239 AXNT20 KNHC 181157 TWDAT Tropical Weather Discussion NWS National Hurricane Center Miami FL 757 AM EDT Wed Jul 18 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 1100 UTC. ...SPECIAL FEATURES... SW Caribbean Sea gale warning: A gale goes into effect at 19/0000 UTC along the coast of Colombia from 11N to 12.5N between 74W and 76W, with seas of 10 to 13 ft. The gale ends at 19/1200 UTC. Please read the High Seas Forecast, under the AWIPS/WMO headers HSFAT2/FZNT02 KNHC, for more details. ...TROPICAL WAVES... A tropical wave is nearing the Cabo Verde Islands, with an axis extending from 20N20W to 05N23W, moving westward at 10 to 15 kt. A well defined 700 mb trough exiting the coast of western Africa is noted in the model guidance. Satellite imagery shows scattered moderate convection from 07N to 10N between 17W and 28W. 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 has an axis that extends from 22N54W to 06N56W, moving westward at 15 to 20 kt. The wave is depicted on GOES-16 RGB imagery as an inverted-V shape envelope of broken to overcast stratocumulus clouds covering the area. 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 240 nm of either side of the wave. A tropical wave axis is over the western Caribbean that extends from 22N82W to S of Panama at 06N81W, moving westward at 15 to 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 S of 12N W of 81W. ...MONSOON TROUGH/ITCZ... The monsoon trough axis extends from W Africa near 20N16W to 07N28W to 06N37W, where latest scatterometer data indicates that the ITCZ begins and continues to 06N37W to 10N55W. The ITCZ then resumes west of a tropical wave at 09N58W and continues to South America near 08N62W. Aside from convection associated with the tropical waves discussed in the tropical waves section, scattered moderate convection is from 06N to 10N between 28W and 41W, and from 01N to 05N between 41W and 48W. ...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 prevails over the region, outside of thunderstorms. Current NWS mosaic radar and satellite imagery show scattered thunderstorms over the northern gulf primarily north of 28N E of 92W. This activity is being enhanced by a mid- to upper- level trough over and to the NE of the area. The base of the trough and associated thunderstorm activity is expected to remain over this area for at least the next day or so. Weak high pressure is forecast to prevail across the northern gulf waters through Thu night. Thereafter, a frontal trough will cross the NE gulf, forcing the ridge axis farther south over the SE Gulf. Increasing winds are expected over the NE gulf with this trough in the area. Another surface trough will move westward off the Yucatan Peninsula the next few evenings, enhancing nocturnal winds over the eastern section of the SW Gulf. CARIBBEAN SEA... Please read the Special Features section for details about the pulsing gale force winds 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 again develop over the interior sections of Hispaniola this afternoon and Thu afternoon. ATLANTIC OCEAN... A mid- to upper-level trough over the SE United States is helping to enhance shower and thunderstorm activity NW of a line from 31N70W to 25N80W. These thunderstorms will remain active through Thu as a portion of the trough moves off the eastern seaboard. High pressure will build in the wake of this trough late this week. A 1031 mb high well north of the area, centered near 38N41W is dominating much of the central and eastern Atlantic discussion waters. A large area of Saharan Air Layer (SAL) dust is currently over the central Atlantic from 09N to 25N and between 51W and 65W, and over the eastern Atlantic S of 25N E of 51W. The dust will continue to translate westward for the rest of the week. For additional information please visit http://www.hurricanes.gov/marine $$ MRF/Latto