Tropical Weather Discussion
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135
AXNT20 KNHC 211805
TWDAT

Tropical Weather Discussion
NWS National Hurricane Center Miami FL
205 PM EDT Fri Jul 21 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 1200 UTC surface analysis and satellite imagery through
1745 UTC.

...TROPICAL WAVES...

A tropical wave over the eastern Atlantic extends from near 17N32W
to 04N34W, moving westward near 20 kt. The wave crosses monsoonal
southwesterly flow that exists off the coast of Africa, and where
deepening moisture is evident. The GFS guidance shows a well-
defined 700 mb trough with this wave. Scattered moderate
convection is east of the wave within 120 nm north of the monsoon
trough. METEOSAT imagery along with SSMI Total Precipitable Water
(TPW) imagery show an extensive area of Saharan dry air and dust
northwest through northeast of the wave.

A large amplitude tropical wave is over the central Atlantic with
its axis extending from near 24N57W to 16N58W, and becomes ill-
defined to 09N57W. It is moving westward at 15 kt. Total
Precipitable Water imagery depicts a bulge of very deep moisture
within 180 nm east of the wave from 17N-21N. This wave has been
exhibiting a very well-defined inverted-v shape structure the
past several days. Latest satellite imagery shows and increasing
area of scattered moderate isolated strong convection from 19N-
22N between 55W-60W. Scattered showers and isolated thunderstorms
are elsewhere within 180 nm either side of the wave. A portion of
this wave is forecast to move across the eastern Caribbean during
the upcoming weekend, bringing increasing moisture that will
produce scattered showers and thunderstorms with gusty winds.

A pretty well-defined tropical wave, which in recent days was
part of the large cyclonic envelop that encompassed the wave
to its east described in above paragraph, is identified over the
eastern Caribbean this morning with its axis from just east of
Puerto Rico to 14N67W to inland Venezuela at 11N67W. It is
moving westward at 20 kt. SSMI Total Precipitable Water imagery
moisture to the west of the wave to near 71W, and within 120 nm
east of the wave from 13N-17N. There is a well defined 700 mb
trough associated with this wave. Scattered showers and isolated
thunderstorms are seen west of the wave to 75W and north of 15N.
Scattered showers are within 120 nm east of the wave north of 13N.
Any of the shower and thunderstorm may produce strong gusty winds
as the wave moves westward across the remainder of the eastern
Caribbean this afternoon and evening, then across the central
Caribbean on Saturday and Saturday night.

A tropical wave is over the western Caribbean with its axis near
83W south of 18N to the eastern Pacific waters near 04N84W. It is
moving westward near 15 kt. This wave is the remnant of former
Tropical Storm Don. Abundant low level moisture is to the south
of 15N west of 80W, and has resulted in scattered moderate to
isolated strong convection from 10.5N-13N west of 80W. Scattered
showers and thunderstorms are elsewhere south of 14N west of 79W.
The wave portion south of 15N will move inland Central America
this afternoon, while the portion north of 15N will move inland
the far southern Yucatan Peninsula and Belize on Saturday. Expect
scattered to numerous showers and thunderstorms to affect much of
the interior portions of Honduras and Nicaragua through Sunday
and showers and thunderstorms to increase over the southern
Yucatan Peninsula and Belize late Saturday through Sunday. This
activity may contain gusty winds and produce locally heavy
rainfall.

...ITCZ/MONSOON TROUGH...

The monsoon trough axis extends from 15N17W to 13N24W to 12N32W
to 09N38W The ITCZ then extends from 09N38W to a weak low pressure
of 1014 mb near 09N42W to 06N45W and to 10N59W. It resumes just
west of the central Atlantic tropical wave to 10N62W. Aside from
the convection associated with the tropical waves, scattered
moderate isolated strong convection is just offshore the coast
of W Africa from 11N-14N between 17W-20W. Scattered moderate
isolated strong convection is within 240 nm north of the ITCZ
axis between 42W-50W and also within 120 nm south of the ITCZ
axis between 43W-48W. Scattered moderate convection is south
of the ITCZ axis within 60 nm of 08N58W.

...DISCUSSION...

GULF OF MEXICO...

In the upper levels, a small upper level low is centered over
Mexico near 23N100W. Another small upper level low is centered
over the Yucatan Peninsula near 21N91W. A third and larger upper
level low has dropped southwest from offshore the southeastern
U.S. coast where it was yesterday to the present location of the
NE Gulf. Plenty of moisture and instability is in place over much
of the central and eastern Gulf. This has lead to the development
of scattered to numerous showers and thunderstorms over these
areas of the Gulf, with the added assistance of a weak surface
trough over the north-central Gulf as well. The low over the NE
Gulf will retrograde westward to the north-central Gulf by
tonight. With weak pressure in place maintaining a relatively
weak pressure pattern and forecast to continue through the next
48 hours, expect for the shower and thunderstorm to be the norm
through the weekend. Some of this activity may contain frequent
lightning and strong gusty winds.

A surface trough is over the central Bay of Campeche. Isolated
showers and thunderstorms are within 180 nm west of the trough,
and elsewhere over the Gulf. This activity will change little
through Sunday.

CARIBBEAN SEA...

The main features in the basin are the two tropical waves moving
through the basin. See the section above for discussions on
these features. In between the tropical waves there ares of
dry air aloft as noted in water vapor imagery. In addition, some
Saharan dry air has advected in over portions of the Caribbean
during the recent days. These conditions are allowing for
mainly fair weather conditions outside the shower and
thunderstorm activity related to the waves. They are forecast
to persist through the weekend, with at most isolated showers
quickly moving westward in the trade wind flow.

...HISPANIOLA...

A tropical wave moving quickly to the south of the island. An
area of moisture to the northwest of the wave as noted in
the SSMI Total Precipitable Water imagery is situated just
to the south of Hispaniola, and is being enhanced by an upper
level low located just east of the central Bahamas. Scattered
showers and isolated thunderstorms within this area of moisture
are just south of the central and eastern portions of the island
as of early this afternoon. Scattered to broken low clouds are
developing over much of the interior portions of the island
as daytime heating combines with the presence of moisture and
instability. Surface observations indicate that shower and
thunderstorm activity may be developing from some of these
clouds. Moisture from the aforementioned tropical wave is expected
to come into play to allow for the formation of more showers and
thunderstorms over much of the Hispaniola through early on
Saturday, then diminish later in Saturday as the wave pulls off to
the west of the Hispaniola regional waters.


ATLANTIC OCEAN...

Two tropical wave are moving through the far southern waters,
an a tropical wave is over the far eastern Atlantic region. See
the Tropical Waves section above for discussions on these
features. details. In the upper levels, latest water vapor imagery
shows a large upper level low just east of the central Bahamas,
with a trough extending south-southwestward to Haiti and to
across the central Caribbean Sea. Another large upper level
low moving southeastward is noted over the north-central portion
of the basin near 31N45W, with a trough stretching southwestward
to 23N50W and to near 19N57W. At the surface, a 1023 mb high is
north of the area near 34N39W with a ridge southwestward to a 1024
mb high at 29N54W, and a ridge extending from it to the central
Bahamas. Aside from convection associated with the above described
tropical waves, scattered to numerous showers and thunderstorms
are observed over the far western part of the basin north of 24N
and west 76W. Scattered showers and isolated thunderstorms
associated with the upper level low near 31N45W exists from 25N-
31N between 40W-50W. Isolated showers and thunderstorms are seen
elsewhere north of 20N west of 40W. The remainder of the basin is
under very stable conditions as dry air aloft persists. The
METEOSAT-9 imagery is depicting extensive plumes of Saharan dust
overspreading the eastern portion of the basin, and much of the
central portion outside moisture associated with the tropical
waves. The most western extent of the dust has reached from the
north-central and NW Caribbean Sea areas north to much of the
Bahamas of early this afternoon.

For additional information please visit
http://www.hurricanes.gov/marine

$$
Aguirre



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