171 AXNT20 KNHC 211339 AAA TWDAT Tropical Weather Discussion...Updated NWS National Hurricane Center Miami FL 939 AM EDT Fri Sep 21 2018 Updated Caribbean Sea section Tropical Weather Discussion for North America, Central America Gulf of Mexico, Caribbean Sea, northern sections of South America, and Atlantic Ocean to the African coast from the Equator to 32N. The following information is based on satellite imagery, weather observations, radar and meteorological analysis. Based on 0600 UTC surface analysis and satellite imagery through 1300 UTC. ...TROPICAL WAVES... A tropical wave is likely to be analyzed today near the coast of Africa based on model guidance and satellite imagery. Presently, satellite imagery shows a broad canopy of multilayer clouds with scattered moderate to strong convection ahead of the wave from 04N to 13N between 16W and 21W. The imagery also shows pretty good upper-level outflow with this activity denoted by the cirrus clouds fanning outward from the convection. A far eastern Atlantic Ocean has its axis along 26W/27W from 03N to 20N, moving westward at 10 to 15 kt. Only isolated showers and weak thunderstorms are within 60 nm either side of the wave axis from 04N to 08N. A central Atlantic tropical wave has its axis along 51W from 05N to 19N, moving westward at 10 to 15 kt. This feature was analyzed as a low 24 hours, but during the past 12 hours weakened to wave. The first visible images show a clearly exposed low-level center moving westward at 13N52W. The imagery shows scattered moderate to isolated strong convection within 60 nm either side of the wave from 11N to 14N. Scattered moderate convection is within 30 nm of 12N48W and within 30 nm of 11N49W. Isolated showers are within 30 nm of the exposed low. Dry mid- level air and strong upper-level winds are expected to hinder any development of this disturbance while it moves toward the WNW. A central Caribbean Sea tropical wave has its axis along 74W south of 20N, moving westward at 10 to 15 kt. Scattered showers and isolated thunderstorms are seen within 240 nm east of the wave axis north of 15N to across the Dominican Republic. Isolated showers and thunderstorms are north of 15N and west of the wave to 75W. Satellite imagery is depicting Saharan dust to the east of the wave over much of the eastern Caribbean, and east from there to over the tropical Atlantic waters. Surface observations from Puerto Rico and the U.S. Virgin Islands indicate visibilities of 6 sm in haze. ...MONSOON TROUGH/ITCZ... The monsoon trough axis extends from the coastal border sections of Guinea and Sierra Leone near new to 05N20W. The ITCZ continues from 05N20W to 06N28W, to 05N38W, and 08N44W. Other than convection associated with the wave along the coast of Africa, and with the one along 26W/27W, scattered moderate convection is seen within 210 nm north of the ITCZ between 40W and 45W. ...DISCUSSION... GULF OF MEXICO... An upper-level cyclonic circulation center is moving westward over S Florida near 26N81W. The low extends a trough southward to across central Cuba and to the western Caribbean Sea. Associated upper-level cyclonic wind flow covers Florida, the Gulf of Mexico, and the Atlantic Ocean from the Florida Straits to 32N between 75W and 85W. Scattered showers and thunderstorms are over the eastern gulf waters from 24N to 27N east of 85W. Isolated showers and thunderstorms are elsewhere over the basin. High pressure building in the eastern U.S.A. will support gentle to moderate E to SE winds, and sea heights ranging from 1 foot to 4 feet, across the basin through Sun. It is possible that typical nocturnal surface trough, that will be moving westward across the southwestern corner of the Gulf of Mexico, may produce locally fresh winds through Sat CARIBBEAN SEA...Updated Broad upper-level cyclonic flow is present over the western section of the sea north of 15N and west of 78W in association with the upper trough that extends from the upper low over S Florida to a base near 16N82W. Scattered showers and thunderstorms from 14N to 19N west of 75W are increasing in coverage as the upper trough acting on a very unstable atmosphere is resulting in ample instability there. A patch of low-level moisture advecting westward over the northeastern Caribbean is accompanied by scattered showers. These showers should continue over the section of the Caribbean into most of this afternoon. The eastern Pacific monsoon trough axis extends across Costa Rica to along 09N and to northwestern Colombia near 75W. This along with low-level wind speed convergence is leading to scattered moderate to strong convection over the far western portion of the southwestern Caribbean south of 14N and west of 80W. Isolated showers and thunderstorms are elsewhere west of 70W. The central Caribbean tropical wave will move westward into Central America through Sun. The pressure gradient between central Atlantic high pressure and the lower pressure near the wave will support moderate to fresh trades today. Trades will diminish over much of the basin this weekend. ATLANTIC OCEAN... An upper-level cyclonic circulation center is near 28N80W, moving from the Atlantic Ocean toward Florida. Upper level cyclonic wind flow covers Florida, the Gulf of Mexico, and the Atlantic Ocean from the Florida Straits to 32N between 75W and 85W. Isolated moderate rainshowers are in the area of upper level cyclonic wind flow. Other isolated moderate rainshowers are in the Atlantic Ocean from 25N to 30N between 70W and 75W. A 1008 mb low is just north of the area at 33N63W, with a rather weak cold front extending to 28N70W and to 29N80W. The low has significantly weaken since yesterday. The combination of dry air and strong upper-level winds is expected to inhibit any significant development of this low for the next several days while it moves southward today, and then drifts westward to the southwest of Bermuda Sat through Sun. Scattered showers and isolated thunderstorms are seen within 90 nm south of the front west of 77W. A surface trough extends from near 32N34W to a 1017 mb low, remnants of former tropical cyclone Joyce, at 30.5N35W and to near 24N36W. Isolated showers and thunderstorms are within 30 nm of the low. An upper-level trough extends from an upper-level low at 31N24W to 29N30W, where it becomes a cyclonic shear axis to another upper-level low near 29N41W. A trough extends from this low to 22N45W to 17N52W and to a base at 10N58W. Scattered moderate convection is within 30 nm either side of a line from 28N42W to 29N39W. Isolated showers are seen from near 20N to 28N between 40W and 50W. The aforementioned cold front is forecast to drift slightly southward today, then dissipate by early on Sat. The 1017 mb ow will meander across in a clockwise motion through early next week. NE swell is forecast to affect the waters E of the Bahamas through early next week. For additional information please visit http://www.hurricanes.gov/marine $$ Aguirre