868 AXNT20 KNHC 150603 TWDAT Tropical Weather Discussion NWS National Hurricane Center Miami FL 203 AM EDT Sat Sep 15 2018 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 0545 UTC. ...SPECIAL FEATURES... Tropical Storm Florence is centered near 33.8N 79.1W at 15/0300 UTC, or about 10 nm NNE of Myrtle Beach South Carolina moving WSW at 4 kt. Estimated minimum central pressure has risen slightly to 980 mb. Maximum sustained winds are 55 kt with gusts to 65 kt. Latest satellite imagery shows that Florence is being affected by drier air into its S and SW quadrants as noted by the lack of deep convection there. Scattered moderate to isolated strong convection is present within 300 nm of the center in the NE quadrant and within 180 nm of the center in the NW quadrant. Scattered moderate convection within an outer rainband is within 30 nm of a line from 32N78W to 34N77W and to 34.5N77.5W. Latest NHC advisory has Florence to continue with its slow motion through early today, then is forecast to turn westward and then northward through the Carolinas and to the Ohio Valley by Mon. Gradual weakening is forecast while Florence moves farther inland during the next couple of days, and it is likely to weaken to a tropical depression by tonight. The PUBLIC ADVISORIES for Florence are transmitted via the WMO header WTNT31 KNHC, and via the AWIPS header MIATCPAT1. The FORECAST/ADVISORIES for Florence are transmitted via the WMO header WTNT21 KNHC, and via the AWIPS header MIATCMAT1. Tropical Storm Helene is centered near 35.8N 35.8W at 15/0300 UTC, which is about 450 nm WSW of Lajes Air Base In The Azores moving NNE at 16 kt. Estimated minimum central pressure is 988 mb. Maximum sustained winds are 60 kt with gusts to 75 kt. Latest satellite imagery shows scattered moderate convection from 35N to 39N between 31W and 39W. Helene is under strong vertical as indicated by its center being located just to the south of its related deep convection. This convection consists of the scattered moderate isolated strong type from 36N to 39N between 34W and 38W. Scattered moderate convection is seen from 38N to 42N between 32W and 38W. Helene is forecast by the latest NHC advisory to turn toward the north-northeast expected by tonight or on Sun. On the forecast track, Helene will pass near or over the Azores late Sat or Sun. The PUBLIC ADVISORIES for Helene are transmitted via the WMO header WTNT33 KNHC, and via the AWIPS header MIATCPAT3. The FORECAST/ADVISORIES for Helene are transmitted via the WMO header WTNT23 KNHC, and via the AWIPS header MIATCMAT3. Tropical Storm Joyce is centered near 31.6N 43.6W at 15/0300 UTC, or about 900 nm WSW of the Azores moving E at 5 kt. Estimated minimum central pressure is 997 mb. Maximum sustained winds are 45 kt with gusts to 55 kt. Joyce is being hampered by srong southwesterly wind shear. Latest satellite imagery shows scattered moderate convection within 60 nm of the center, and to the northeast of the cyclone from 32N to 34N, and also within 30 nm of a line from 30N44W to 32N42W. Joyce is forecast by the latest NHC advisory to turn northeastward with an increase in forward speed today, with some slight strengthening possible. Gradual weakening is forecast to begin by Sun. The PUBLIC ADVISORIES for Joyce are transmitted via the WMO header WTNT3S KNHC, and via the AWIPS header MIATCPAT5. The FORECAST/ADVISORIES for Joyce are transmitted via the WMO header WTNT25 KNHC, and via the AWIPS header MIATCMAT5. Tropical Storm Isaac is centered near 15.4N 69.5W at 15/0300 UTC, or about 190 nm S of Santo Domingo Dominican Republic moving W at 13 kt. Estimated minimum central pressure is 1002 mb. Maximum sustained winds are 35 kt with gusts to 45 kt. Latest satellite imagery shows that the overall structure of the cyclone is rather ill-defined and not too symmetrical in nature. The imagery shows scattered moderate to strong convection to the southwest of the center within 30nm either side of a line 14N70W to 15.5N70.5W. Scattered moderate convection is to the east of Isaac from 17N to 17.5N between 64W and 67W. Isaac is forecast by the Latest NHC advisory to decrease its forward speed over the next couple of days with a turn to the west-northwest possible by the end of the weekend, if Isaac has not degenerated into a trough of low pressure by then. The PUBLIC ADVISORIES for Isaac are transmitted via the WMO header WTNT34 KNHC, and via the AWIPS header MIATCPAT4. The FORECAST/ADVISORIES for Isaac are transmitted via the WMO header WTNT24 KNHC, and via the AWIPS header MIATCMAT4. ...TROPICAL WAVES... A far eastern Atlantic tropical wave has its axis along 31W from 05N to 21N, moving westward at 10 to 15 kt. The wave is surrounding by a rather dry environment, which is inhibiting deep convection from forming along or near it. Only isolated showers and weak isolated thunderstorms are within 120 nm either side of the wave axis where it cross the monsoon trough. ...MONSOON TROUGH/ITCZ... The monsoon trough axis extends from off the coast of Guinea Bissau, Africa to 09N25W to 08N32W. The ITCZ continues from 08N32W to 09N41W to 10N46W to 12N57W. Aside from the shower and thunderstorm activity associated with the tropical wave along 30W, scattered moderate isolated strong convection is within 180 nm south of the ITCZ from 41W and 54W. Scattered moderate convection is within 180 nm south of the monsoon trough from 21W to 25W, and also well south of the monsoon trough from 04N to 09N between 15W and 18W. ...DISCUSSION... GULF OF MEXICO... An upper-level cyclonic circulation center is over the north- central gulf near 26N90W, with an inverted trough to an upper low over the northwestern Caribbean at 20N84W that is moving westward. Upper-level cyclonic wind flow covers the central part of the gulf. A surface trough extends from just inland central near Freeport southwestward to a 1009 mb low near Baffin Bay and to Ciudad Camargo and to northeastern Mexico near Ciudad Mante. Both satellite iamgey and NWS dopler radar imagery depict increasing scattered moderate to strong convection that is becoming more concentrated over and just inland the Texas coast from Brownsville to just south of Corpus Christi, and eastward from these two locations to near 95W. This activity is rotating cyclonically around the low as the troug/low system slowly moves further inland through today and this evening. The activity will likely produce heavy rainfall and possible strong gusty winds as swell. Scattered showers and thunderstorms are over northeastern Mexico. Scattered showers and isolated thunderstorms are elsewhere south of 28N and west of 92W. Isolated showers are possible east of 92W. A weak ridge extends northwest to southeast from SE Louisiana to Florida Bay will shift slightly south on Sun with the ridge extending southeast to the Yucatan Basin. A weak surface trough, extending southwest from the remnants of tropical cyclone Florence, will pass slowly east across the north-central gulf waters through early next week. CARIBBEAN SEA... Please refer to the Special Feature section above for details on Tropical Depression Isaac. An upper level cyclonic circulation center is about 100 nm to the west of Jamaica, in the NW corner of the Caribbean Sea. Isolated showers and thunderstorms are north of 15N and west of 80W. Tropical Depression Isaac near 15.4N 69.5W 1002 mb at 0300 UTC is moving W at 13 kt. Maximum sustained winds 35 kt gusts 45 kt. Isaac will move more WNW and reach near 15.7N 73.3W Sat evening, then gradually weaken to a tropical depression near 16.7N 77.4W Sun evening, before shifting NW and dissipating by late Mon. Otherwise, gentle to moderate trades are forecast west of Isaac, while moderate to locally fresh trades are forecast to the east of Isaac. Strong nocturnal trades are forecast off the northwest coast of Colombia on Mon and Tue nights. ATLANTIC OCEAN... Please read the Special Features section, for details on Tropical Storm Florence, Tropical Storm Helene, and Tropical Storm Joyce. An upper-level trough extends from near 32N45W to 26N48W to 20N54W, and to 16N59W. Upper level cyclonic wind flow is within 360 nm on either side of the trough. Scattered showers and isolated thunderstorms are within 30 nm ether side of a line from 22N44W to 22N50W to 22N57W. Isolated showers and thunderstorms are seen elsewhere to the west of 56W. A surface ridge will remain about stationary from near 32N68W to southeastern astern Florida through Sun. The ridge will shift northward, to a position from 31N74W to central Florida by the middle of the next week. For additional information please visit http://www.hurricanes.gov/marine $$ McElroy