555 AXNT20 KNHC 212207 AAA TWDAT Tropical Weather Discussion...Updated NWS National Hurricane Center Miami FL 607 PM EDT Fri Sep 21 2018 Updated to add a Special Feature 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 1200 UTC surface analysis and satellite imagery through 2100 UTC. ...Special Features... A central Atlantic tropical wave has its axis along 52W between 07N-17N moving W around 10 kt. The wave runs through a 1009 mb low near 12N52W, or about 435 nm east of the Windward Islands. Visible imagery shows an exposed low-level center due to moderate to strong westerly wind shear. Satellite imagery shows that scattered moderate convection has increased within 90 nm of the low in the NE quadrants. Scattered moderate convection is also seen from 12N to 14N between 48W and 50.5W. This system has a medium chance for developing into a tropical cyclone through Sat. Strong upper-level winds and dry air are likely to prevent additional development of this system by late this weekend while it moves WNW at 8 kt. Please refer to the latest Tropical Weather Outlook under AWIPS/WMO headers MIATWOAT/ABNT20 KNHC for more information. ...TROPICAL WAVES... Showers and thunderstorms about 520 nm SSE of the Cabo Verde Islands are showing some signs of organization. This activity is associated with a tropical wave, analyzed along 17W from 03N-16N. An ASCAT pass from around 10Z this morning shows a partially closed low-level circulation near 07N18W with winds of 25-30 kt in the eastern semicircle. Visible and IR satellite images depict a coherent cluster of convection with a banding pattern from 04N- 10N, between 16W-23W, where numerous moderate and isolated strong convection is noted. The environment is forecast to be conducive for slow development, and a tropical depression could form early next week while the system moves west around 15 kt across the low latitudes of the eastern and central tropical Atlantic Ocean. The system has a low chance of tropical cyclone formation within the next 48 hours. The tropical wave that was previously along 26W/27W has been relocated to 36W from 04N-16N and is moving west at 10-15 kt. The wave corresponds nicely with a maximum in TPW imagery. Model analyses depict an associated 700 mb trough along 39W, or just ahead of the wave axis. Isolated showers are noted within 120 nm of the wave axis from 09N-13N. Little change is expected with this wave over the next 24-48 hours. A central Caribbean Sea tropical wave has been relocated to 78W/79W based on surface observations. This location is a little to the west of the analysis from 6 hours ago, which had it along 74W. Movement is estimated to be westward around 10-15 kt. Scattered showers are noted south of Jamaica from 16N-18N between 76W-80W. A maximum in TPW is noted just east of the wave axis. ...MONSOON TROUGH/ITCZ... The monsoon trough axis extends from the coast of Guinea near 10N14W to 07N18W to 07N27W. The ITCZ continues from 07N27W to 08N40W to 10N56W to NE Venezuela. Other than convection associated with the tropical waves, isolated weak to moderate convection is located within 60 nm south and 150 nm north of the ITCZ between 40W-47W. Scattered moderate convection is also noted over NE Venezuela as well as Trinidad and Tobago. ...DISCUSSION... GULF OF MEXICO... An upper-level cyclonic circulation is moving slowly westward over the far eastern Gulf and the west coast of Florida. Satellite imagery and NWS Doppler Radar indicate scattered showers and thunderstorms over the eastern Gulf, mainly from 24N-29N, between 81W-87W, including portions of the Florida Keys and the Florida Peninsula. A ridge extends from a high pressure over the eastern U.S. to the northern Gulf of Mexico. The ridge will support gentle to moderate easterly winds and seas 3 ft or less across most of the Gulf of Mexico through the middle of next week. It is possible that a 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... A tropical wave is moving across the Central Caribbean Sea. Please see the Tropical Waves section for more details. The eastern Pacific monsoon trough combined with diffluence aloft is helping to induce scattered showers and thunderstorms over portions of Costa Rica, Nicaragua and eastern Honduras. Plenty of moisture will persist over the southwest Caribbean through Saturday as the tropical wave, currently along 78W/79W, reaches the area and moves inland into central America late Sat or early Sun. The pressure gradient between central Atlantic high pressure and the lower pressure near the central Caribbean tropical wave will support fresh trades today. Winds will diminish over most of the Caribbean Sea this weekend. ATLANTIC OCEAN... Currently, there are three tropical waves between the west coast of Africa and the Lesser Antilles. Please see the Tropical Waves section for more details. A cold front extends from a 1010 mb low just SE of Bermuda and enters the forecast area near 31N64W and extends westward to 28N70W to the east coast of FL near Cape Canaveral. Isolated showers are noted along the frontal boundary. The front will drift slightly southward today, then dissipate by Saturday morning. The low is currently producing moderate to fresh winds north of 28N between 60W-65W. The low is expected to drift south and then west over the forecast area between 60W-70W while weakening. The remainder of the Atlantic forecast area is under the influence of a ridge, anchored by a 1022 mb surface high located W of the Madeiras Islands near 34N23.5W. Looking ahead, a non-tropical low is forecast to develop tonight or on Saturday to the north of the forecast area about midway between Bermuda and the Azores. Conditions are expected to be conducive for the low to acquire subtropical or tropical characteristics, and a subtropical or tropical cyclone is expected to form late this weekend or early next week while the low meanders over the central Atlantic Ocean. There is a low chance of development within the next 48 hours but a high chance of development within 5 days. For additional information please visit http://www.hurricanes.gov/marine $$ Hagen/GR/Aguirre