAHAMAS... Ascat data from late last night highlighted SW to W winds in the range of 20-30 kt N of 30N between 71W and 76W. These winds are along the outer perimeter of stronger winds occurring just N of the forecast waters that are associated with Tropical Storm Maria that is now centered well N of the area. Seas are in the 10-14 ft with these winds. Swell produce 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 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 in the wake of Maria will extend from the N-central forecast waters SW to near the Central Bahamas through Thursday night, then become diffuse Thursday 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 model signals suggest that the front will most likely wash out over or very near the far NW waters on Saturday as continental high pressure builds southward and strengthens across it. This will lead to increasing NE winds over 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 over portions of the NW waters. Will await for future guidance to decide if a gale warning needs to be issued for some portions of the NW waters for the upcoming weekend time frame. Expect seas to build to large values over 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 Aguirre. National Hurricane Center.