Atlantic Tropical Weather Discussion (Text)

AXNT20 KNHC 280605

Tropical Weather Discussion
NWS National Hurricane Center Miami FL
205 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 
0545 UTC.


An eastern Atlantic tropical wave extends its axis from 19N34W to
broad low pressure of 1012 mb at 10N34W and to 06N34W, moving 
westward at 10 kt. Scattered moderate convection is noted from 
09N-12N between 32W-38W. 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 within
45 nm of 12N34W. Scattered moderate convection is within 30 nm of
line from 12N34W to 11N37W.

A central Atlantic tropical wave extends its axis from 19N43W to 
12N45W to 03N45W, 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 is 
suppressed to the south of 12N within the region where the monsoon
trough transitions to the ITCZ. Satellite imagery shows scattered
moderate isolated strong convection west of the wave within 30 nm
of a line from 08N46W to 09N47W. Scattered moderate convection is
east of the wave to 43W from 05N-09N. This wave is forecast to 
approach the Lesser Antilles late on Sunday.

An eastern Caribbean tropical wave is near 63W, and extends 
northward to the Atlantic near 21N. Water vapor imagery depicts 
dry subsident air over the wave north of 15N. 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 is just east of Jamaica with
its axis along 76W 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 over the wave. The southern portion of the wave is 
aiding strong type convective complex observed 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. This 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. 


The monsoon trough axis extends from 19N16W southwest to 13N24W
to a 1012 mb low at 10N34W and to 07N45W, where scatterometer data 
indicates the ITCZ then begins and continues to 07N50W to 
near 06N56W. Aside from convection related to the tropical
waves as described above, scattered moderate isolated strong
convection is within 60 nm south of the monsoon trough between 
38W-40W, and also to the south of the monsoon trough from 06N-09N
between 26W-30W. Scattered moderate convection is within 180 nm 
south of the trough between 40W-43W.

Scattered moderate isolated strong convection is south of the 
monsoon trough from 06N-09N. Scattered moderate convection is 
well to the southeast of the monsoon trough off the coast of 
Africa from 05N-09N between 14W-19W.



In the upper levels, a small upper low moving west is over the 
north-central Gulf at 27N91W. 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 weak pressure pattern over the area. 
A 1014 mb low is over northern Florida near 30N84W with a trough 
extending southwest to 27N88W. 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.
Scattered showers and isolated thunderstorms are mainly over 
the far SE Gulf and Straits of Florida, and are moving to the 
west. Isolated showers and thunderstorms are over the NE Gulf near 
the surface trough, and also from 26N-28N east of 87W. 

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.


Two tropical waves are moving across the basin. Refer to the
section above for details. Scattered showers and isolated 
thunderstorms continue over the western Caribbean elsewhere to 
the west of that 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 has quickly developed over the SW Caribbean from 10N-
14N west of 80W. 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.


The scattered showers showers and thunderstorms that developed 
over the interior portions of the island on Thursday have 
dissipated. Only 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. With 
dry air aloft nearby, this may limit the activity some. Moisture
should increase on Saturday as the present eastern Caribbean
tropical wave passes through the central Caribbean.


In the upper levels, a large upper-level low lifting northward
is identified to be near 26N58W, with a trough stretching south-
southwestward to 21N59W and to the northeastern portion of the 
Caribbean. This low is reflected at the surface as a trough that 
extends along a position from 31N56W to 23N59W. Isolated showers 
and thunderstorms are seen north of 22N between 53W-62W. Scattered
showers and thunderstorms are north of 28N between 63W-70W, and
are ahead of a stationary front north of the area. Scattered 
showers and isolated thunderstorms are over and near the Bahamas.
Otherwise, higher surface 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 55W and is resulting 
in very stable conditions there.

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Page last modified: Friday, 28-Jul-2017 06:06:03 UTC