683 AXNT20 KNHC 121805 TWDAT Tropical Weather Discussion NWS National Hurricane Center Miami FL 205 PM EDT Wed Sep 12 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 29.8N 71.3W at 12/1500 UTC or about 420 nm SE of Wilmington North Carolina and 450 nm ESE of Myrtle Beach South Carolina, moving NW or 305 degrees at 13 kt. Estimated minimum central pressure is 943 mb. Maximum sustained winds are 115 kt with gusts to 140 kt. Latest satellite imagery continues to show a rather distinct eye feature. The imagery shows numerous moderate to strong convection within 75 nm of the center in the NE quadrant, within 60 nm of the center in the SE and NW quadrants and within 30 nm of the center in the SW quadrant. Scattered moderate to isolated strong convection is elsewhere within 90 nm of center in the E semicircle and within 90 nm of the center in the W semicircle. The present NW motion of Florida is forecast to continue through Sat with a gradual decrease in forward speed. On the forecast track, the center of Florence will move over the southwestern Atlantic Ocean between Bermuda and the Bahamas today, and approach the coast of North Carolina or South Carolina in the hurricane warning area on Thu and Fri, then move slowly near the coastline through Sat. Florence is a category 4 hurricane on the Saffir-Simpson Hurricane Wind Scale. Further strengthening is forecast through tonight. While some weakening is expected on Thu, Florence is still forecast to be an extremely dangerous major hurricane when it nears the U.S. coast on Friday. 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 15.0N 54.7W at 12/1500 UTC or 365 nm E of Martinique or 395 nm E of Guadeloupe moving W or 275 degrees at 15 kt. Estimated minimum central pressure is 1000 mb. Maximum sustained winds have diminished some to 50 kt with gusts to 60 kt. Latest satellite imagery shows scattered moderate convection removed to the SE of Isaac as it undergoes upper northerly shear. The cyclone is exhibiting a fully exposed center on visible imagery. The observed convection is located from 12N to 15N between 49W and 55W. On the present forecast track, Isaac is forecast to move across the central Lesser Antilles and into the eastern and central Caribbean Sea on Thu, and move into the eastern and central Caribbean Sea Thu night through Sat. Heavy rainfall, with the possibility of life-threatening flash flood flooding, is expected across Martinique, Dominica, Guadeloupe, Puerto Rico and the southern United States Virgin Islands. Gradual weakening is forecast during the next few days. 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 20.3N 36.5W at 12/1500 UTC, which is about 755 nm WNW of the Cabo Verde Islands and about 1170 nm SSW of the Azores. It is moving WNW at 12 kt. Estimated minimum central pressure is 977 mb. Maximum sustained winds are 80 kt with gusts to 100 kt. Latest satellite imagery shows that the eye has become less distinct, but the overall cloud pattern remains well organized and symmetric. Scattered moderate to strong convection is within 90 nm of the center, except 240 nm of the center in the NE quadrant. Helene is forecast to turn to the north and northeast with an increase in forward speed during the next few days. The cyclone is forecast to weaken to a tropical storm by late Thu. 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. Satellite images and surface observations indicate that a broad area of low pressure has formed in the central Gulf of Mexico. Although the shower activity is still disorganized, upper-level winds are forecast to become more conducive for development, and it is likely that a tropical depression will form on Thu, before the system reaches the western Gulf coast. Another reconnaissance plane is scheduled to investigate the disturbance tomorrow. Regardless of development, heavy rainfall and gusty winds are expected across portions of northeastern Mexico, Texas, and Louisiana late this week, and interests there should monitor the progress of this system, and refer to products from their local weather office. This system has a high chance of tropical cyclone development over the next couple of days. 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. Satellite images indicate that a strong area of low pressure located a little more than 600 miles west-southwest of the Azores is rapidly becoming better organized and a subtropical or tropical storm appears to be forming. If this trend continues, advisories will be initiated later today or tonight. The low is forecast to move toward the southwest for the next couple of days, and then turn to the northeast behind Hurricane Helene. For more information, see High Seas Forecasts issued by the National Weather Service. The satellite imagery shows scattered moderate convection within 60 nm of the center, except 30 nm in the NW quadrant. Similar convection is within 30 nm of a line from 37.5N38W to 38N40W, and from 32W to 37N between 35W and 40W. This system has a high chance for tropical or subtropical cyclone development over the next 48 hours. For more information, see High Seas Forecasts issued by the National Weather Service. Please read the latest Tropical Weather Outlook under WMO header ABNT20 KNHC, and under the AWIPS header MIATWOAT for updates on the chances for development of this system. ...TROPICAL WAVES... A new tropical wave has its axis just offshore the coast of Africa along 18W from 03N to 20N, moving westward at 10 to 15 kt. The wave is supported by well pronounced 700 mb troughing as depicted in the GFS model guidance. Scattered moderate convection is seen within 120 nm west and 60 nm east of the wave axis from 15N to 19N. Scattered moderate convection is occurring within 120 nm west of the wave axis from 08N to 10N. Similar activity is within 180 nm inland the coast of Africa from 10N to 18N. ...MONSOON TROUGH/ITCZ... The monsoon trough extends from W Africa near 11N15W to 06N17W to 04N22W. Scattered moderate convection is within 30 nm of the trough between 17W and 22W. Similar convection is north of the monsoon trough within 60 nm of a line from 09N19W to 09N23W, and within 30 nm of a line from 08N23W to 08N29W. ...DISCUSSION... GULF OF MEXICO... See Special Features above for details on an area of disturbed weather over the southeastern Gulf of Mexico. Within the area of disturbed area of weather described above under Special Features, an upper-level low moving S to SW just north of the Yucatan Peninsula is providing further instability over and near the surface trough related to the aforementioned disturbance. The trough is analyzed from near 26N90W to a 1010 mb low at 23.5N 92.5W and to inland southeastern Mexico. Deep convection has increased over the south-central gulf waters and Yucatan Channel and vicinity waters during the morning and early afternoon hours. This convection consists of the scattered moderate to strong type intensity from 22N to 25N between 88W and 93W. Scattered moderate isolated strong convection is noted south of 24N between 85W and 88W, and is lifting northward with time. Scattered showers and isolated thunderstorms are elsewhere south of 27N east of 92W. A stationary front is just inland the Texas and SW Louisiana coasts to a 1012 mb low over far southern Texas, and continues to far NE Mexico. In the upper levels, a narrow trough stretches from northeastern Texas southward to near 24N96W. Plenty of available atmospheric moisture is being tapped by these features leading to ample instability to exist over the far western gulf. Scattered showers and thunderstorms are present north of 20N and west of 94W. CARIBBEAN SEA... Widely scattered moderate convection is over the NW Caribbean and Central America W of 85W. Elsewhere, scattered moderate convection is over S Haiti. The eastern extent of the eastern Pacific monsoon trough is over Costa Rica and Panama along 09N. Scattered moderate convection is within 120 nm S of the monsoon trough. A 1009 mb low is centered over N Colombia near 09N74W. In the upper-levels, the upper-level low just north of the Yucatan Peninsula extends a trough to inland the eastern Yucatan Peninsula. The trough energy is acting on a very moist and unstable atmosphere over the far western Caribbean. This has resulted in scattered moderate to isolated strong convection north of 17N and west of 85W. This activity is capable of producing very heavy rainfall and strong gusty winds over these waters as well within about 60 nm inland the Yucatan Peninsula. Isolated showers and thunderstorms are elsewhere north of 15N west of 71W and east of 71W. Elsewhere, an upper level-low is centered just along the northwest coast of Colombia near 11N75W. An upper-level anticyclone is centered over the northern Leeward Islands near 17N63W. The eastern extent of the eastern Pacific monsoon trough is over Costa Rica and Panama along 09N. Isolated showers and thunderstorms are within 60 nm S of the monsoon trough. A 1010 mb low is centered over northern Colombia near 10N75W. Isolated showers and thunderstorms are within 60 nm S of the monsoon trough. Tropical Storm Isaac near 15.0N 54.7W 1000 mb at 11 AM EDT moving W at 15 kt. Maximum sustained winds 50 kt gusts 60 kt. Isaac will move to 15.1N 56.7W this evening, 15.3N 59.4W Thu morning, 15.3N 62.6W Thu evening, 15.4N 65.6W Fri morning, and 15.5N 71.0W Sat morning. 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 stretches from an upper-level low north of the area at 35N40W southwestward to 25N43W, to a small upper- level low at 22N45W and to 15N50W where it becomes a cyclonic shear axis to 13N50W to 09N54W. At the surface, a cold front extends from the surface low north of the area near 37N41W 1011 mb associated with the above described special feature, to 32N44W and to 30N50W. Isolated showers and possible weak thunderstorms are along and within 60 nm of the front. In the upper-levels, a small upper level low is centered N of the NW Bahamas near 28N71W. Isolated showers and thunderstorms are seen west of 73W as well as over and near the NW Bahamas. Hurricane Florence near 29.8N 71.3W 943 mb at 11 AM EDT moving NW at 13 kt. Maximum sustained winds 115 kt gusts 140 kt. Florence will move north of the area to 31.1N 73.1W this evening and reach 32.6N 75.1W Thu morning, Outside of the path of Florence, large swell generated by the hurricane will affect the waters around the Bahamas and Florida the next few days. For additional information please visit http://www.hurricanes.gov/marine $$ Aguirre