Tropical Weather Discussion
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AXNT20 KNHC 202340

Tropical Weather Discussion
NWS National Hurricane Center Miami FL
740 PM EDT Thu Jul 20 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 1800 UTC surface analysis and satellite imagery through
2330 UTC.


A tropical wave over the far eastern Atlantic has its axis
extending from near 18N29W to 11N29W to 04N29W, moving westward
at 20 kt. This wave is along the leading edge of a deep pool of
moisture associate with a well pronounced monsoon trough off the
coast of Africa. With this, scattered showers are observed along
the wave`s axis from 09N-12N. The METEOSAT-9 imagery along with
the Total Precipitable Water (TPW) imagery show an extensive area
of Saharan dry air and dust surrounding the wave which is
limiting deep convection. The GFS guidance continues to show
well-defined 700 mb with this wave.

A large amplitude tropical wave is over the central Atlantic with
axis from 25N49W to 08N51W, moving west at around 15 kt. The
Total Precipitable Water (TPW) imagery animation depicts deep
moisture south of 20N between 48W-54W. In this area, scattered
moderate convection prevails. A portion of this wave is forecast
to move across the eastern Caribbean during the upcoming weekend,
bringing increasing moisture and probabilities for scattered
showers and thunderstorms with gusty winds.

A tropical wave crossed the Lesser Antilles this morning, and
observations showed a wind gusts up to 31 kt at Martinique at
around 1230Z. The wave`s axis is located now from 19N62W to
07N62W, moving westward around 20 kt. No significant convection
is noted at this time, as the feature is moving across an area
where abundant Saharan dust prevails. This wave is capable to
produce strong gusty winds as it continues moving west across the
eastern Caribbean late tonight and into Friday morning, and
across the central Caribbean on Saturday and Saturday night.

A tropical wave is over the central Caribbean Sea. Its axis
extends from 17N77W to inland NW Colombia at 07N76W. The wave is
under the southern portion of an upper-level trough. With this,
scattered moderate convection is observed along and to the west of
the wave south of 15N between 77W-83W. The wave will quickly
move across the rest of the central Caribbean through this
evening, and across the western Caribbean on Friday. Strong gusty
winds will continue to be possible with the associated shower and
thunderstorm activity.

A western Caribbean tropical wave has its axis extending from
22N84W to inland northeastern Honduras and to over eastern
Nicaragua. It is moving westward at 15-20 kt. The TPW imagery
shows abundant low/mid-level moisture content within the wave.
Scattered moderate convection prevails inland affecting Central
America and adjacent waters south of 17N.


The monsoon trough extends from 13N17W to 09N32W. The ITCZ then
extends from 09N32W to 07N42W. Aside from the convection
associated with the tropical waves, scattered moderate convection
is observed within 180 nm north of the ITCZ.



A surface ridge extends across the basin anchored by a 1020 mb
high centered near 26N93W. An upper-level low is centered over the
Florida Peninsula enhancing convection across the area and
adjacent waters mainly within 100 nm of the coastline. To the
south, a tropical wave is approaching from the western Caribbean
enhancing convection across the Yucatan Peninsula, western Cuba
and adjacent waters. The upper-level low will shift southwest to
the northeast Gulf Friday night, and retrograde westward to the
western Gulf by Saturday while weakening to a trough. The instability
associated with this feature should help set-off scattered
showers and thunderstorms over some areas of the eastern and
central Gulf through this period. In addition, expect
scattered showers and thunderstorms to remain active over the Bay
of Campeche through Saturday with the tropical wave. Surface
ridging will remain over the central to eastern Gulf through the
next 24 hours, with its associated gradient supporting a gentle
to moderate anticyclonic flow.


The main features in the basin are the now three tropical waves
moving through the basin. See the section above for details.
Scattered showers and isolated thunderstorms are seen over the far
SW Caribbean within 120 nm of the coasts of Costa Rica and
Panama due to the proximity of the monsoon trough that currently
extends along 10N. Mostly fair weather conditions are observed
elsewhere, with areas of Saharan dust spreading westward across
much of the waters mainly north of about 15N.


Dry air and dust aloft have moved in over the Caribbean north of
15N including Hispaniola supporting fair weather. A tropical wave
currently over the far eastern Caribbean will pass just to the
south of the island during the next 24 hours. Moisture associated
with this wave may help erode some of the dry air in place as it
advects towards the island leading to increase chances for
scattered showers and isolated thunderstorms across the island.


The northern portions of two tropical wave are moving through the
far southern waters. See the Tropical Waves section above for
details. An upper-level low is developing across the western
Atlantic supporting scattered moderate convection mainly west of
74W. A surface trough was noted extending from 31N77W to 28N79W.
Weather conditions are very stable elsewhere as an extensive
area of Saharan dust continues to migrate westward over the
basin. The remainder of the basin is under the influence of a surface
ridge anchored by a 1025 mb high centered near 31N41W. A similar
weather pattern is expected trough the next 24 hours.

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