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Atlantic Tropical Weather Discussion

AXNT20 KNHC 250605

Tropical Weather Discussion
NWS National Hurricane Center Miami FL
205 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 0000 UTC surface analysis and satellite imagery through 
0545 UTC.


A far eastern Atlantic tropical wave has its axis along 19W from
16N to 06N, 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 east of the wave within 
30 nm of a line from 13.5N19W to the coast of Africa at 14N17W. 
Scattered moderate convection is within 30 nm either side of a 
line from 10N19W 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 extending from 15N30W to 09N31W 
to 03N31W, moving westward around 15 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 dry air and related 
dust as clearly seen on satellite 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 23N45W to 16N43W to 09N43W, moving westward at 10-15 
kt. The wave lies under the eastern edge of 700 mb troughing. The 
wave remains obsolete 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 west of the wave at 24N49W is moving westward in tandem with 
the wave, and it is assisting with development of the shower and 
thunderstorm activity. 

A tropical wave is approaching the Lesser Antilles, with its axis 
extending from near 18N55W to 11N56W to the coast of South America 
at 06N56W. 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 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 shower and thunderstorm activity 
within an area from 11N-14N between 53W-61W. The wave will cross
the Lesser Antilles tonight, 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 analyzed tropical waves, scattered
moderate convection exists within 120 nm north of the axis 
between 33W-35W, within 90 nm south of the trough between 20W-26W,
and north of the ITCZ within 30 nm of 10N49W.



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 far south-central Gulf. Isolated showers 
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 seen. Similar 
activity is over the northwest portion of the sea. 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 tonight. 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 
west across much of the island. 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 


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 24N49W is moving westward. 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 ridge 
anchored by a 1025 mb high centered near 32N58W, and a 1032 mb 
high well northeast of the area near 37N29W. A trough extends from
near 31N52W to 26N55W. Isolated showers are within 60 nm of the 
trough. Scattered showers and thunderstorms are expected to 
develop again over much of the northwestwestern portion of the 
area today and continue into this evening.

In the tropical Atlantic, four tropical waves are moving across 
that portion of the discusion area. See above for discussions 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 mid-week.

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