AXNT20 KNHC 271052

Tropical Weather Discussion
NWS National Hurricane Center Miami FL
652 AM EDT Thu Jul 27 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.


An eastern Atlantic tropical wave has its axis along 30W from 04N to 17N, moving westward at an estimated 24 hour motion of about 10 kt. First visible METEOSAT images show what appears to be a low to mid-level cyclonic circulation just west of the wave near 11N31W. Satellite images from during the overnight hours showed and are still showing increasing convection over and near this circulation. The convection is of the scattered moderate intensity just north of where the wave crosses the monsoon trough from 11N-12N between 29W-31W. However, the METEOSAT-9 imagery shows abundant dense Saharan dry air and dust across the wave north of 13N. Isolated showers and thunderstorms are elsewhere within 120 nm either side of the wave south of 13N.

A central Atlantic tropical wave has its axis extending from 16N39W to 11N40W to 03N40W, moving westward at 10-15 kt. This wave is moving through a very stable region of the Atlantic where METEOSAT-9 imagery is exhibiting abundant Saharan dry and dust northwest through northeast of this wave. The CIMSS total precipitable water (TPW) imagery animation shows moisture in relation to this wave confined to south of 12N near and along the location of the monsoon trough. Latest satellite imagery shows scattered moderate isolated strong convection within 300 nm east of the wave axis along and north of the monsoon trough from 06N- 11N. Scattered moderate convection is west of the wave within 30 nm of line from 09N40W to 09N42W. Scattered showers and isolated thunderstorms are elsewhere within 180 nm west of the wave from 06N-10N.

A tropical wave approaching the Lesser Antilles is along 58W from 11N-24N, moving westward at about 13 kt. This wave continues to exhibit a pretty well-defined cloud pattern on latest satellite imagery. Both latest satellite imagery and the TPW imagery animation depict a swath of moisture along and within 120 nm east of the wave axis from 13N-24N and a larger area of deeper moisture south of 13N within 240 nm east and 180 nm west of the wave axis. Scattered showers and thunderstorms are occurring within the moisture area north of 13N, while the activity is isolated south of 13N. The wave is forecast to move across the Lesser Antilles and into the far eastern Caribbean Sea today, and move across the rest of the eastern Caribbean Sea though Friday night, then through the central Caribbean Saturday and Saturday night. Scattered showers and isolated thunderstorms are possible with this wave.

An eastern Caribbean Sea tropical wave is along 72W south of 19N, moving westward at 15-20 kt. Relatively drier air east of the wave north of mainly 15N is allowing for only isolated showers and thunderstorms to exist to the east of this wave, with a little in the way of more shower and thunderstorm over the far eastern portion of the Caribbean Sea. Similar activity is within 60-90 nm west of the wave. This wave will move across the rest of the Caribbean through tonight, and across the western Caribbean Sea Friday through Saturday.


The monsoon trough axis extends from 13N17W to 11N29W to 07N39W to 07N45W, where scatterometer winds indicate the ITCZ begins and extends to 08N50W and continues to 09N61W. Aside from the convection mentioned in the tropical wave section, scattered moderate isolated strong convection is within 120 nm north of the monsoon trough between 33W-35W. Scattered moderate convection is within 120 nm north of the monsoon trough between 31W-33W.



In the upper levels, a small upper level low is over the NW Gulf at 28N96W, with a trough extending southward to the SW Gulf. A mid-upper level trough is along the far northern coastal sections of the Gulf. Plenty of deep moisture and instability is present throughout the area. At the surface, a 1019 mb high is centered over the north central Gulf of Mexico near 28N91W. The associated anticyclonic flow consists of gentle to moderate winds across the basin. A surface trough that moved off the Yucatan Peninsula last night is over the eastern Bay of Campeche along 92W. Scattered moderate convection that moved offshore the central Florida coast last night and earlier this morning has reached the far SE Gulf from 24N-26N between 82W-84W. Scattered showers and isolated thunderstorms are elsewhere over the eastern Gulf south of 27N and east of 86W, including the Straits of Florida. Similar activity is over the NE Gulf associated with a surface to low- level trough that extends from along the southeastern Georgia coast southwest to the NE Gulf waters near 28N86W. Isolated showers and thunderstorms are seen over the remainder of the Gulf, except over the central section from north of 24N to between 88W- 92W where drier air is present deep through the atmosphere.

Little change is expected through Saturday for the majority of the Gulf. On Saturday, a weak front is forecast to drop south over the southeastern United States to near the Florida panhandle by late Saturday may bring an increase of moisture along with an increase in shower and thunderstorm activity to the NE Gulf at that time.


A tropical wave is moving across the central Caribbean, while another one is approaching the Lesser Antilles. See above for discussions on this feature that includes associated shower and thunderstorm activity. Scattered showers and isolated thunderstorms are over the western Caribbean from 12N-16N west of 79W. Similar activity is developing north of 18N between Jamaica and 84W due to an upper trough that trails southward to over Cuba and the NW Caribbean Sea from an upper level low located between the central Bahamas and Cuba. Isolated showers and thunderstorms being steered by the easterly trades are elsewhere over the basin.

The eastern extension of the Pacific monsoon trough in combination with additional instability provided by an upper level low near 12N81W is expected to set-off scattered to numerous showers and thunderstorms over the SW Caribbean Sea later this morning. The pressure gradient throughout supports 15-25 kt trade winds across the sea, with the strongest winds over the central Caribbean. The central Caribbean tropical wave may help to further enhance convection over the SW Caribbean late Friday through Saturday.


Scattered moderate convection is dissipating over Haiti early this morning. With a tropical wave along 72W, expect a slight increase in moisture across the island today as dry air is noted east of the wave over the northern portion of the Caribbean. The moisture should combine with daytime heating and local island effects and lead to the development of scattered moderate to strong convection over some areas of the island. This activity should last into tonight. Moisture is expected to decrease on Friday and Friday night, then increase on Saturday as another tropical wave passes through the central Caribbean Sea.


In the upper levels, a large and elongated upper level low is centered over the western Atlantic near 28N59W, with a trough stretching southwestward to the NE Caribbean Sea, then westward as cyclonic shear axis to Hispaniola. Scattered moderate convection is noted from 22N-24.5N between 53W-56W. Similar activity is north of 30N west of 79W. Scattered showers and isolated thunderstorms are occurring south of 25N and west of 75W, and are mainly being aided by an upper level low noted between the central Bahamas and Cuba. This activity is moving westward, and reaches to the Straits of Florida.

In the tropics, there are three tropical waves. See above for discussions on these features. Elsewhere at the surface, a 1022 mb high is centered over the central Atlantic near 31N45W, with a ridge extending southwestward to central Florida. The ridge is forecast to shift southward this weekend as a trough begins to move across northern Florida. Expect for showers and thunderstorms to become more widespread over the northwest portion of the area this weekend. Otherwise, the high pressure is maintaining rather stable and dry conditions throughout. The METEOSAT-9 satellite imagery shows abundant Saharan dry and dust present over the eastern Atlantic and over much of the central Atlantic. Some broken areas of dust are over the western Atlantic.

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

$$ Aguirre