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Atlantic Tropical Weather Discussion

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 

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.

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