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000
AXNT20 KNHC 281046
TWDAT

Tropical Weather Discussion
NWS National Hurricane Center Miami FL
646 AM EDT Fri Jul 28 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.


...TROPICAL WAVES...

An eastern Atlantic tropical wave extends its axis from 18N35W to low pressure of 1012 mb centered on the wave at 10N35W and to 05N35W, moving westward at 10 kt. This system is broad in structure, and is forecast to remain like this through at least the next 24 to 48 hours. First visible METEOSAT-9 images reveal what appears to be a low-mid level cyclonic circulation west-northwest of the 1012 mb low near 11N37W. Both satellite imagery and the total precipitable water (TPW) imagery animation both reveal that moisture has increased within the surrounding environment of the this system, and seems to have overtaken most of the Saharan dust that has been present over its northern portion the past several days. Scattered moderate isolated strong convection is seen east of the wave within 30 nm of 12.5N34W. Scattered moderate convection is along the wave within 30 nm of 12N35W. Isolated showers and thunderstorms are within 90 nm of circulation near 11N37W.

A central Atlantic tropical wave has its axis from 20N44W to 12N46W to 03N46W, moving west at 10 to 15 kt. This wave is moving through a very stable atmospheric environment marked by the presence of a very pronounced African Saharan Air Layer that covers the eastern and central Atlantic north of the tropics. Latest water vapor imagery confirms that strong subsidence and resultant dry air prevails over the the northern and central portions of this wave. The total precipitable water (TPW) imagery animation shows that moisture associated with this wave remains suppressed to the south of 12N within the region where the monsoon trough transitions to the ITCZ. Satellite imagery shows scattered moderate convection east of the wave to 43W from 05N- 09N, and also west of the wave within 30 nm of 09N48W. The wave is forecast to approach the Lesser Antilles late on Sunday.

An eastern Caribbean tropical wave has its axis along 64W, and extends northward to the Atlantic near 21N. Water vapor imagery depicts dry subsident air over the wave north of 14N. No deep convection is occurring with this wave. Only isolated showers and weak isolated thunderstorms moving quickly westward in the trade wind flow are noted within 150 nm either side of the wave. This wave will move across the remainder of the eastern Caribbean through tonight, then through the central Caribbean on Saturday.

A central Caribbean tropical wave has its axis along 77W/78W south of 18.5N. It is moving west at about 17 kt. Moisture with this wave remains rather shallow at the low- mid levels to its, while at the upper levels water vapor imagery shows a thin moisture in the form of scattered high clouds streaming eastward across the wave. The southern portion of the wave is aiding scattered moderate isolated strong convection over northwestern Colombia and its adjacent waters. Otherwise, only isolated showers and thunderstorms moving quickly westward in the trade wind flow are within 120 nm east and 300 nm west of the wave north from 14N- 18N. The wave will move across the rest of the central Caribbean through most of this morning, then across the western Caribbean through tonight and inland central America on Saturday.


...ITCZ/MONSOON TROUGH...

The monsoon trough axis extends from 14N16W to 09N24W to 11N30W to a 1012 mb low at 10N35W and to 08N45W, where scatterometer data indicates the ITCZ then begins and continues to just east of the tropical wave along 46W, and resumes to the west of the same wave to 06N56W. Aside from convection related to the tropical wave features as described above, scattered moderate isolated strong convection is within 240 nm south of the monsoon trough between 14W-19W, between 90-240 nm south of monsoon trough between 26W-31W, and within 60 nm south of the monsoon trough between 38W-40W. Scattered moderate convection is within 60 nm either side of the monsoon trough between 40W-43W.


...DISCUSSION...

GULF OF MEXICO...

In the upper levels, a small upper low moving west-southwestward is over the north-central Gulf at 27N92W. The southern periphery of strong riding aloft associated with a large anticyclone over Oklahoma covers the northern portion of the Gulf. Dry air aloft is present over some areas of the central and western Gulf. At the surface, a weak 1016 mb high is centered over the NW Gulf at 28N92W. It is maintaining a rather weak pressure pattern over the area. A 1013 mb low is over southeastern Georgia with a trough trailing southwest to across NE Florida and to 29N83W to 27N87W. Latest satellite imagery and NWS mosaic radar animation are indicating less shower and thunderstorm activity over the basin than what was seen the past few nights. Isolated showers and thunderstorms are mainly over the far SE Gulf and Straits of Florida, and are moving to the west. Isolated showers are over the NE Gulf near the surface trough, and also south of 26N west of 85W, with the exception of an isolated thunderstorm over the eastern Bay of Campeche near 19N92W. This thunderstorm was initiated by a small upper low noted over the eastern Bay of Campeche. Little change is expected through the weekend 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.

CARIBBEAN SEA...

Two tropical waves are moving across the basin. Refer to the section above for details on these features. Scattered showers and isolated thunderstorms continue over the western Caribbean elsewhere to the west of the activity associated with the tropical wave along 76W. This activity is located on the southern periphery of an upper-level low centered just south of western Cuba as seen in water vapor imagery. Scattered moderate isolated strong convection is noted over the SW Caribbean from 09N-13N west of 80W. This activity is along the eastern extension of the Pacific monsoon trough that protrudes eastward to northwestern Colombia, and is being further aided by upper diffluent flow found on the southern extent of an upper trough located over the NW Caribbean that is associated with the aforementioned upper-level low. Isolated showers and possible isolated thunderstorms are elsewhere over the western Caribbean. The present pressure gradient will allow for fresh to strong trades to continue over the south-central Caribbean through the weekend, with moderate to locally fresh trades elsewhere.


...HISPANIOLA...

Isolated showers and thunderstorms are noted over some of the interior locations of the island as well as over its adjacent waters. Scattered showers and thunderstorms are expected to develop again today with daytime heating and local effects in play over some of the interior locations. However, dry air aloft nearby may limit the activity some. Moisture should increase slightly tonight into Saturday as the present eastern Caribbean tropical wave passes through the central Caribbean. Some pockets of dry air behind the wave will follow in behind the wave.

ATLANTIC OCEAN...

In the upper levels, a large upper-level low lifting northward is identified to be near 27N58W, with a trough stretching south to 21N58W and southwestward to the northeastern portion of the Caribbean. This low is reflected at the surface as a trough that extends along a position from near 27N59W to 21N62W. Isolated showers and thunderstorms are seen north of 22N between 53W-62W, and also to the southeast of the trough from 17N-20N between 55W and the northern Leeward Islands. Scattered showers and thunderstorms are north of 28N between 62W-70W, and are ahead of of a mid-upper level trough that is along 32N63W to near 29N68W. Scattered showers and isolated thunderstorms aided by upper divergence present to the east-northeast of the upper low just south of western Cuba are occurring south of 23N between 70W- 77W, including the SE Bahamas. This activity is expected to linger into tonight, then begin to lift northward on Saturday in response to a cold front that pushes off the southeastern United States and to the northwest portion Saturday through Sunday. Otherwise, high pressure ridging along 28N is supporting light to gentle breezes and slight seas. Farther east, high pressure over the central Atlantic is maintaining gentle to moderate trade winds, with Saharan dust dominating east of about 59W and is maintians very stable conditions there.

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

$$ Aguirre

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