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AXNT20 KNHC 161148

Tropical Weather Discussion
NWS National Hurricane Center Miami FL
748 AM EDT Sun Sep 16 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 0600 UTC surface analysis and satellite imagery through
1145 UTC.


Tropical Depression Florence is centered near 33.8N 81.4W at
16/0900 UTC or 20 nm SW of Columbia South Carolina moving W at 7
kt. Estimated minimum central pressure is 1000 mb. Maximum
sustained wind speed is 30 kt with gusts to 40 kt. Scattered
moderate convection is removed from the center of the cyclone
from 29N to 37N between between 76W and 83W. The latest NHC
advisory has Florence turning toward the northwest, with an
increase in forward speed today, followed by a turn toward the
northeast with an additional increase in forward speed on Mon. On
this forecast track, Florence’s center will move across the
western Carolinas on Sun and then recurve over the Ohio Valley and
northeastern U.S. Mon and Tue. See the latest NHC
forecast/advisory under AWIPS/WMO headers MIATCPAT1/WTNT31 KNHC
for more details.

Tropical Storm Helene is centered near 42.5N 28.4W at 16/0900 UTC
or 230 nm N of Faial Island in the central Azores moving NE at 22
kt. Estimated minimum central pressure is 993 mb. Maximum
sustained wind speed is 45 kt with gusts to 55 kt. Scattered showers
and isolated thunderstorms are from 42N to 50N between 18W and
32W. Helene is forecast to continue on its present motion, with
an increase in forward speed expected during the next couple of
days. On the current forecast track, Helene should move away from
the Azores overnight and then approach Ireland and the United
Kingdom tonight and Mon. See the latest NHC forecast/advisory
under AWIPS/WMO headers MIATCPAT3/WTNT33 KNHC for more details.

Tropical Storm Joyce centered near 34.0N 36.1W at 16/0900 UTC or
500 nm WSW of the Azores moving ENE at 15 kt. Estimated minimum
central pressure is 1002 mb. Maximum sustained wind speed is 35
kt with gusts to 45 kt. Scattered moderate convection is noted
to the northeast of the center from 35N to 38N between 30W and
33W. Joyce is forecast by the latest NHC advisory to continue on
its current motion over the next day or so, with a decrease in
forward speed. After that time, Joyce should turn east-southeast
away from the Azores. See the latest NHC forecast/advisory under
AWIPS/WMO headers MIATCPAT5/WTNT35 KNHC for more details.


A far eastern Atlantic tropical wave has its axis extending along
37W between 05N and 20N, moving westward at 15 kt. The wave is
supported by broad troughing at the 700 mb level, but is
surrounded by a rather dry environment from dust that moved off
the coast of Africa a few days ago. The dust is is spreading
westward across the eastern Atlantic. These dry conditions are
inhibiting convection from developing along or near the wave. Only
scattered showers are within 150 nm east and 120 nm w of the wave
axis from 06N to 08N.


The monsoon trough axis extends from well inland Africa southwestward
to the coast of Guinea-Bissau and continues to 09N10W and to
08N26W. The ITCZ extends from 08N26W to just east of the above
described tropical wave, and resumes just west of the same
tropical wave to 09N43W to 09N53W. Aside from the shower and
thunderstorm activity associated with the above described tropical
wave, scattered showers are within 120 nm south and 90 nm north
of the ITCZ.



An upper-level low located north of the Yucatan Peninsula near
23N89W is moving westward. A trough extends from the low to
inland the central Yucatan Peninsula. Its cyclonic flow covers the
central gulf section. Upper-level high pressure ridging stretches
from Oklahoma southward to eastern Texas and eastern Mexico north
of 22N. Northerly flow produced by the anticyclonic flow over the
ridge is channeling broken to overcast mid and high level clouds
southward across much of the western gulf west of 92W. Isolated
showers and thunderstorms are possible underneath these clouds.
Areas of light to moderate rain along with scattered to numerous
showers and thunderstorms over deep southern Texas associated with
a surface trough and attendant weak low pressure that are slowly
moving westward.


A 1007 mb low analyzed over the central Caribbean near 16N73W has
a trough extending north-northeast to the border of Haiti and the
Dominican Republic and south to near 13N74W. This system depicts
is associated with the remnants of recent tropical cyclone Isaac.
Scattered moderate isolated strong convection is seen within 30 nm
either side of a line from 15N72W to 15N73W. Scattered moderate
convection is within 30 nm either side of a line from 15N71W to
17N71W. This area of low pressure is forecast to bring locally
heavy rains and gusty winds across portions of Hispaniola,
Jamaica, and eastern Cuba during the next couple of days, with the
possibility of it undergoing some slow development. The Ascat
pass from Sat afternoon showed mainly moderate to fresh winds with
the low, and with a few winds of strong intensity within the
aforementioned convection. Gentle trades are west of the low and
trough, while gentle to moderate trades are elsewhere east of the
low and trough. A far eastern tropical wave will move across the
tropical waters east of the Windward Islands on Tue accompanied by
fresh to locally strong trades and building seas.

The tail-end of a broad central Atlantic trough extends
southwestward to the eastern Caribbean from just east of Puerto
Rico to near 16N66W. Trough energy is helping to trigger off
scattered moderate convection from 15N to 17N between 65W and
68W. Scattered showers and isolated thunderstorms are elsewhere
north of 15N and east of 70W.

Over the far southwestern Caribbean, the eastern extent of the
eastern Pacific monsoon trough is bringing scattered showers and
thunderstorms over the waters south of 12N from northwestern
Colombia westward to 76W.


Please refer to the Special Features section above, for details
on Tropical Storms Helene and Joyce and on Tropical Depression

A broad deep-layer trough extends across the central Atlantic
from a large and elongated upper-level low that is north of the
area near 36N52W southwestward to 32N58W to 25N64W, to just east
of Puerto and the far northeast Caribbean Sea. Plenty of
instability exists with this trough as it acts on a very moist
and unstable atmosphere. The trough is forecast to move westward
during the next few days. with an upper-level low forecast to
evolve from this trough near 28N65W by Wed.

At the surface, a 1011 mb low is near 28N64W, with a trough from
31N61W to 24N66W. A 1013 mb low is near 24N57W, with a trough
extending east to southeastward to 23N51W. Increasing scattered
moderate convection quickly moving south-southeastward is seen
from 22N to 26N between 60W and 65W, and from 28N to 30N between
54W and 60W. Scattered showers and isolated thunderstorms are
elsewhere from 18N to 31N between 50W and 66W. Isolated showers
are west of 66W, except for scattered showers and thunderstorms
that are over a small section of the far northwestern waters of
the area from 28N to 32N between 77W and 80W due to a surface
trough that extends from just east of Tropical Depression Florence
south to 32N80W, and southwestward from there to inland north-
central Florida.

Otherwise, a rather weak pressure pattern is in place over the
central and western Atlantic, while stronger high pressure is
present over the eastern Atlantic.

An area of African dust is over the eastern Atlantic from 11N to
25N east of 37W, and is gradually migrating westward.

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