AXNT20 KNHC 171806

Tropical Weather Discussion
NWS National Hurricane Center Miami FL
205 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 1200 UTC surface analysis and satellite imagery through 1745 UTC.


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.


A tropical wave is over far western Africa with its axis extending from 19N16W to 10N16W to near 05N16.5W. 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 along with 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 has its axis along a position from near 23N45W to 16N48W to near 08N51W, 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 eastern Cuba to 15N77W and inland to the Panama/Colombian border near 07N75W, moving westward around 20 kt. Saharan dust is following in behind this wave axis as observed in GOES-16 satellite imagery. No deep convection is presently noted with this wave over the Caribbean waters. The earlier noted scattered moderate to strong convective activity has shifted westward to Panama and weakened to scattered moderate convection over central Panama and its immediate adjacent waters. Isolated showers are possible elsewhere within 60-90 nm of the wave axis.


The monsoon trough axis extends from W Africa along the coast of Mauritania near 19N16W southwestward to 11N20W and to 07N30W, where latest scatterometer data indicates that the ITCZ begins and continues to 09N39W to 09N49W. It resumes west of the above described tropical wave at 09N51W and to 07N59W. Scattered moderate convection is within 120-180 nm east-southeast of the trough between 18W-21W. Similar activity is also within 60 nm south of the ITCZ between 32W-37W, and within 30 nm of a line from 06N38W to 05N45W.



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-91W. 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 24-24 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.


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, with a shear axis stretching from it eastward to east-central Cuba and to the SE Bahamas. Mainly 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.


A weak surface trough over the western Atlantic extends from near 32N61W to 30N63W. 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, and the present strong high pressure ridging that stretches southwestward from a 1033 mb high well north of the area near 39N41W to 29N60W to the NW Bahamas becomes the dominate feature of the area where the trough is located. 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. It spreads southwestward to over much of Cuba, with some of it spreading westward through the Straits of Florida.

For additional information please visit http://www.hurricanes.gov/marine

$$ Aguirre