000 AXNT20 KNHC 212347 TWDAT Tropical Weather Discussion NWS National Hurricane Center Miami FL 647 PM EST WED DEC 21 2016 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. ...SPECIAL FEATURES... .None. ...THE ITCZ/THE MONSOON TROUGH... The ITCZ emerges from Liberia on the western coast of Africa at 06N11W and immediately transitions to the ITCZ and extends through 03N20W to 02N30W to the equator at 45W. Scattered moderate isolated strong convection is noted from the equator to 05N between 20W and 27W and from 03S to the equator between 28W and 35W. ...DISCUSSION... ...THE GULF OF MEXICO... An upper-level high is centered over northeastern Mexico near 24N99W with associated anticyclonic flow dominating the Gulf of Mexico W of 85W. The base of an sharp upper level shortwave trough extends from 30N76W across north-central Florida to 27N84W. This trough was digging SE around the eastern edge of the anticyclone. Very strong deep layered subsidence was sliding southward across Florida. The upper- level high over Mexico will gradually shift eastward during the next couple of days, keeping the ridge over the Gulf of Mexico. At the surface... a weakening stationary front extended from Naples, FL through 26N85W to 24N95W. A surface trough extended from 24N95W to the Bay of Campeche at 18N94W. A 1548 UTC ASCAT pass indicated Fresh NW winds W of and light to moderate E winds E of the trough. Elsewhere over the Gulf Ne to E winds have decreased to 10 to 15 KT over most areas...except 15 to 20 KT over the far eastern Gulf and the straits of Florida. Broken to overcast low and mid clouds were noted over the Gulf S of 27N. The front is expected to dissipate during the next 24 hours. Another frontal boundary will sink southward toward the northern Gulf coast on Friday, then stall. Moderate to fresh E to SE winds will generally prevail over the Gulf into the weekend. ...THE CARIBBEAN SEA... In the upper levels, convergent upper-level flow from the NW is producing strong subsidence over the Caribbean W of 70W. An upper- level low is centered over the central Caribbean south of Hispaniola near 17N70W. Divergent upper-level winds on the east side of the low are triggering scattered showers and isolated thunderstorms mainly south of Puerto Rico from 14N to 18N between 66W and 70W. The upper- level low will weaken toward the end of the week, which will cause the associated convection to diminish as it shifts westward in tandem with the low.Scatterometer data indicated NE to E winds of 15 to 25 KT across most of the basin with the exception of lighter winds in the vicinity of the monsoon trough over the extreme SW Caribbean just north of Panama and Costa Rica. The convection is expected to slowly spread W to the central Caribbean during the next day or so. ...HISPANIOLA... A sharp transition between deep moisture and pronounced drying and subsidence is currently oriented southwestward from the Mona Passage to between Colombia and Haiti along 72W. 12Z rawinsondes from San Juan and Santo Domingo showed weaker than normal trade wind inversions with moderate precipitable water values of around 1.4 inches. The area of moisture will shift W during the next day or so and allow showers over Puerto Rico to spread across the Dominican Republic tonight and Thursday, then possibly Haiti on Friday before the upper-level low weakens. ...THE ATLANTIC OCEAN... Broad troughiness defined the overall flow regime over the Atlantic with troughs over the central Atlantic and eastern Atlantic associated with mid to upper- level lows near 23N25W and 23N57W. Additionally, a sharp upper level shortwave trough extending from 30N76W across north-central Florida was digging SE around the eastern edge of an anticyclone over the Gulf. Very strong deep layered subsidence was sliding southward across Florida and the adjacent waters in the NW Atlantic. At the surface a quasi-stationary front extended over the western Atlantic from 32N70W to between Miami and Fort Lauderdale Florida. A weak surface low was noted about 40 nm east of Fort Lauderdale. Scattered showers were noted along and up to 120 nm east of the front E of 75W and within the small low pressure area itself. Another weak surface trough is over the central Atlantic from 22N56W to 28N55W. Scattered showers are within 120 nm of the trough. This trough has little in the way of associated convection. Another surface trough is over the eastern Atlantic from 22N25W to 27N28W. A 1035 mb high was centered over the Azores near 38N25W. A ridge axis extends WSW from the high to 32N58W. Expect the front to dissipate over the next 24 hours before a new cold front moves eastward from the southeastern United States on Friday. For additional information please visit http://www.hurricanes.gov/marine $$ Cobb