[Tropical] Tropical Weather Discussion

[EMWIN Server]

AXNT20 KNHC 211758
TWDAT

Tropical Weather Discussion
NWS National Hurricane Center Miami FL
158 PM EDT Fri Sep 21 2018

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
1715 UTC.

...TROPICAL WAVES...

Showers and thunderstorms about 520 nm SSE of the Cabo Verde
Islands are showing some signs of organization. This activity is
associated with a tropical wave, analyzed along 17W from 03N-16N.
An ASCAT pass from around 10Z this morning shows a partially
closed low-level circulation near 07N18W with winds of 25-30 kt
in the eastern semicircle. Visible and IR satellite images depict
a coherent cluster of convection with a banding pattern from 04N-
10N, between 16W-23W, where numerous moderate and isolated strong
convection is noted. The environment is forecast to be conducive
for slow development, and a tropical depression could form early
next week while the system moves west around 15 kt across the low
latitudes of the eastern and central tropical Atlantic Ocean. The
system has a low chance of tropical cyclone formation within the
next 48 hours.

The tropical wave that was previously along 26W/27W has been
relocated to 36W from 04N-16N and is moving west at 10-15 kt. The
wave corresponds nicely with a maximum in TPW imagery. Model
analyses depict an associated 700 mb trough along 39W, or just
ahead of the wave axis. Isolated showers are noted within 120 nm
of the wave axis from 09N-13N. Little change is expected with this
wave over the next 24-48 hours.

A central Atlantic tropical wave has its axis along 52W between
07N-17N moving W around 10 kt. The wave runs through a 1012 mb
low near 12N52W, or about 435 nm east of the Windward Islands.
Visible imagery shows an exposed low-level center due to moderate
to strong westerly wind shear. Scattered moderate convection is
limited to the eastern semicircle from 10N-15N, between 48W-52W.
The combination of dry mid-level air and strong upper-level winds
is expected to hinder any further development of the low while it
moves WNW.

A central Caribbean Sea tropical wave has been relocated to 78W/79W
based on surface observations. This location is a little to the
west of the analysis from 6 hours ago, which had it along 74W.
Movement is estimated to be westward around 10-15 kt. Scattered
showers are noted south of Jamaica from 16N-18N between 76W-80W. A
maximum in TPW is noted just east of the wave axis.

...MONSOON TROUGH/ITCZ...

The monsoon trough axis extends from the coast of Guinea near
10N14W to 07N18W to 07N27W. The ITCZ continues from 07N27W to
08N40W to 10N56W to NE Venezuela. Other than convection
associated with the tropical waves, isolated weak to moderate
convection is located within 60 nm south and 150 nm north of the
ITCZ between 40W-47W. Scattered moderate convection is also noted
over NE Venezuela as well as Trinidad and Tobago.

...DISCUSSION...

GULF OF MEXICO...

An upper-level cyclonic circulation is moving slowly westward
over the far eastern Gulf and the west coast of Florida. Satellite
imagery and NWS Doppler Radar indicate scattered showers and
thunderstorms over the eastern Gulf, mainly from 24N-29N, between
81W-87W, including portions of the Florida Keys and the Florida
Peninsula.

A ridge extends from a high pressure over the eastern U.S. to the
northern Gulf of Mexico. The ridge will support gentle to moderate
easterly winds and seas 3 ft or less across most of the Gulf of
Mexico through the middle of next week. It is possible that a
typical nocturnal surface trough, that will be moving westward
across the southwestern corner of the Gulf of Mexico, may produce
locally fresh winds through Sat.

CARIBBEAN SEA...

A tropical wave is moving across the Central Caribbean Sea. Please
see the Tropical Waves section for more details.

The eastern Pacific monsoon trough combined with diffluence aloft
is helping to induce scattered showers and thunderstorms over
portions of Costa Rica, Nicaragua and eastern Honduras. Plenty of
moisture will persist over the southwest Caribbean through
Saturday as the tropical wave, currently along 78W/79W, reaches
the area and moves inland into central America late Sat or early
Sun.

The pressure gradient between central Atlantic high pressure and
the lower pressure near the central Caribbean tropical wave will
support fresh trades today. Winds will diminish over most of the
Caribbean Sea this weekend.

ATLANTIC OCEAN...

Currently, there are three tropical waves between the west coast
of Africa and the Lesser Antilles. Please see the Tropical Waves
section for more details.

A cold front extends from a 1010 mb low just SE of Bermuda and
enters the forecast area near 31N64W and extends westward to
28N70W to the east coast of FL near Cape Canaveral. Isolated
showers are noted along the frontal boundary. The front will drift
slightly southward today, then dissipate by Saturday morning. The
low is currently producing moderate to fresh winds north of 28N
between 60W-65W. The low is expected to drift south and then west
over the forecast area between 60W-70W while weakening.

The remainder of the Atlantic forecast area is under the influence
of a ridge, anchored by a 1022 mb surface high located W of the
Madeiras Islands near 34N23.5W.

Looking ahead, a non-tropical low is forecast to develop tonight
or on Saturday to the north of the forecast area about midway
between Bermuda and the Azores. Conditions are expected to be
conducive for the low to acquire subtropical or tropical
characteristics, and a subtropical or tropical cyclone is expected
to form late this weekend or early next week while the low
meanders over the central Atlantic Ocean. There is a low chance of
development within the next 48 hours but a high chance of
development within 5 days.

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

$$
Hagen/GR
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