834 AXNT20 KNHC 111748 TWDAT Tropical Weather Discussion NWS National Hurricane Center Miami FL 148 PM EDT Tue Sep 11 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 1200 UTC surface analysis and satellite imagery through 1745 UTC. ...SPECIAL FEATURES... Hurricane Florence is centered near 26.4N 64.6W at 11/1500 UTC or 340 nm S of Bermuda and about 790 nm ESE of Cape Fear North Carolina moving WNW or 295 degrees at 14 kt. The estimated minimum central pressure is 950 mb. The maximum sustained winds are 115 kt with gusts to 140 kt. Latest satellite imagery shows scattered moderate to strong convection within 120 nm of the center in the NE quadrant, and within 120 nm of the center in the SE quadrant. Scattered moderate convection is within 240 nm of the center in the NW quadrant, and within 120 nm of the center in the SW quadrant. Florence is forecast to continue on a west- northwestward to northwestward motion with a slight increase in forward speed are expected during the next couple of days. On the forecast track, the center of Florence will move over the southwestern Atlantic Ocean between Bermuda and the Bahamas through Wednesday, and approach the coast of North Carolina or South Carolina on Thu. Interests elsewhere in the southeastern and mid-Atlantic states need to monitor the progress of Florence. Public Advisories for Florence are issued under WMO header WTNT31 KNHC and under AWIPS header MIATCPAT1. Forecast/Advisories for Florence are issued under WMO header WTNT21 KNHC and under AWIPS header MIATCMAT1. Tropical Storm Isaac is centered near 14.6N 49.7W at 11/1500 UTC, which is about 675 nm E of the Lesser Antilles moving W or 270 degrees at 14 kt. The estimated minimum central pressure is 996 mb. The maximum sustained wind are 60 kt with gusts to 75 kt. Latest satellite imagery shows scattered moderate to strong convection within 90 nm of the center in the NW and SE quadrants and within 120 of the center in the NE quadrant. Scattered moderate convection is within 30 nm of the center in the SW quadrant. The present motion is expected to continue through the end of the week. On is present motion, Isaac is expected move near or over the central Lesser Antilles on Thu and move into the eastern Caribbean Sea Thu night. Swells generated by Isaac will begin to affect portions of the Lesser Antilles on Wed afternoon. These swells are likely to cause life-threatening surf and rip current conditions. Please consult products from your local weather office. Public Advisories for Isaac are issued under WMO header WTNT34 KNHC and under AWIPS header MIATCPAT4. Forecast/Advisories for Isaac are issued under WMO header WTNT24 KNHC and under AWIPS header MIATCMAT4. Hurricane Helene is centered near 16.5N 34.3W at 11/1500 UTC, which is about 585 nm west of the southernmost Cabo Verde Islands or 1350 nm south-southwest of the Azores moving NW or 310 degrees at 10 kt. The estimated minimum central pressure is 966 mb. The maximum sustained winds are 95 kt with gusts to 115 kt. Latest satellite imagery shows that Helene's cloud pattern has not changed much over the psst several hours. Scattered moderate isolated strong convection is within 180 nm of the center in the eastern semicircle and SW quadrant, and within 150 nm of the center in the NW quadrant. Helene is forecast to maintain its present motion today, with a turn toward north-northwest and northward thereafter on Wed and Thu. Gradual weakening should begin tonight, and Helene is forecast to become a tropical storm on Thu near 22.5N 38.2W. Public Advisories for Helene are issued under WMO header WTNT33 KNHC and under AWIPS header MIATCPAT3. Forecast/Advisories for Helene are issued under WMO header WTNT23 KNHC and under AWIPS header MIATCMAT3. A surface trough extends from the southeastern Gulf of Mexico near 25N84W to inland the eastern Yucatan Peninsula. An upper-level low is identified to be just north of the Yucatan Peninsula at 23N88W. Satellite visible imagery indicates that a weak surface low pressure may be forming near 23N87W. The surface trough is interacting with the upper-level low. Satellite imagery shows numerous moderate to strong convection convection from 18N to 22N, including the Yucatan Channel and western half of Cuba from 81W to 85W. Scattered moderate isolated strong convection is seen from 22N to over the far southeastern Gulf of Mexico near 26N between 82W and 88W, including the western section of the Straits of Florida. This entire area of convection is gradually becoming better organized. Ship reports indicate that this system is producing strong gusty winds over the Yucatan Channel. Upper-level winds are forecast to become more conducive for development, and a tropical depression is likely to form by Thu night while the disturbance moves across the western Gulf of Mexico. If necessary, an Air Force Reconnaissance plane will investigate the system tomorrow. This area of disturbed weather has a medium chance for tropical cyclone formation over the next couple of days. Interests across northeastern Mexico and the coasts of Texas and Louisiana should monitor the progress of this system. Regardless of development, heavy rainfall and gusty winds are expected to continue over western Cuba and portions of the northeastern Yucatan Peninsula during the rest of today. Please read the latest Tropical Weather Outlook under WMO header ABNT20 KNHC, and under the AWIPS header MIATWOAT for more details on this evolving system. ...TROPICAL WAVES... Tropical waves are not presently over water, however, the next tropical wave expected to move across the discussion area waters in the short-term is presently over the interior of Africa near 10W. Ahead of this wave, scattered moderate isolated strong convection is observed from 09N to 13N east of 17W to well inland the African coast. ...MONSOON TROUGH/ITCZ... The monsoon trough is non-continuous over the interior of Africa. The western segment of the trough extends southwestward from coast of Sierra Leone to 06N18W. The ITCZ is not analyzed at this time. Scattered moderate convection is within 180 nm south and 120 nm north of the monsoon trough between 17W and 18W. ...DISCUSSION... GULF OF MEXICO... See Special Features above for details on an area of disturbed weather over the southeastern Gulf of Mexico. A narrow upper-level trough extends from the northeastern part of the Gulf of Mexico to the eastern central Gulf of Mexico and to an upper-level low at 23N88W, It continues from the low to inland the eastern Yucatan Peninsula. Associated upper cyclonic flow is present south of 29N and east of 93W. Upper-level anticyclonic wind flow covers the Gulf of Mexico west of 93W and the Atlantic Ocean to 70W, and the Caribbean Sea from 15N northward from 70W westward. Aside from the convection associated with the above described area of disturbed weather over the southeastern gulf waters, isolated showers and thunderstorms are noted elsewhere east of 91W. A stationary front stretches from southwestern Louisiana to inland the southern Texas coast. Plenty of deep atmospheric moisture is present over the far western gulf as a rather strong upper trough moves across central Texas. Scattered moderate isolated strong convection is noted north of 22N and west of a line from 30N91W to 25N94W to 22N95W. Scattered showers and isolated thunderstorms are elsewhere west of 91W. A surface trough in the western Caribbean Sea will move across the Yucatan peninsula on Tue, and into the Gulf of Mexico on Wed. The trough is expected to move into the western Gulf on Thu and to the west of the area on Sat. CARIBBEAN SEA... A surface trough extends from the southeastern Gulf of Mexico near 25N84W to just inland the eastern section of the Yucatan Peninsula. An upper-level trough extends from an upper-level low just north of the Yucatan Peninsula south-southwestward to just inland the peninsula. Plenty of available deep tropical moisture along with the presence of these features has resulted in plenty of atmospheric instability, leading to numerous moderate to strong convection north of 18N and west of 81W, including western Cuba and the Yucatan Channel. This activity is likely to be accompanied by strong to near gale gusty winds and very heavy rainfall as it gradually lifts northward over the eastern and central Gulf of Mexico through Thu. See Special Features section above for more details on this area of disturbed weather. Scattered showers and isolated thunderstorms are elsewhere north of 15N and west of 81W. Tropical Storm Isaac near 14.6N 49.7W 996 mb at 11 AM EDT moving W at 14 kt. Maximum sustained winds 60 kt gusts 75 kt. Isaac will move to 14.6N 51.8W this evening, strengthen to a hurricane near 14.7N 54.4W Wed morning, then weaken to a tropical storm near 14.9N 57.1W Wed evening, reach to near 15.1N 59.9W Thu morning and to 15.5N 65.5W Fri morning. Isaac will change little in intensity as it moves to near 15.5N 71.0W early Sat. Expect winds and seas to increase over the far eastern Caribbean beginning late Wed into Thu. ATLANTIC OCEAN... Please refer to the Special Features section above for details on Hurricanes Florence and Helene and on Tropical Storm Isaac. An upper-level trough extends from a nearly stationary upper- level low near 31N34W southwestward to 26N40W, where it becomes a cyclonic shear axis to a small upper-level low dropping south at 24N48W and to near 23N55W. Upper-level cyclonic flow covers the area generally to the north of 23N between 26W and 54W. Scattered showers and isolated thunderstorms are within 120 nm in the NE and 90 nm in the SW semicircles of the low near 24N48W, and also north of 30N from 34W to 38W. Isolated showers and thunderstorms are elsewhere west of 40W, except for scattered showers and isolated thunderstorms north of 26N west of 74W enhanced by the combiation of an inverted surface trough that is offshore the coast of Florida from 26N79W to 31N81W, and an upper trough axis that extends into the far northwest corner of the area from 32N79W to northeastern Florida. Hurricane Florence near 26.7N 65.3W 950 mb at 11 AM EDT moving WNW at 14 kt. Maximum sustained winds 115 kt gusts 140 kt. Florence will move to 27.7N 67.6W this evening, 29.4N 70.6W Wed morning, 31.1N 73.1W Wed evening, and will move to a position north of the area near 32.6N 75.2W Thu morning. For additional information please visit http://www.hurricanes.gov/marine $$ Aguirre