865 AXNT20 KNHC 170956 AAA TWDAT Tropical Weather Discussion...Updated NWS National Hurricane Center Miami FL 556 AM EDT Mon Sep 17 2018 Updated Special Features section for latest on Tropical Depression Joyce 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 0000 UTC surface analysis and satellite imagery through 0930 UTC. ...SPECIAL FEATURES...Updated Tropical Depression Joyce is centered near 34.2N 329.0W at 17/0900 UTC or 250 nm SW of the Azores moving E at 15 kt. Estimated minimum central pressure is 1008 mb. Maximum sustained wind speed is 30 kt with gusts to 40 kt. Latest satellite imagery shows that Joyce is a sheared system under strong west to northwest vertical shear as noted by its deep convection being sheared off to the northeast of its center. The convection is defined as the moderate type intensity within 30 nm either side of a line from 35N25W to 35N29W. Overcast to broken low and mid-level clouds with embedded scattered showers and isolated thunderstorms are well to the east and northeast of Joyce from 34N to 38N between 20W and 25W. Strong southwest to west winds aloft and dry air in its surrounding environment will continue to hamper Joyce over the next few days causing it to gradually weaken. Joyce is forecast by the latest NHC advisory to slow its forward motion while turning toward the southeast today, south on Tue, and southwest on Wed. See the latest NHC forecast/advisory under AWIPS/WMO headers MIATCPAT5/WTNT35 KNHC for more details. ...TROPICAL WAVES... A central Atlantic tropical wave has its axis extending along 43W from 04N and 20N, moving westward at 15-20 kt. Model guidance indicates that the wave is moving through a pretty well pronounced 700 mb jet stream branch accompanied by a jet core of 30-45 kt east winds to the east of the wave axis. The wave marks the leading edge of a quite an extensive area of Saharan Air (SAL) for this time during the season. The African dust extends from 10N to 25N east of the wave to Africa. Earlier dust that surrounded the wave has thinned out since yesterday as now a surge of low-level moisture is surging westward on either side of the wave south of 15N to the region of the ITCZ. This moisture surge consists of scattered showers and thunderstorms moving quickly westward from 10N to 15N between 38W and 44W, and within 120 nm either side of the wave in the vicinity of the ITCZ region from 04N to 09N. ...MONSOON TROUGH/ITCZ... The monsoon trough axis extends from well inland Africa southwestward to the coastline of northwest Senegal near 17N16W and continues to 09N22W. The ITCZ extends from 09N22W to 06N30W to just west of the tropical wave along 43W. It resumes at 08N43W to 08N58W. Aside from the shower and thunderstorms activity associated with the tropical wave described above, scattered moderate isolated strong convection is seen within 210 nm north of the ITCZ between 53W and 58W, within 180 nm south of the ITCZ between 45W and 49W, and within 60 nm south of the ITCZ between 51W and 53W. Similar convection is northwest of the ITCZ within 60 nm of a line from 09N58W to the coast of S America at 08N60W. Scattered moderate convection is within 150 nm north of the ITCZ between 48W and 51W. ...DISCUSSION... GULF OF MEXICO... In the upper-levels, and elongated upper low is over the western gulf near 25N93W, with a trough extending northeastward to inland southeastern Louisiana and continues well northeastward of the area. Another trough extends from the low southeastward to the eastern Bay of Campeche and over the southwest section of the Yucatan Peninsula. Upper-level cyclonic flow covers the basin west of 86W. Anticyclonic flow aloft associated with an upper- level anticyclone over the northwestern Caribbean near 20N84W is over the gulf east of 86W. At the surface, a trough is just inland the coast of Mexico from 20N to near San Fernando. Plenty of atmospheric instability along with deep moisture in place is leading to scattered showers and isolated thunderstorms south of 25N and west of 94W to inland Mexico, where the activity increases to scattered in coverage. At the surface, high pressure of 1015 mb is centered near 27N93W. Relatively weak high pressure is across the basin. A surface trough extends southwestward from what is now Tropical Depression Florence located over the border of eastern Georgia, southwestward to over the Florida Panhandle. Scatterometer data from last night highlighted light to gentle anticyclonic flow west of 87W, and light to moderate south to southwest flow east of 87W. Little change in these winds is expected through the middle part of the upcoming week. CARIBBEAN SEA... A surface trough extends across the central Caribbean from just west of Haiti to a 1009 mb low near 17.5N76W, and southeastward to near 13N74W. These features and related convection denote a broad area of low pressure associated with former tropical cyclone Isaac. Abundant moisture and atmospheric instability, that is Father enhanced by the tail-end of a central Atlantic deep layer trough that extends to Haiti and to near 17N85W, remains entrenched across the central Caribbean and much of the eastern Caribbean. This is resulting in scattered moderate to strong convection from 16N to 18N between 75W and 76W, including the eastern portion of Jamaica. Scattered showers and isolated thunderstorms are elsewhere between 65W and 80W, while isolated showers and thunderstorms are east of 65W. The broad area low pressure remains disorganized. Only slow development will be possible during the next day or so due to proximity to dry air and land interaction with Jamaica. By Wed, environmental conditions are forecast to become less conducive for re-development to occur. Regardless of development, locally heavy rains and gusty winds will be possible over portions of Hispaniola, Jamaica, and eastern Cuba during the next couple of days while the system moves west-northwestward to westward toward the northwestern Caribbean Sea. Gentle trades are west of the trough and low, while gentle to moderate trades are elsewhere east of the trough. The central Atlantic tropical wave will move across the tropical waters east of the Windward Islands Tue and Tue night, and through the east Caribbean on Wed and Thu accompanied by fresh to strong trades and building seas. ATLANTIC OCEAN... Please refer to the Special Features section above, for details on Tropical Depression Joyce. A rather strong and broad deep-layer trough extends across the central Atlantic from a large upper-level low (TUTT), that is observed to be near 27N65W, southwestward to Haiti. Plenty of instability exists with this trough as it acts on a very moist atmosphere. The upper-level low is forecast to gradually move westward today, then in a northward direction tonight through Tue. At the surface, a nearly stationary 1013 mb low is near 24N65W, with a trough extending northeastward to near 30N59W, and southwestward to near 20N70W. To the east of this system, another surface trough extends from 29N50W to near 24N61W. All of these features acting on the present very moist and unstable atmosphere are leading to the development of numerous showers and thunderstorms over the waters from 19N to 26N between 59W and 65W, and from 18N to 23N between 65W and 74W, including the Windward Passage and vicinity waters. Scattered showers and thunderstorms are also evident from 18N to 23N between 54W and 59W, and from 26N to 32N between 50W and 56W. Any of this activity is capable of producing very heavy rainfall as well as frequent lightning strikes. Drier air is present to the west of the upper-level trough and TUTT low, where only isolated showers and thunderstorms are noted, except in the far northwest corner of the discussion area where scattered showers and thunderstorms moving quickly to the northeast are occurring due to a very moist and unstable southerly flow in combination with surface convergence along a surface trough that is over southeastern Georgia to the north/central Florida, and that is present to the south of Tropical Depression Florence located over western South Carolina. A 1023 mb high center is near 28N39W, with a ridge extending southwestward to near 20N60W. Rather dry and stable conditions are present east of 50W, except near the tropical wave along 43W as described above. Also as stated above, a rather extensive batch of Saharan Air (SAL) is observed over the eastern Atlantic from 10N to 25N and east to Africa. These conditions are contributing to the already dry and stable conditions supported by the high pressure that in place over that part of the Atlantic. The dust will continue to migrate westward during the next few days. For additional information please visit http://www.hurricanes.gov/marine $$ Aguirre