609 AXNT20 KNHC 172357 TWDAT Tropical Weather Discussion NWS National Hurricane Center Miami FL 757 PM EDT Tue Jul 17 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 1800 UTC surface analysis and satellite imagery through 2315 UTC. ...SPECIAL FEATURES... SW Caribbean Sea gale warning: The pressure gradient is forecast to tighten between strong central Atlantic high pressure and broad low pressure over the southwestern Caribbean Sea beginning late Wed night increasing the present ongoing strong NE-E winds over the area from 11N-15N between 70W-77W, including the waters along the coast of Colombia, to increase to minimal gale force. These winds will weaken to just below gale force in the early morning hours of Thu. Seas are expected to build to the range of 10-14 ft with the gale force winds, then subside to 8-11 ft Thu morning. Please read the High Seas Forecast, under the AWIPS/WMO headers HSFAT2/FZNT02 KNHC, for more details. ...TROPICAL WAVES... A tropical wave is over far western Africa with its axis extending from 20N17W to 06N17W. It is moving westward near 13 kt. A well defined 700 mb trough is noted in the model guidance over far western Africa. Latest satellite imagery is showing increasing cloudiness and convection in the vicinity of the wave. The observed cloud pattern is more typically characteristic of the presence of an ongoing W African monsoon trough, with the wave passing through it. The imagery shows scattered moderate convection within 120 nm east and 60 nm west of the wave from 08N- 10N. A major outbreak of Saharan Air Layer (SAL) dust envelops the wave environment north of 10N as seen in GOES-16 images. A central Atlantic tropical wave extends from 22N49W to 15N51W to 07N52W, moving westward at 15-20 kt. The wave is depicted on GOES-16 RGB imagery as having a rather broad inverted-V shape envelope of broken to overcast stratocumulus clouds covering the area from 09N-23N between 38W-57W. The wave is being intruded upon by Saharan dust limiting significant convection from developing near it. Only scattered showers and isolated thunderstorms are seen within 180 nm to the east of the wave from 10N-12N, while isolated showers are within 30 nm of a line from 16N47W to 15N50W. The GOES-16 RGB images of the far eastern Atlantic are depicting yet another massive plume of Saharan dry air and associated dust following in behind the wave from 08N-27N, to well inland W Africa including the environment of the tropical wave that is analyzed from 19N16W to 10N16N to 05N16.5W as described above. A tropical wave axis over the central Caribbean extends from central Cuba at 22N78W to S of Panama at 06N78W, moving westward at 20 kt. Saharan dust is following in behind this wave axis as observed in GOES-16 satellite imagery. Scattered moderate to isolated strong convection is over Panama. ...MONSOON TROUGH/ITCZ... The monsoon trough axis extends from W Africa along the coast of Mauritania near 18N16W southwestward to 09N22W to 07N30W, where latest scatterometer data indicates that the ITCZ begins and continues to 09N50W. The ITCZ resumes west of a tropical wave at 10N53W and continues to Venezuela near 09N60W. Scattered moderate convection is from 07N-09N between 22W-26W. Similar convetion is from 03N-06N between 33W-44W. ...DISCUSSION... GULF OF MEXICO... The western periphery of Atlantic high pressure ridging extends E to W over the central gulf waters. Light to moderate anticyclonic flow are over the gulf waters. Current NWS mosaic radar shows scattered moderate isolated strong convection over the northern gulf north of 28N between 86W-92W. Scattered moderate convection is over the far northeastern waters north of 28N and east of 84W. This activity is associated with the tail end of a mid-level trough that extends to over the northeast and north- central gulf waters, with additional support from upper- level disturbances that are riding NE-SW across the eastern periphery of a mid to upper-level anticyclone that situated over northeastern Texas. The activity is expected to remain active over the next 24-48 hrs. Isolated showers and thunderstorms are over the remainder of the gulf waters. Expect rather weak high pressure to prevail across the northern gulf waters through Thu. A surface trough will move westward off the Yucatan Peninsula each evening through Thu, enhancing nocturnal winds over the eastern section of the SW Gulf. CARIBBEAN SEA... Please read the Special Features section for details about the next pulsing gale force wind event in the far SW Caribbean near the coast of Colombia. Aside from the tropical waves mentioned above, a small upper-level low is seen on water vapor imagery over the extreme northeastern part of the Yucatan Peninsula. Mostly fresh E winds will continue over the central Caribbean Sea, and strong NE-E winds will be over the SW and S central sections of the Caribbean, through Thu. Low-cloud streamers with brief isolated showers will continue over the far eastern Caribbean through Thu. Scattered showers and thunderstorms will develop over the interior sections of Hispaniola each afternoon through Thu. ATLANTIC OCEAN... A weak surface trough over the western Atlantic extends from near 32N58W to 30N62W. Scattered showers and isolated thunderstorms are ahead of the trough from 27N to 32N between 55W-59W and also from 28N-30N between 59W-61W. The trough will dissipate Wed as the supporting upper trough continues to lift northeastward away from it. A 1032 mb high is well north of the area near 38N41W. Scattered showers and thunderstorms are north of 30N between 75W- 80W are associated with a mid/upper level trough that stretches along the U.S. eastern seaboard. This activity will remain active through Thu as a strong deep-layer trough moves off the eastern seaboard. High pressure will build in the wake of the trough. A large area of Saharan Air Layer (SAL) dust that is currently over the central Atlantic from 09N-24N and between 48W-65W will continue to translate westward through the rest of the week. A more subtle batch of dust tracking westward is observed from 18N- 27N between 60W and the Bahamas. For additional information please visit http://www.hurricanes.gov/marine $$ Formosa