Tropical Weather Discussion
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AXNT20 KNHC 280837 CCA

Tropical Weather Discussion
NWS National Hurricane Center Miami FL
205 AM EDT Fri Jul 28 2017

corrected surface analysis time

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.


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. The 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 across 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. 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


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 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 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 has quickly developed over the SW
Caribbean from 09N-13N 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


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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