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Tropical Weather Discussion


AXNT20 KNHC 281046

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.


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.


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.



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.


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.


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.


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.

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