000 AXNT20 KNHC 282342 TWDAT Tropical Weather Discussion NWS National Hurricane Center Miami FL 742 PM EDT Fri Jul 28 2017 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 1800 UTC surface analysis and satellite imagery through 2315 UTC. ...TROPICAL WAVES... A central Atlantic tropical wave extends from 17N38W to a 1012 mb surface low centered near 11N38W to 04N37W, moving westward at 10 kt. Both satellite imagery and the total precipitable water (TPW) imagery animation reveal that significant moisture was noted in a narrow swath within 120-180 nm either side of the wave axis. A large area of Saharan Air lies to the east of the wave axis. Scattered low topped showers are noted within 120 nm of the low circulation. A west-central Atlantic tropical wave extends from 20N50W to 04N51W, moving west at 10 to 15 kt. This wave is moving through a very stable atmospheric environment marked by the presence of a very pronounced African Saharan Air Layer (SAL) that covers the eastern and central Atlantic north of the tropics. Latest water vapor imagery confirms that strong subsidence and resultant dry air prevails over the the northern and central portions of this wave. Isolated showers and thunderstorms were noted east of the wave's axis S of 10N between 40W-48W. An eastern Caribbean tropical wave along 67W, and extends northward to the Atlantic near 21N. Water vapor imagery depicts dry subsident air over the wave. No convection is occurring with this wave at this time. This wave will move across the remainder of the eastern Caribbean through tonight, then through the central Caribbean on Saturday. A western Caribbean tropical wave has is along 81W south of 20N. The wave is moving west at about 15 kt. The southern portion of the wave is aiding scattered moderate to strong convection over Central America and the adjacent Caribbean west of 81W. The wave will move across the rest of the western Caribbean through tonight and inland over central America on Saturday. ...ITCZ/MONSOON TROUGH... The monsoon trough extends from 14N16W to 12N20W to the 1012 mb low at 11N38W to 09N43W. A segment of the ITCZ extends from 09N43W to 09N51W. Aside from convection related to the tropical wave described above, scattered moderate isolated strong convection is within 240 nm to the south of the monsoon trough from the African coast to 25W. ...DISCUSSION... GULF OF MEXICO... A surface ridge prevails across the basin anchored by a 1017 mb high centered near 27N93W. A broad weak surface trough extended from Pensacola, Florida to near 26N88W. Water vapor imagery and satellite derived winds indicated anticyclonic flow covered most of the Gulf anchored to an anticyclone over eastern Oklahoma. Deep layered subsidence accompanied the flow over the Gulf west of 90W while upper level moisture was sweeping across areas east of 90W. An upper-level low centered over western Cuba was enhancing convection over the island while upper level diffluent flow between the two features coupled with the afternoon seabreeze was enhancing convection over east-central Florida. Scatterometer and surface observational data depicted light to gentle winds across the basin with seas generally 1 to 2 FT. A fairly deep mid to upper level trough for mid-summer will push a cold front southward to the deep south by late Sat with an increase in showers and thunderstorms along the Gulf coast. The front itself will enter the northern Gulf on Sun with winds shifting to the north. Increasing SW to W winds will develop south of the front over the Gulf to 25N. CARIBBEAN SEA... Two tropical waves are moving across the basin. Refer to the section above for details on these features. Isolated showers are observed across the western Caribbean on the southern periphery of an upper-level low centered over central Cuba, as seen in water vapor imagery. Scattered to numerous moderate and strong convection is noted over the southwest Caribbean south of 12N between 80W-83W. This activity is occurring along the eastern extension of the Pacific monsoon trough and the tropical wave along 81W. This activity is being further aided by upper diffluent flow between the southern extent of an upper trough located over the NW Caribbean and an anticyclone over Panama. Other areas of orographically forced convection were noted over Cuba, Hispaniola and Puerto Rico. The activity appears to have peaked with weakening trends in place. Scatterometer data indicated fresh to strong trades over the south- central Caribbean with generally light winds over the western portion of the basin. Little change in the overall conditions is expected through the weekend. ...HISPANIOLA... The 1200 UTC rawindsonde from Santo Domingo showed limited moisture with generally light winds through the entire column up to 200 mb. The profile suggests any thunderstorm activity that develops would be primarily orographically forced and quasi- stationary. Currently clusters of quasi-stationary strong thunderstorms over the interior of Haiti and the Dominican Republic are in the process of weakening. Moisture should increase slightly Saturday as the present eastern Caribbean tropical wave passes through the central Caribbean. Some pockets of dry air behind the wave will follow in behind the wave. ATLANTIC OCEAN... Two tropical waves are moving across the basin. Refer to the section above for details. The upper-level low centered over western Cuba extends northeast enhancing convection across the southern Bahamas and Florida Straits. This activity is expected to lift northward on Saturday in response to a frontal boundary that will push off the southeastern United States and move to the northwest portion of the basin late Saturday through Sunday. To the east, another upper- level low near 27N58W is reflected at the surface as a trough that extends from 31N56W to 25N57W. Widely scattered showers are observed along this trough. The upper low is sliding eastward at 10 kt. The remainder of the basin is under the influence of a surface ridge anchored by a 1024 mb high centered near 30N38W. For additional information please visit http://www.hurricanes.gov/marine $$ Cobb