AXNT20 KNHC 260845 AAA

Tropical Weather Discussion...updated
NWS National Hurricane Center Miami FL
445 AM EDT Wed Jul 26 2017

updated Caribbean Sea section to include additional convection

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 0000 UTC surface analysis and satellite imagery through 0545 UTC, except 0745 UTC Caribbean Sea section.


A far eastern Atlantic tropical wave has its axis extending from 16N23W to 07N24W, moving westward at 10 kt. Although satellite imagery shows that the wave lacks deep moisture as it remains under Saharan dry air and dust, there appears to be a broad area of mid-level clouds rotating counterclockwise over the area from 12N-15N between 22W-26W. The only convection related to the wave is near where it crosses the monsoon trough. This convection consists of the scattered moderate type intensity within 30-60 nm north of the trough within 120 nm either side of the wave.

An eastern Atlantic tropical wave has its axis extending from 14N34W to 04N35W, moving westward 10-15 kt. This wave is moving through a very stable surrounding atmospheric environment marked by Saharan dry air and dust north of 11N. The SSMI Total Precipitable Water imagery (TPW) imagery indicates that deep moisture with this wave is confined to south of 11N. Satellite imagery shows minimal deep convection with this wave. Only scattered moderate convection is where the wave crosses the monsoon trough within 60 nm east of the wave.

A large amplitude central tropical wave has its axis extending from near 24N51W to 18N51W to 12N51W, moving westward at 15 kt. The northern portion of this wave is observed as a very noticeable area of moisture as seen on SSMI TPW imagery. The wave has a distinct signal at 650-700 mb as suggested in global model guidance. Saharan dry air and dust is following in behind the wave as seen in the latest METEOSAT-9 imagery. Scattered showers and isolated thunderstorms are within 180 nm east and 240 nm west of the wave from 10N-13N. Isolated showers and thunderstorms are within 60 nm elsewhere east of the wave.

An eastern Caribbean tropical wave has its axis along 64W south of 19N, moving westward at 10-15 kt. The wave has a distinct moist area as seen on SSMI TPW imagery south of 14N and north of 16N. Broad 700 mb troughing is much of the eastern Caribbean. Since dry aloft is evident across the central portion of this wave as seen in water vapor imagery, scattered showers and isolated thunderstorms are mainly confined to within the moisture areas highlighted by the TPW imagery. The wave will move across the remainder of the Caribbean through this evening, the central Caribbean Thursday and Thursday night and the eastern portion of the western Caribbean on Friday.


The monsoon trough axis extends from 13N16W to 10N24W to 08N35W to 08N42W, where scatterometer data indicates the ITCZ begins and continues to 10N50W and briefly ends. It begins just to the west of the tropical wave along 51W, and continues to 13N60W. Besides convective activity described above in association with the tropical waves, scattered moderate convection is within 60 nm north of the monsoon trough between 26W-30W, within 60 nm south of the monsoon trough between 38W-40W, within 30 nm of the ITCZ between 42W-46W and also within 60 nm south of the ITCZ between 54W-56W.



In the upper levels, an upper level low is centered over the western Gulf near 26N93W, with a trough extending south-southwestward to 18N93W, and another trough extending northeastward to near 30N92W. Water vapor imagery shows a markedly increased area of moisture covering the much of the western portion of the Gulf. Ample moisture and instability associated with the upper level low and first trough has resulted in numerous showers and scattered thunderstorms across the western half of the Gulf south of 27N. Scattered showers and isolated thunderstorms are noted south of 26N across the central and eastern Gulf. Isolated showers and thunderstorms are Yucatan Peninsula, while scattered showers and thunderstorms are over much of southeastern Mexico. At the surface, a 1020 mb centered at 28N88W dominates the wind flow pattern across the eastern and central portions of the Gulf, with light to gentle anticyclonic winds. The typical Yucatan Peninsula thermal trough has moved to the eastern Bay of Campeche. Little change is expected with the current synoptic pattern over the next couple of days.


A tropical wave is over the eastern Caribbean. See above for a discussion on this feature. In the upper levels, an upper level low is centered over the SW Caribbean near 15N79W, with a trough southwest to northern Panama. An anticyclone is just north of the coast Venezuela at 11N65W, with associated anticyclonic flow covering much of the eastern and central Caribbean except for the northeast portion where the southern portion of a central Atlantic upper trough is evident. Scattered strong convection is increasing over the far SW Caribbean south of 12N and west of 79W, and also south of 10N between the coast of Colombia and 79W. Scattered showers and isolated thunderstorms are seen north of 15N between 67W-72W, and over some areas of the eastern Caribbean associated with a tropical wave there as discussed above. Isolated showers and thunderstorms are elsewhere. The tropical wave moving through the eastern Caribbean should provide additional moisture and instability that portion of the sea and also the central and eastern portion of the western Caribbean over the next 48 hours.


Strong convection that occurred over western Haiti on Tuesday evening has dissipated. Scattered showers and thunderstorms are increasing over the northeast section of the Dominican Republic as an upper cyclonic shear axis nearby is providing additional instability to an already very moist and unstable environment present over the island. Water vapor imagery depicts that moisture has increased in the upper levels compared to the previous days. With plenty of deep layer moisture and instability in place, and with the added factor of the eastern Caribbean tropical wave forecast to pass near the island within the next 48 hours, expect for deep convection to become scattered to locally numerous over most sections of Hispaniola through at least Thursday night. Some of this activity may produce locally heavy rain and frequent lightning.


In the upper levels, a large upper level low retrograding in a general westward direction is identified to be over the central Atlantic at 27N56W, with a trough stretching southwestward to across the NE Caribbean Sea and to Hispaniola. Scattered moderate convection is occurring from 20N-24N between 53W-58W. Scattered showers and isolated thunderstorms are seen south of 27N between 58W-75W, and are moving westward. Isolated showers and thunderstorms are elsewhere west of 70W. At the surface, a 1026 mb high is centered over the central Atlantic near 33N54W. To its east, a 1028 mb high is centered over the eastern Atlantic near 33N37W. A broad ridge covers most of the central and eastern Atlantic. A large area of Saharan dust is present over the eastern Atlantic, and muchn of the central Atlantic behind the tropical wave along 51W. In the tropics, there are three tropical waves. See above for discussions on these features. No major changes expected through next couple of days.

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$$ Aguirre