AXNT20 KNHC 251032

Tropical Weather Discussion
NWS National Hurricane Center Miami FL
632 AM EDT Tue Jul 25 2017

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 0600 UTC surface analysis and satellite imagery through 1015 UTC.


A far eastern Atlantic tropical wave has its axis from 16N20.5W to 06N20W, moving westward at 10-15 kt. This wave remains embedded within a very moist southwesterly wind flow regime associated with the monsoon trough that has becomes well established off the coast of Africa. Model guidance from the GFS indicates that broad 700 mb troughing exists over the area where the wave is located. Latest satellite imagery along with the Total Precipitable Water (TPW) imagery depict an increasing deep atmospheric moisture layer enveloping the wave. Scattered moderate isolated strong convection is increasing within 180 nm either side of the wave to the south of the monsoon trough. Scattered moderate convection is within 30 nm either side of a line from 09N18W to 10N15W. Scattered showers and isolated thunderstorms are elsewhere east of the wave, and within 120 nm west of the wave from 10N-16N.

A tropical wave has its axis along 31W from 03N to 15N, moving westward near 10 kt. This wave remains suppressed, convectively and structurally speaking, as it moves through a region of the Atlantic where a very stable environment is present, with the added factor of Saharan dust as clearly seen on the METEOSAT-9 imagery. The only portion of the wave that remains moist and unstable is that to the south of 09N and across the monsoon trough. Scattered moderate convection is within 180 nm either side of the wave from 06N-08N.

A broad central Atlantic tropical wave has its axis extending from near 23N46W to 16N45W to 09N43W, moving westward 15 kt. The wave lies under the eastern edge of 700 mb troughing. The wave remains void of deep moisture, with the exception of the portion from 17N-23N between 44W-47W as seen in TPW imagery. Latest satellite imagery shows scattered showers and isolated thunderstorms within this area of moisture. An upper level low just to the northwest of the wave at 25N51W and moving in a west-northwest direction. Upper level divergence to the east of the low is helping fuel the aforementioned shower and thunderstorm activity.

A tropical wave is approaching the Lesser Antilles, with its axis extending from near 18N56W to 11N57W to the coast of South America at 06N57W. It is moving westward at 10-15 kt. Broad 700 mb troughing is indicated by the models to be present over the northern and central portions of the wave, however, the presence of dry air aloft as noted in water vapor imagery and Saharan dust as seen in the METEOSAT-9 imagery is only allowing for isolated showers and weak isolated thunderstorms near those portions of the wave. A diffluent flow aloft provided by an upper trough that exists across the northern part of the wave, and ridging across the southern part of the wave is helping to support increasing scattered moderate isolated strong convection within 120 nm east and 60 nm west of the wave axis from 11N-13N. The wave will cross the Lesser Antilles this evening, and move across the eastern Caribbean on Wednesday. Moisture associated with this wave is forecast to bring increasing chances for scattered showers and thunderstorms to the the eastern Caribbean on Wednesday, however, before the wave arrives there scattered showers and thunderstorms ahead of it will move across portions of mainly the Leeward Islands today. Some of this activity may be attendant by gusty winds.


The monsoon trough axis extends from 12N16W to 08N27W to 07N37W, where scatterometer data indicates the ITCZ begins and continues to 06N42W to 07N46W to 10N53W. Besides the convection mentioned described above in relation to the tropical waves, scattered moderate convection exists within 120 nm either side of the ITCZ between 34W-37W, within 120 nm north of the ITCZ between 49W-53W, and within 60 nm south of the trough axis between 23W-28W.



In the upper levels, an upper level low is centered near 25N93W. A cyclonic shear axis extends from the low east to 25N84W, and southwest from the low to 22N98W. Deep moisture has increased across just about the entire Gulf during the past 24 hours. The earlier scattered moderate to isolated strong convection that was over the Yucatan Peninsula and Cuba is quickly dissipating. Scattered showers and isolated thunderstorms over some areas of the SW Gulf, and the far south-central Gulf. Isolated showers and thunderstorms are elsewhere. At the surface, a 1019 mb high is near 27N84W. The related anticyclonic flow consists of gentle to light and variable winds E of 91W and moderate easterly to southeasterly flow over the W Gulf. Little overall change is forecast in the current synoptic set-up through the next couple of days. Scattered moderate to isolated strong convection is again expected to develop over the the Yucatan Peninsula and Cuba this afternoon and last into this evening.


Just about the entire the Caribbean is under moderate subsidence aloft and dry sinking air, with the exception of the southern portion of the eastern Caribbean where scattered showers and isolated thunderstorms moving quickly westward are observed. Similar activity is over the northwest portion of the sea, and just recently developed over the waters just south of the central section of Hispaniola. In the far SW part of the sea, the eastern extension of the eastern Pacific monsoon trough along with low- level speed convergence is supporting isolated showers and thunderstorms along and just inland the coast of northern Panama. The pressure gradient across the area will maintain fresh to strong winds across the central Caribbean through tonight, except for near gale force winds along the NW coast of Colombia through early this morning. The pressure gradient will then relax on Wednesday, with strong nocturnal trades expected mainly along the coast of Colombia. Moderate to locally fresh trades will prevail elsewhere across the Caribbean at that time. A tropical wave will move across the eastern Caribbean on Wednesday. See above for a discussion on this feature.


Scattered showers and thunderstorms that were over most of Haiti last night have dissipated. Isolated showers are possible over the waters just west of Haiti. Dry air aloft continues to advect westward over much of Hispaniola. This should keep deep convection to a minimum over the island this afternoon and evening, and again on Wednesday. Some moisture may approach the eastern part of the island late on Wednesday as a tropical wave passes just to the south. This may bring some shower and thunderstorm activity to mainly the southeastern and central sections of the island.


A mid to upper level trough stretches from well north of the area southwestward to 32N70W and to the central Bahamas. Broad mid/upper ridging is east of the trough to 53W. A large upper level low near 25N51W is moving west-northwest. Its broad circulation covers the area north of 19N between 43W-53W. Scattered showers and isolated thunderstorms are noted from 20N- 26N between 47W-53W. They are moving westward. At the surface, broad surface ridging anchored by a 1026 mb high centered near 32N57W, and a 1030 mb high well northeast of the area near 36N29W. A trough extends from near 31N53W to 26N55W. Isolated showers and thunderstorms are within 180 nm west of the trough north of 28N, and from 18N-28N between 56W-65W. Isolated showers and thunderstorms are developing over the waters west of the Bahamas, including the Straits of Florida. Scattered showers and thunderstorms are expected to again form over much of the northwestwestern portion of the area today and continue into this evening as an upper trough along the SE United States acts upon deep layer moisture present over that portion of the basin.

In the tropical Atlantic, four tropical waves are moving across that portion of the discussion area. See above for details on these features. Aside from shower and thunderstorm activity associated with the tropical waves, the remainder of the basin is under very stable and dry air derived from the Saharan Air Layer that is present there. These conditions are expected to change very little through Thursday.

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

$$ Aguirre