Atlantic Tropical Weather Discussion (Text)

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

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 


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.

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Page last modified: Tuesday, 25-Jul-2017 10:32:46 UTC